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Many thanks to R. Calaga and T. Mastoridis for help, material and comments. Cavity Control. P. Baudrenghien CERN BE-RF 6th Crab Cavity workshop (CC13 ) Dec 9-11, 2013. Content. Phasing the CC with the ACS Keeping the CC tilt aligned with the individual bunch centers …or not - PowerPoint PPT Presentation
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P Baudrenghien CERN BE-RF
6th Crab Cavity workshop (CC13)
Dec 9-11 2013
CAVITY CONTROL
Many thanks to R Calaga and T Mastoridis for help material and comments
Content
bull Phasing the CC with the ACSKeeping the CC tilt aligned with the individual bunch centers hellipor not
bull Impedance at the fundamentalSame threshold as for the HOM or not
bull Machine protectionMechanisms for fast CC field changes LLRF mitigations
bull Operational scenarioFilling and ramping with transparent CC Physics with crabbing
bull Conclusions
Dec 9 2013 6th Crab Cavity workshop (CC13) 2
PHASING CC WITH THE ACS
Keeping the CC tilt aligned with the individual bunch centers hellipor not
Dec 9 2013 6th Crab Cavity workshop (CC13) 3
Phasing the CC with the bunch centre
bull A phase error (wrt bunch centre) causes an offset of the bunch rotation axis This results in a transverse offset Dx at the IP
bull 1 deg RF phase (7 ps) leads to Dx = 1 mm (15h transverse beam size)bull The stringent requirement concerns transverse emittance growth caused
by RF phase noise at frequencies intersecting one of the betatron bands Much larger offset can be accepted if static or at frequencies significantly below 3 kHz (first betatron band)
Dec 9 2013 6th Crab Cavity workshop (CC13)
RFRF
cx
4
A DETOUR PHASE MODULATION IN THE MAIN CAVITIES
Dec 9 2013 6th Crab Cavity workshop (CC13) 5
Present schemebull RFLLRF is currently setup for extremely
stable RF voltage (minimize transient beam loading effects) Less than 1 RF phase modulation over the turn with 035 A DC (7 ps)
bull To continue this way we would need at least 300 kW of klystron forward power at ultimate intensity (17E11 ppb)
bull Klystrons saturate at 200 kW with present DC parameters (ultimately 300 kW)
bull Sufficient margin necessary for reliable operation additional RF manipulations etc
bull At 7 TeV 11 A DC the synchrotron radiation loss is 14 keV per turn or 27 pW
bull All RF power is dissipated in the circulator loads
bull The present scheme cannot be extended much beyond nominal
Required klystron power for 115e11 ppb 25 ns 7 TeV 17e11 ppb 25 ns 450 GeV 17e11 ppb 25 ns 7 TeV
6
P Baudrenghien T Mastoridis ldquoProposal for an RF Roadmap Towards Ultimate Intensity in the LHC IPAC 2012
Dec 9 2013 6th Crab Cavity workshop (CC13)
Proposed RF phase modulation schemebull In physics
bull We will accept the modulation of the cavity phase by the beam current (transient beam loading) and adapt the voltage set point for each bunch accordingly
bull The klystron drive is kept constant over one turn (amplitude and phase)
bull The cavity is detuned so that the klystron current is aligned with the average cavity voltage
bull Needed klystron power becomes independent of the beam current For QL=60k we need 105 kW only for 12 MV total
bull Stability is not modified we keep the strong RFfdbk and OTFB
bull The resulting displacement of the luminous region is acceptable
bull During filling bull It is desirable to keep the cavity phase constant for
clean capture Thanks to the reduced total voltage (6 MV) the present scheme can be kept with ultimate
Modulation of the cavity phase by the transient beam loading in physics 2835 bunches 17 1011 pbunch 15 MVcavity QL=60k full detuning (-78 kHz)
7Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CCbull If the CC follows the phase modulation
bull Forcing the CC to follow the fast phase modulation (-10 degrees 400 MHz in the 32 ms long abort gap) results in huge power requirement
bull With the HiLumi parameters (2808 bunches 22E11 pbunch 111 A DC 32 ms long abort gap ) assuming 3 MV per crab cavity 300 W RQ we need an absolute minimum of 170 kW per cavity This minimum is achieved with a QL=44000
bull With QL=500000 we need 950 kW
8Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CC (contrsquod)bull Fixed CC phase
bull Keeping the Crab Cavity phase constant over the turn will result in a phase error df with respect to the individual bunch center
bull This phase error causes an offset of the bunch rotation axis resulting in a transverse displacement Dx at the IP
bull For a phase drift of 30 ps the transverse displacement is 5 mm approximately equal to the transverse beam size
bull Fortunately the filling patterns are identical for both rings (except for the first six or twelve bunches batch) and the phase errors will be equal for colliding pairs in IP1 (ATLAS) and IP5 (CMS) because the bucket numbering convention makes the bucket one of both rings (first bucket after the abort gap) ldquocolliderdquo in IP1 and IP5
bull There will therefore be no loss of luminosity only a modulation of the transverse position of the vertex over one turn This is acceptable by the experiments
9
RF
cx
Dec 9 2013 6th Crab Cavity workshop (CC13)
IMPEDANCE AT THE FUNDAMENTAL
ldquoBestrdquo parking position during fillingramping
Impedance reduction with cavities in operation (on resonance)
Dec 9 2013 6th Crab Cavity workshop (CC13) 10
Machine Impedance bull The HighLumi LHC will accelerate 11 A DC current per beam (compared
to 035 A DC in 2012)bull The Crab Cavities will introduce a series of Narrow-Band resonators in the
machine (fundamental plus HOMs)bull Control of the fundamental is the responsibility of the LLRF bull At the fundamental one cavity presents a transverse impedance around
25 GWm
bull At the fundamental frequency the effective impedance can be reduced by an active RF feedback
bull But what is the max impedance at the fundamental
Dec 9 2013 6th Crab Cavity workshop (CC13)
LRR QQc
11
Instability growth rate (12)bull A resonant transverse impedance with resistive part ZT [Ohmm] at resonant
frequency ωr=2πfr will drive coupled-bunch mode (n m) fr
=(n+pM+Qβ)f0+mfs with the growth rate
f0 and fs are revolution and synchrotron frequency
M is number of (symmetric) bunches
Qβ is betatron tune ωξ = Qβ ω0ξη ξ is chromaticity
Formfactor F(x) for water-bag bunch F(0)=1 (the worst mode m=0) F(x gt 05) asymp 05
Dec 9 2013 126th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Instability growth rate (22)bull Growth rate ~ 1E rarr maximum at low energy
bull At 450 GeV for nominal intensity and ZT =1 MOhmm (ξ=0)
minimum τinst = 015 [s]
bull Condition τinst gt τd gives
ZT [MOhmm] lt 015 (1-x16)τd x = (fr - fξ)τ lt 08
ZT [MOhmm] lt 03 (05+x)τd x gt 08
τd [s] is the damping time by transverse damper
τ is the bunch length typically 10 ns lt τ lt 15 ns
bull For ultimate intensity factor 23
Dec 9 2013 136th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Fortunately the situation looks better at the fundamental because bull we can control the cavity tune vs betatron bandbull we can act on the beam induced voltage via active RF
feedback
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Content
bull Phasing the CC with the ACSKeeping the CC tilt aligned with the individual bunch centers hellipor not
bull Impedance at the fundamentalSame threshold as for the HOM or not
bull Machine protectionMechanisms for fast CC field changes LLRF mitigations
bull Operational scenarioFilling and ramping with transparent CC Physics with crabbing
bull Conclusions
Dec 9 2013 6th Crab Cavity workshop (CC13) 2
PHASING CC WITH THE ACS
Keeping the CC tilt aligned with the individual bunch centers hellipor not
Dec 9 2013 6th Crab Cavity workshop (CC13) 3
Phasing the CC with the bunch centre
bull A phase error (wrt bunch centre) causes an offset of the bunch rotation axis This results in a transverse offset Dx at the IP
bull 1 deg RF phase (7 ps) leads to Dx = 1 mm (15h transverse beam size)bull The stringent requirement concerns transverse emittance growth caused
by RF phase noise at frequencies intersecting one of the betatron bands Much larger offset can be accepted if static or at frequencies significantly below 3 kHz (first betatron band)
Dec 9 2013 6th Crab Cavity workshop (CC13)
RFRF
cx
4
A DETOUR PHASE MODULATION IN THE MAIN CAVITIES
Dec 9 2013 6th Crab Cavity workshop (CC13) 5
Present schemebull RFLLRF is currently setup for extremely
stable RF voltage (minimize transient beam loading effects) Less than 1 RF phase modulation over the turn with 035 A DC (7 ps)
bull To continue this way we would need at least 300 kW of klystron forward power at ultimate intensity (17E11 ppb)
bull Klystrons saturate at 200 kW with present DC parameters (ultimately 300 kW)
bull Sufficient margin necessary for reliable operation additional RF manipulations etc
bull At 7 TeV 11 A DC the synchrotron radiation loss is 14 keV per turn or 27 pW
bull All RF power is dissipated in the circulator loads
bull The present scheme cannot be extended much beyond nominal
Required klystron power for 115e11 ppb 25 ns 7 TeV 17e11 ppb 25 ns 450 GeV 17e11 ppb 25 ns 7 TeV
6
P Baudrenghien T Mastoridis ldquoProposal for an RF Roadmap Towards Ultimate Intensity in the LHC IPAC 2012
Dec 9 2013 6th Crab Cavity workshop (CC13)
Proposed RF phase modulation schemebull In physics
bull We will accept the modulation of the cavity phase by the beam current (transient beam loading) and adapt the voltage set point for each bunch accordingly
bull The klystron drive is kept constant over one turn (amplitude and phase)
bull The cavity is detuned so that the klystron current is aligned with the average cavity voltage
bull Needed klystron power becomes independent of the beam current For QL=60k we need 105 kW only for 12 MV total
bull Stability is not modified we keep the strong RFfdbk and OTFB
bull The resulting displacement of the luminous region is acceptable
bull During filling bull It is desirable to keep the cavity phase constant for
clean capture Thanks to the reduced total voltage (6 MV) the present scheme can be kept with ultimate
Modulation of the cavity phase by the transient beam loading in physics 2835 bunches 17 1011 pbunch 15 MVcavity QL=60k full detuning (-78 kHz)
7Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CCbull If the CC follows the phase modulation
bull Forcing the CC to follow the fast phase modulation (-10 degrees 400 MHz in the 32 ms long abort gap) results in huge power requirement
bull With the HiLumi parameters (2808 bunches 22E11 pbunch 111 A DC 32 ms long abort gap ) assuming 3 MV per crab cavity 300 W RQ we need an absolute minimum of 170 kW per cavity This minimum is achieved with a QL=44000
bull With QL=500000 we need 950 kW
8Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CC (contrsquod)bull Fixed CC phase
bull Keeping the Crab Cavity phase constant over the turn will result in a phase error df with respect to the individual bunch center
bull This phase error causes an offset of the bunch rotation axis resulting in a transverse displacement Dx at the IP
bull For a phase drift of 30 ps the transverse displacement is 5 mm approximately equal to the transverse beam size
bull Fortunately the filling patterns are identical for both rings (except for the first six or twelve bunches batch) and the phase errors will be equal for colliding pairs in IP1 (ATLAS) and IP5 (CMS) because the bucket numbering convention makes the bucket one of both rings (first bucket after the abort gap) ldquocolliderdquo in IP1 and IP5
bull There will therefore be no loss of luminosity only a modulation of the transverse position of the vertex over one turn This is acceptable by the experiments
9
RF
cx
Dec 9 2013 6th Crab Cavity workshop (CC13)
IMPEDANCE AT THE FUNDAMENTAL
ldquoBestrdquo parking position during fillingramping
Impedance reduction with cavities in operation (on resonance)
Dec 9 2013 6th Crab Cavity workshop (CC13) 10
Machine Impedance bull The HighLumi LHC will accelerate 11 A DC current per beam (compared
to 035 A DC in 2012)bull The Crab Cavities will introduce a series of Narrow-Band resonators in the
machine (fundamental plus HOMs)bull Control of the fundamental is the responsibility of the LLRF bull At the fundamental one cavity presents a transverse impedance around
25 GWm
bull At the fundamental frequency the effective impedance can be reduced by an active RF feedback
bull But what is the max impedance at the fundamental
Dec 9 2013 6th Crab Cavity workshop (CC13)
LRR QQc
11
Instability growth rate (12)bull A resonant transverse impedance with resistive part ZT [Ohmm] at resonant
frequency ωr=2πfr will drive coupled-bunch mode (n m) fr
=(n+pM+Qβ)f0+mfs with the growth rate
f0 and fs are revolution and synchrotron frequency
M is number of (symmetric) bunches
Qβ is betatron tune ωξ = Qβ ω0ξη ξ is chromaticity
Formfactor F(x) for water-bag bunch F(0)=1 (the worst mode m=0) F(x gt 05) asymp 05
Dec 9 2013 126th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Instability growth rate (22)bull Growth rate ~ 1E rarr maximum at low energy
bull At 450 GeV for nominal intensity and ZT =1 MOhmm (ξ=0)
minimum τinst = 015 [s]
bull Condition τinst gt τd gives
ZT [MOhmm] lt 015 (1-x16)τd x = (fr - fξ)τ lt 08
ZT [MOhmm] lt 03 (05+x)τd x gt 08
τd [s] is the damping time by transverse damper
τ is the bunch length typically 10 ns lt τ lt 15 ns
bull For ultimate intensity factor 23
Dec 9 2013 136th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Fortunately the situation looks better at the fundamental because bull we can control the cavity tune vs betatron bandbull we can act on the beam induced voltage via active RF
feedback
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
PHASING CC WITH THE ACS
Keeping the CC tilt aligned with the individual bunch centers hellipor not
Dec 9 2013 6th Crab Cavity workshop (CC13) 3
Phasing the CC with the bunch centre
bull A phase error (wrt bunch centre) causes an offset of the bunch rotation axis This results in a transverse offset Dx at the IP
bull 1 deg RF phase (7 ps) leads to Dx = 1 mm (15h transverse beam size)bull The stringent requirement concerns transverse emittance growth caused
by RF phase noise at frequencies intersecting one of the betatron bands Much larger offset can be accepted if static or at frequencies significantly below 3 kHz (first betatron band)
Dec 9 2013 6th Crab Cavity workshop (CC13)
RFRF
cx
4
A DETOUR PHASE MODULATION IN THE MAIN CAVITIES
Dec 9 2013 6th Crab Cavity workshop (CC13) 5
Present schemebull RFLLRF is currently setup for extremely
stable RF voltage (minimize transient beam loading effects) Less than 1 RF phase modulation over the turn with 035 A DC (7 ps)
bull To continue this way we would need at least 300 kW of klystron forward power at ultimate intensity (17E11 ppb)
bull Klystrons saturate at 200 kW with present DC parameters (ultimately 300 kW)
bull Sufficient margin necessary for reliable operation additional RF manipulations etc
bull At 7 TeV 11 A DC the synchrotron radiation loss is 14 keV per turn or 27 pW
bull All RF power is dissipated in the circulator loads
bull The present scheme cannot be extended much beyond nominal
Required klystron power for 115e11 ppb 25 ns 7 TeV 17e11 ppb 25 ns 450 GeV 17e11 ppb 25 ns 7 TeV
6
P Baudrenghien T Mastoridis ldquoProposal for an RF Roadmap Towards Ultimate Intensity in the LHC IPAC 2012
Dec 9 2013 6th Crab Cavity workshop (CC13)
Proposed RF phase modulation schemebull In physics
bull We will accept the modulation of the cavity phase by the beam current (transient beam loading) and adapt the voltage set point for each bunch accordingly
bull The klystron drive is kept constant over one turn (amplitude and phase)
bull The cavity is detuned so that the klystron current is aligned with the average cavity voltage
bull Needed klystron power becomes independent of the beam current For QL=60k we need 105 kW only for 12 MV total
bull Stability is not modified we keep the strong RFfdbk and OTFB
bull The resulting displacement of the luminous region is acceptable
bull During filling bull It is desirable to keep the cavity phase constant for
clean capture Thanks to the reduced total voltage (6 MV) the present scheme can be kept with ultimate
Modulation of the cavity phase by the transient beam loading in physics 2835 bunches 17 1011 pbunch 15 MVcavity QL=60k full detuning (-78 kHz)
7Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CCbull If the CC follows the phase modulation
bull Forcing the CC to follow the fast phase modulation (-10 degrees 400 MHz in the 32 ms long abort gap) results in huge power requirement
bull With the HiLumi parameters (2808 bunches 22E11 pbunch 111 A DC 32 ms long abort gap ) assuming 3 MV per crab cavity 300 W RQ we need an absolute minimum of 170 kW per cavity This minimum is achieved with a QL=44000
bull With QL=500000 we need 950 kW
8Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CC (contrsquod)bull Fixed CC phase
bull Keeping the Crab Cavity phase constant over the turn will result in a phase error df with respect to the individual bunch center
bull This phase error causes an offset of the bunch rotation axis resulting in a transverse displacement Dx at the IP
bull For a phase drift of 30 ps the transverse displacement is 5 mm approximately equal to the transverse beam size
bull Fortunately the filling patterns are identical for both rings (except for the first six or twelve bunches batch) and the phase errors will be equal for colliding pairs in IP1 (ATLAS) and IP5 (CMS) because the bucket numbering convention makes the bucket one of both rings (first bucket after the abort gap) ldquocolliderdquo in IP1 and IP5
bull There will therefore be no loss of luminosity only a modulation of the transverse position of the vertex over one turn This is acceptable by the experiments
9
RF
cx
Dec 9 2013 6th Crab Cavity workshop (CC13)
IMPEDANCE AT THE FUNDAMENTAL
ldquoBestrdquo parking position during fillingramping
Impedance reduction with cavities in operation (on resonance)
Dec 9 2013 6th Crab Cavity workshop (CC13) 10
Machine Impedance bull The HighLumi LHC will accelerate 11 A DC current per beam (compared
to 035 A DC in 2012)bull The Crab Cavities will introduce a series of Narrow-Band resonators in the
machine (fundamental plus HOMs)bull Control of the fundamental is the responsibility of the LLRF bull At the fundamental one cavity presents a transverse impedance around
25 GWm
bull At the fundamental frequency the effective impedance can be reduced by an active RF feedback
bull But what is the max impedance at the fundamental
Dec 9 2013 6th Crab Cavity workshop (CC13)
LRR QQc
11
Instability growth rate (12)bull A resonant transverse impedance with resistive part ZT [Ohmm] at resonant
frequency ωr=2πfr will drive coupled-bunch mode (n m) fr
=(n+pM+Qβ)f0+mfs with the growth rate
f0 and fs are revolution and synchrotron frequency
M is number of (symmetric) bunches
Qβ is betatron tune ωξ = Qβ ω0ξη ξ is chromaticity
Formfactor F(x) for water-bag bunch F(0)=1 (the worst mode m=0) F(x gt 05) asymp 05
Dec 9 2013 126th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Instability growth rate (22)bull Growth rate ~ 1E rarr maximum at low energy
bull At 450 GeV for nominal intensity and ZT =1 MOhmm (ξ=0)
minimum τinst = 015 [s]
bull Condition τinst gt τd gives
ZT [MOhmm] lt 015 (1-x16)τd x = (fr - fξ)τ lt 08
ZT [MOhmm] lt 03 (05+x)τd x gt 08
τd [s] is the damping time by transverse damper
τ is the bunch length typically 10 ns lt τ lt 15 ns
bull For ultimate intensity factor 23
Dec 9 2013 136th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Fortunately the situation looks better at the fundamental because bull we can control the cavity tune vs betatron bandbull we can act on the beam induced voltage via active RF
feedback
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Phasing the CC with the bunch centre
bull A phase error (wrt bunch centre) causes an offset of the bunch rotation axis This results in a transverse offset Dx at the IP
bull 1 deg RF phase (7 ps) leads to Dx = 1 mm (15h transverse beam size)bull The stringent requirement concerns transverse emittance growth caused
by RF phase noise at frequencies intersecting one of the betatron bands Much larger offset can be accepted if static or at frequencies significantly below 3 kHz (first betatron band)
Dec 9 2013 6th Crab Cavity workshop (CC13)
RFRF
cx
4
A DETOUR PHASE MODULATION IN THE MAIN CAVITIES
Dec 9 2013 6th Crab Cavity workshop (CC13) 5
Present schemebull RFLLRF is currently setup for extremely
stable RF voltage (minimize transient beam loading effects) Less than 1 RF phase modulation over the turn with 035 A DC (7 ps)
bull To continue this way we would need at least 300 kW of klystron forward power at ultimate intensity (17E11 ppb)
bull Klystrons saturate at 200 kW with present DC parameters (ultimately 300 kW)
bull Sufficient margin necessary for reliable operation additional RF manipulations etc
bull At 7 TeV 11 A DC the synchrotron radiation loss is 14 keV per turn or 27 pW
bull All RF power is dissipated in the circulator loads
bull The present scheme cannot be extended much beyond nominal
Required klystron power for 115e11 ppb 25 ns 7 TeV 17e11 ppb 25 ns 450 GeV 17e11 ppb 25 ns 7 TeV
6
P Baudrenghien T Mastoridis ldquoProposal for an RF Roadmap Towards Ultimate Intensity in the LHC IPAC 2012
Dec 9 2013 6th Crab Cavity workshop (CC13)
Proposed RF phase modulation schemebull In physics
bull We will accept the modulation of the cavity phase by the beam current (transient beam loading) and adapt the voltage set point for each bunch accordingly
bull The klystron drive is kept constant over one turn (amplitude and phase)
bull The cavity is detuned so that the klystron current is aligned with the average cavity voltage
bull Needed klystron power becomes independent of the beam current For QL=60k we need 105 kW only for 12 MV total
bull Stability is not modified we keep the strong RFfdbk and OTFB
bull The resulting displacement of the luminous region is acceptable
bull During filling bull It is desirable to keep the cavity phase constant for
clean capture Thanks to the reduced total voltage (6 MV) the present scheme can be kept with ultimate
Modulation of the cavity phase by the transient beam loading in physics 2835 bunches 17 1011 pbunch 15 MVcavity QL=60k full detuning (-78 kHz)
7Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CCbull If the CC follows the phase modulation
bull Forcing the CC to follow the fast phase modulation (-10 degrees 400 MHz in the 32 ms long abort gap) results in huge power requirement
bull With the HiLumi parameters (2808 bunches 22E11 pbunch 111 A DC 32 ms long abort gap ) assuming 3 MV per crab cavity 300 W RQ we need an absolute minimum of 170 kW per cavity This minimum is achieved with a QL=44000
bull With QL=500000 we need 950 kW
8Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CC (contrsquod)bull Fixed CC phase
bull Keeping the Crab Cavity phase constant over the turn will result in a phase error df with respect to the individual bunch center
bull This phase error causes an offset of the bunch rotation axis resulting in a transverse displacement Dx at the IP
bull For a phase drift of 30 ps the transverse displacement is 5 mm approximately equal to the transverse beam size
bull Fortunately the filling patterns are identical for both rings (except for the first six or twelve bunches batch) and the phase errors will be equal for colliding pairs in IP1 (ATLAS) and IP5 (CMS) because the bucket numbering convention makes the bucket one of both rings (first bucket after the abort gap) ldquocolliderdquo in IP1 and IP5
bull There will therefore be no loss of luminosity only a modulation of the transverse position of the vertex over one turn This is acceptable by the experiments
9
RF
cx
Dec 9 2013 6th Crab Cavity workshop (CC13)
IMPEDANCE AT THE FUNDAMENTAL
ldquoBestrdquo parking position during fillingramping
Impedance reduction with cavities in operation (on resonance)
Dec 9 2013 6th Crab Cavity workshop (CC13) 10
Machine Impedance bull The HighLumi LHC will accelerate 11 A DC current per beam (compared
to 035 A DC in 2012)bull The Crab Cavities will introduce a series of Narrow-Band resonators in the
machine (fundamental plus HOMs)bull Control of the fundamental is the responsibility of the LLRF bull At the fundamental one cavity presents a transverse impedance around
25 GWm
bull At the fundamental frequency the effective impedance can be reduced by an active RF feedback
bull But what is the max impedance at the fundamental
Dec 9 2013 6th Crab Cavity workshop (CC13)
LRR QQc
11
Instability growth rate (12)bull A resonant transverse impedance with resistive part ZT [Ohmm] at resonant
frequency ωr=2πfr will drive coupled-bunch mode (n m) fr
=(n+pM+Qβ)f0+mfs with the growth rate
f0 and fs are revolution and synchrotron frequency
M is number of (symmetric) bunches
Qβ is betatron tune ωξ = Qβ ω0ξη ξ is chromaticity
Formfactor F(x) for water-bag bunch F(0)=1 (the worst mode m=0) F(x gt 05) asymp 05
Dec 9 2013 126th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Instability growth rate (22)bull Growth rate ~ 1E rarr maximum at low energy
bull At 450 GeV for nominal intensity and ZT =1 MOhmm (ξ=0)
minimum τinst = 015 [s]
bull Condition τinst gt τd gives
ZT [MOhmm] lt 015 (1-x16)τd x = (fr - fξ)τ lt 08
ZT [MOhmm] lt 03 (05+x)τd x gt 08
τd [s] is the damping time by transverse damper
τ is the bunch length typically 10 ns lt τ lt 15 ns
bull For ultimate intensity factor 23
Dec 9 2013 136th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Fortunately the situation looks better at the fundamental because bull we can control the cavity tune vs betatron bandbull we can act on the beam induced voltage via active RF
feedback
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
A DETOUR PHASE MODULATION IN THE MAIN CAVITIES
Dec 9 2013 6th Crab Cavity workshop (CC13) 5
Present schemebull RFLLRF is currently setup for extremely
stable RF voltage (minimize transient beam loading effects) Less than 1 RF phase modulation over the turn with 035 A DC (7 ps)
bull To continue this way we would need at least 300 kW of klystron forward power at ultimate intensity (17E11 ppb)
bull Klystrons saturate at 200 kW with present DC parameters (ultimately 300 kW)
bull Sufficient margin necessary for reliable operation additional RF manipulations etc
bull At 7 TeV 11 A DC the synchrotron radiation loss is 14 keV per turn or 27 pW
bull All RF power is dissipated in the circulator loads
bull The present scheme cannot be extended much beyond nominal
Required klystron power for 115e11 ppb 25 ns 7 TeV 17e11 ppb 25 ns 450 GeV 17e11 ppb 25 ns 7 TeV
6
P Baudrenghien T Mastoridis ldquoProposal for an RF Roadmap Towards Ultimate Intensity in the LHC IPAC 2012
Dec 9 2013 6th Crab Cavity workshop (CC13)
Proposed RF phase modulation schemebull In physics
bull We will accept the modulation of the cavity phase by the beam current (transient beam loading) and adapt the voltage set point for each bunch accordingly
bull The klystron drive is kept constant over one turn (amplitude and phase)
bull The cavity is detuned so that the klystron current is aligned with the average cavity voltage
bull Needed klystron power becomes independent of the beam current For QL=60k we need 105 kW only for 12 MV total
bull Stability is not modified we keep the strong RFfdbk and OTFB
bull The resulting displacement of the luminous region is acceptable
bull During filling bull It is desirable to keep the cavity phase constant for
clean capture Thanks to the reduced total voltage (6 MV) the present scheme can be kept with ultimate
Modulation of the cavity phase by the transient beam loading in physics 2835 bunches 17 1011 pbunch 15 MVcavity QL=60k full detuning (-78 kHz)
7Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CCbull If the CC follows the phase modulation
bull Forcing the CC to follow the fast phase modulation (-10 degrees 400 MHz in the 32 ms long abort gap) results in huge power requirement
bull With the HiLumi parameters (2808 bunches 22E11 pbunch 111 A DC 32 ms long abort gap ) assuming 3 MV per crab cavity 300 W RQ we need an absolute minimum of 170 kW per cavity This minimum is achieved with a QL=44000
bull With QL=500000 we need 950 kW
8Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CC (contrsquod)bull Fixed CC phase
bull Keeping the Crab Cavity phase constant over the turn will result in a phase error df with respect to the individual bunch center
bull This phase error causes an offset of the bunch rotation axis resulting in a transverse displacement Dx at the IP
bull For a phase drift of 30 ps the transverse displacement is 5 mm approximately equal to the transverse beam size
bull Fortunately the filling patterns are identical for both rings (except for the first six or twelve bunches batch) and the phase errors will be equal for colliding pairs in IP1 (ATLAS) and IP5 (CMS) because the bucket numbering convention makes the bucket one of both rings (first bucket after the abort gap) ldquocolliderdquo in IP1 and IP5
bull There will therefore be no loss of luminosity only a modulation of the transverse position of the vertex over one turn This is acceptable by the experiments
9
RF
cx
Dec 9 2013 6th Crab Cavity workshop (CC13)
IMPEDANCE AT THE FUNDAMENTAL
ldquoBestrdquo parking position during fillingramping
Impedance reduction with cavities in operation (on resonance)
Dec 9 2013 6th Crab Cavity workshop (CC13) 10
Machine Impedance bull The HighLumi LHC will accelerate 11 A DC current per beam (compared
to 035 A DC in 2012)bull The Crab Cavities will introduce a series of Narrow-Band resonators in the
machine (fundamental plus HOMs)bull Control of the fundamental is the responsibility of the LLRF bull At the fundamental one cavity presents a transverse impedance around
25 GWm
bull At the fundamental frequency the effective impedance can be reduced by an active RF feedback
bull But what is the max impedance at the fundamental
Dec 9 2013 6th Crab Cavity workshop (CC13)
LRR QQc
11
Instability growth rate (12)bull A resonant transverse impedance with resistive part ZT [Ohmm] at resonant
frequency ωr=2πfr will drive coupled-bunch mode (n m) fr
=(n+pM+Qβ)f0+mfs with the growth rate
f0 and fs are revolution and synchrotron frequency
M is number of (symmetric) bunches
Qβ is betatron tune ωξ = Qβ ω0ξη ξ is chromaticity
Formfactor F(x) for water-bag bunch F(0)=1 (the worst mode m=0) F(x gt 05) asymp 05
Dec 9 2013 126th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Instability growth rate (22)bull Growth rate ~ 1E rarr maximum at low energy
bull At 450 GeV for nominal intensity and ZT =1 MOhmm (ξ=0)
minimum τinst = 015 [s]
bull Condition τinst gt τd gives
ZT [MOhmm] lt 015 (1-x16)τd x = (fr - fξ)τ lt 08
ZT [MOhmm] lt 03 (05+x)τd x gt 08
τd [s] is the damping time by transverse damper
τ is the bunch length typically 10 ns lt τ lt 15 ns
bull For ultimate intensity factor 23
Dec 9 2013 136th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Fortunately the situation looks better at the fundamental because bull we can control the cavity tune vs betatron bandbull we can act on the beam induced voltage via active RF
feedback
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Present schemebull RFLLRF is currently setup for extremely
stable RF voltage (minimize transient beam loading effects) Less than 1 RF phase modulation over the turn with 035 A DC (7 ps)
bull To continue this way we would need at least 300 kW of klystron forward power at ultimate intensity (17E11 ppb)
bull Klystrons saturate at 200 kW with present DC parameters (ultimately 300 kW)
bull Sufficient margin necessary for reliable operation additional RF manipulations etc
bull At 7 TeV 11 A DC the synchrotron radiation loss is 14 keV per turn or 27 pW
bull All RF power is dissipated in the circulator loads
bull The present scheme cannot be extended much beyond nominal
Required klystron power for 115e11 ppb 25 ns 7 TeV 17e11 ppb 25 ns 450 GeV 17e11 ppb 25 ns 7 TeV
6
P Baudrenghien T Mastoridis ldquoProposal for an RF Roadmap Towards Ultimate Intensity in the LHC IPAC 2012
Dec 9 2013 6th Crab Cavity workshop (CC13)
Proposed RF phase modulation schemebull In physics
bull We will accept the modulation of the cavity phase by the beam current (transient beam loading) and adapt the voltage set point for each bunch accordingly
bull The klystron drive is kept constant over one turn (amplitude and phase)
bull The cavity is detuned so that the klystron current is aligned with the average cavity voltage
bull Needed klystron power becomes independent of the beam current For QL=60k we need 105 kW only for 12 MV total
bull Stability is not modified we keep the strong RFfdbk and OTFB
bull The resulting displacement of the luminous region is acceptable
bull During filling bull It is desirable to keep the cavity phase constant for
clean capture Thanks to the reduced total voltage (6 MV) the present scheme can be kept with ultimate
Modulation of the cavity phase by the transient beam loading in physics 2835 bunches 17 1011 pbunch 15 MVcavity QL=60k full detuning (-78 kHz)
7Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CCbull If the CC follows the phase modulation
bull Forcing the CC to follow the fast phase modulation (-10 degrees 400 MHz in the 32 ms long abort gap) results in huge power requirement
bull With the HiLumi parameters (2808 bunches 22E11 pbunch 111 A DC 32 ms long abort gap ) assuming 3 MV per crab cavity 300 W RQ we need an absolute minimum of 170 kW per cavity This minimum is achieved with a QL=44000
bull With QL=500000 we need 950 kW
8Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CC (contrsquod)bull Fixed CC phase
bull Keeping the Crab Cavity phase constant over the turn will result in a phase error df with respect to the individual bunch center
bull This phase error causes an offset of the bunch rotation axis resulting in a transverse displacement Dx at the IP
bull For a phase drift of 30 ps the transverse displacement is 5 mm approximately equal to the transverse beam size
bull Fortunately the filling patterns are identical for both rings (except for the first six or twelve bunches batch) and the phase errors will be equal for colliding pairs in IP1 (ATLAS) and IP5 (CMS) because the bucket numbering convention makes the bucket one of both rings (first bucket after the abort gap) ldquocolliderdquo in IP1 and IP5
bull There will therefore be no loss of luminosity only a modulation of the transverse position of the vertex over one turn This is acceptable by the experiments
9
RF
cx
Dec 9 2013 6th Crab Cavity workshop (CC13)
IMPEDANCE AT THE FUNDAMENTAL
ldquoBestrdquo parking position during fillingramping
Impedance reduction with cavities in operation (on resonance)
Dec 9 2013 6th Crab Cavity workshop (CC13) 10
Machine Impedance bull The HighLumi LHC will accelerate 11 A DC current per beam (compared
to 035 A DC in 2012)bull The Crab Cavities will introduce a series of Narrow-Band resonators in the
machine (fundamental plus HOMs)bull Control of the fundamental is the responsibility of the LLRF bull At the fundamental one cavity presents a transverse impedance around
25 GWm
bull At the fundamental frequency the effective impedance can be reduced by an active RF feedback
bull But what is the max impedance at the fundamental
Dec 9 2013 6th Crab Cavity workshop (CC13)
LRR QQc
11
Instability growth rate (12)bull A resonant transverse impedance with resistive part ZT [Ohmm] at resonant
frequency ωr=2πfr will drive coupled-bunch mode (n m) fr
=(n+pM+Qβ)f0+mfs with the growth rate
f0 and fs are revolution and synchrotron frequency
M is number of (symmetric) bunches
Qβ is betatron tune ωξ = Qβ ω0ξη ξ is chromaticity
Formfactor F(x) for water-bag bunch F(0)=1 (the worst mode m=0) F(x gt 05) asymp 05
Dec 9 2013 126th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Instability growth rate (22)bull Growth rate ~ 1E rarr maximum at low energy
bull At 450 GeV for nominal intensity and ZT =1 MOhmm (ξ=0)
minimum τinst = 015 [s]
bull Condition τinst gt τd gives
ZT [MOhmm] lt 015 (1-x16)τd x = (fr - fξ)τ lt 08
ZT [MOhmm] lt 03 (05+x)τd x gt 08
τd [s] is the damping time by transverse damper
τ is the bunch length typically 10 ns lt τ lt 15 ns
bull For ultimate intensity factor 23
Dec 9 2013 136th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Fortunately the situation looks better at the fundamental because bull we can control the cavity tune vs betatron bandbull we can act on the beam induced voltage via active RF
feedback
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Proposed RF phase modulation schemebull In physics
bull We will accept the modulation of the cavity phase by the beam current (transient beam loading) and adapt the voltage set point for each bunch accordingly
bull The klystron drive is kept constant over one turn (amplitude and phase)
bull The cavity is detuned so that the klystron current is aligned with the average cavity voltage
bull Needed klystron power becomes independent of the beam current For QL=60k we need 105 kW only for 12 MV total
bull Stability is not modified we keep the strong RFfdbk and OTFB
bull The resulting displacement of the luminous region is acceptable
bull During filling bull It is desirable to keep the cavity phase constant for
clean capture Thanks to the reduced total voltage (6 MV) the present scheme can be kept with ultimate
Modulation of the cavity phase by the transient beam loading in physics 2835 bunches 17 1011 pbunch 15 MVcavity QL=60k full detuning (-78 kHz)
7Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CCbull If the CC follows the phase modulation
bull Forcing the CC to follow the fast phase modulation (-10 degrees 400 MHz in the 32 ms long abort gap) results in huge power requirement
bull With the HiLumi parameters (2808 bunches 22E11 pbunch 111 A DC 32 ms long abort gap ) assuming 3 MV per crab cavity 300 W RQ we need an absolute minimum of 170 kW per cavity This minimum is achieved with a QL=44000
bull With QL=500000 we need 950 kW
8Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CC (contrsquod)bull Fixed CC phase
bull Keeping the Crab Cavity phase constant over the turn will result in a phase error df with respect to the individual bunch center
bull This phase error causes an offset of the bunch rotation axis resulting in a transverse displacement Dx at the IP
bull For a phase drift of 30 ps the transverse displacement is 5 mm approximately equal to the transverse beam size
bull Fortunately the filling patterns are identical for both rings (except for the first six or twelve bunches batch) and the phase errors will be equal for colliding pairs in IP1 (ATLAS) and IP5 (CMS) because the bucket numbering convention makes the bucket one of both rings (first bucket after the abort gap) ldquocolliderdquo in IP1 and IP5
bull There will therefore be no loss of luminosity only a modulation of the transverse position of the vertex over one turn This is acceptable by the experiments
9
RF
cx
Dec 9 2013 6th Crab Cavity workshop (CC13)
IMPEDANCE AT THE FUNDAMENTAL
ldquoBestrdquo parking position during fillingramping
Impedance reduction with cavities in operation (on resonance)
Dec 9 2013 6th Crab Cavity workshop (CC13) 10
Machine Impedance bull The HighLumi LHC will accelerate 11 A DC current per beam (compared
to 035 A DC in 2012)bull The Crab Cavities will introduce a series of Narrow-Band resonators in the
machine (fundamental plus HOMs)bull Control of the fundamental is the responsibility of the LLRF bull At the fundamental one cavity presents a transverse impedance around
25 GWm
bull At the fundamental frequency the effective impedance can be reduced by an active RF feedback
bull But what is the max impedance at the fundamental
Dec 9 2013 6th Crab Cavity workshop (CC13)
LRR QQc
11
Instability growth rate (12)bull A resonant transverse impedance with resistive part ZT [Ohmm] at resonant
frequency ωr=2πfr will drive coupled-bunch mode (n m) fr
=(n+pM+Qβ)f0+mfs with the growth rate
f0 and fs are revolution and synchrotron frequency
M is number of (symmetric) bunches
Qβ is betatron tune ωξ = Qβ ω0ξη ξ is chromaticity
Formfactor F(x) for water-bag bunch F(0)=1 (the worst mode m=0) F(x gt 05) asymp 05
Dec 9 2013 126th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Instability growth rate (22)bull Growth rate ~ 1E rarr maximum at low energy
bull At 450 GeV for nominal intensity and ZT =1 MOhmm (ξ=0)
minimum τinst = 015 [s]
bull Condition τinst gt τd gives
ZT [MOhmm] lt 015 (1-x16)τd x = (fr - fξ)τ lt 08
ZT [MOhmm] lt 03 (05+x)τd x gt 08
τd [s] is the damping time by transverse damper
τ is the bunch length typically 10 ns lt τ lt 15 ns
bull For ultimate intensity factor 23
Dec 9 2013 136th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Fortunately the situation looks better at the fundamental because bull we can control the cavity tune vs betatron bandbull we can act on the beam induced voltage via active RF
feedback
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Consequences for the CCbull If the CC follows the phase modulation
bull Forcing the CC to follow the fast phase modulation (-10 degrees 400 MHz in the 32 ms long abort gap) results in huge power requirement
bull With the HiLumi parameters (2808 bunches 22E11 pbunch 111 A DC 32 ms long abort gap ) assuming 3 MV per crab cavity 300 W RQ we need an absolute minimum of 170 kW per cavity This minimum is achieved with a QL=44000
bull With QL=500000 we need 950 kW
8Dec 9 2013 6th Crab Cavity workshop (CC13)
Consequences for the CC (contrsquod)bull Fixed CC phase
bull Keeping the Crab Cavity phase constant over the turn will result in a phase error df with respect to the individual bunch center
bull This phase error causes an offset of the bunch rotation axis resulting in a transverse displacement Dx at the IP
bull For a phase drift of 30 ps the transverse displacement is 5 mm approximately equal to the transverse beam size
bull Fortunately the filling patterns are identical for both rings (except for the first six or twelve bunches batch) and the phase errors will be equal for colliding pairs in IP1 (ATLAS) and IP5 (CMS) because the bucket numbering convention makes the bucket one of both rings (first bucket after the abort gap) ldquocolliderdquo in IP1 and IP5
bull There will therefore be no loss of luminosity only a modulation of the transverse position of the vertex over one turn This is acceptable by the experiments
9
RF
cx
Dec 9 2013 6th Crab Cavity workshop (CC13)
IMPEDANCE AT THE FUNDAMENTAL
ldquoBestrdquo parking position during fillingramping
Impedance reduction with cavities in operation (on resonance)
Dec 9 2013 6th Crab Cavity workshop (CC13) 10
Machine Impedance bull The HighLumi LHC will accelerate 11 A DC current per beam (compared
to 035 A DC in 2012)bull The Crab Cavities will introduce a series of Narrow-Band resonators in the
machine (fundamental plus HOMs)bull Control of the fundamental is the responsibility of the LLRF bull At the fundamental one cavity presents a transverse impedance around
25 GWm
bull At the fundamental frequency the effective impedance can be reduced by an active RF feedback
bull But what is the max impedance at the fundamental
Dec 9 2013 6th Crab Cavity workshop (CC13)
LRR QQc
11
Instability growth rate (12)bull A resonant transverse impedance with resistive part ZT [Ohmm] at resonant
frequency ωr=2πfr will drive coupled-bunch mode (n m) fr
=(n+pM+Qβ)f0+mfs with the growth rate
f0 and fs are revolution and synchrotron frequency
M is number of (symmetric) bunches
Qβ is betatron tune ωξ = Qβ ω0ξη ξ is chromaticity
Formfactor F(x) for water-bag bunch F(0)=1 (the worst mode m=0) F(x gt 05) asymp 05
Dec 9 2013 126th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Instability growth rate (22)bull Growth rate ~ 1E rarr maximum at low energy
bull At 450 GeV for nominal intensity and ZT =1 MOhmm (ξ=0)
minimum τinst = 015 [s]
bull Condition τinst gt τd gives
ZT [MOhmm] lt 015 (1-x16)τd x = (fr - fξ)τ lt 08
ZT [MOhmm] lt 03 (05+x)τd x gt 08
τd [s] is the damping time by transverse damper
τ is the bunch length typically 10 ns lt τ lt 15 ns
bull For ultimate intensity factor 23
Dec 9 2013 136th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Fortunately the situation looks better at the fundamental because bull we can control the cavity tune vs betatron bandbull we can act on the beam induced voltage via active RF
feedback
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Consequences for the CC (contrsquod)bull Fixed CC phase
bull Keeping the Crab Cavity phase constant over the turn will result in a phase error df with respect to the individual bunch center
bull This phase error causes an offset of the bunch rotation axis resulting in a transverse displacement Dx at the IP
bull For a phase drift of 30 ps the transverse displacement is 5 mm approximately equal to the transverse beam size
bull Fortunately the filling patterns are identical for both rings (except for the first six or twelve bunches batch) and the phase errors will be equal for colliding pairs in IP1 (ATLAS) and IP5 (CMS) because the bucket numbering convention makes the bucket one of both rings (first bucket after the abort gap) ldquocolliderdquo in IP1 and IP5
bull There will therefore be no loss of luminosity only a modulation of the transverse position of the vertex over one turn This is acceptable by the experiments
9
RF
cx
Dec 9 2013 6th Crab Cavity workshop (CC13)
IMPEDANCE AT THE FUNDAMENTAL
ldquoBestrdquo parking position during fillingramping
Impedance reduction with cavities in operation (on resonance)
Dec 9 2013 6th Crab Cavity workshop (CC13) 10
Machine Impedance bull The HighLumi LHC will accelerate 11 A DC current per beam (compared
to 035 A DC in 2012)bull The Crab Cavities will introduce a series of Narrow-Band resonators in the
machine (fundamental plus HOMs)bull Control of the fundamental is the responsibility of the LLRF bull At the fundamental one cavity presents a transverse impedance around
25 GWm
bull At the fundamental frequency the effective impedance can be reduced by an active RF feedback
bull But what is the max impedance at the fundamental
Dec 9 2013 6th Crab Cavity workshop (CC13)
LRR QQc
11
Instability growth rate (12)bull A resonant transverse impedance with resistive part ZT [Ohmm] at resonant
frequency ωr=2πfr will drive coupled-bunch mode (n m) fr
=(n+pM+Qβ)f0+mfs with the growth rate
f0 and fs are revolution and synchrotron frequency
M is number of (symmetric) bunches
Qβ is betatron tune ωξ = Qβ ω0ξη ξ is chromaticity
Formfactor F(x) for water-bag bunch F(0)=1 (the worst mode m=0) F(x gt 05) asymp 05
Dec 9 2013 126th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Instability growth rate (22)bull Growth rate ~ 1E rarr maximum at low energy
bull At 450 GeV for nominal intensity and ZT =1 MOhmm (ξ=0)
minimum τinst = 015 [s]
bull Condition τinst gt τd gives
ZT [MOhmm] lt 015 (1-x16)τd x = (fr - fξ)τ lt 08
ZT [MOhmm] lt 03 (05+x)τd x gt 08
τd [s] is the damping time by transverse damper
τ is the bunch length typically 10 ns lt τ lt 15 ns
bull For ultimate intensity factor 23
Dec 9 2013 136th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Fortunately the situation looks better at the fundamental because bull we can control the cavity tune vs betatron bandbull we can act on the beam induced voltage via active RF
feedback
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
IMPEDANCE AT THE FUNDAMENTAL
ldquoBestrdquo parking position during fillingramping
Impedance reduction with cavities in operation (on resonance)
Dec 9 2013 6th Crab Cavity workshop (CC13) 10
Machine Impedance bull The HighLumi LHC will accelerate 11 A DC current per beam (compared
to 035 A DC in 2012)bull The Crab Cavities will introduce a series of Narrow-Band resonators in the
machine (fundamental plus HOMs)bull Control of the fundamental is the responsibility of the LLRF bull At the fundamental one cavity presents a transverse impedance around
25 GWm
bull At the fundamental frequency the effective impedance can be reduced by an active RF feedback
bull But what is the max impedance at the fundamental
Dec 9 2013 6th Crab Cavity workshop (CC13)
LRR QQc
11
Instability growth rate (12)bull A resonant transverse impedance with resistive part ZT [Ohmm] at resonant
frequency ωr=2πfr will drive coupled-bunch mode (n m) fr
=(n+pM+Qβ)f0+mfs with the growth rate
f0 and fs are revolution and synchrotron frequency
M is number of (symmetric) bunches
Qβ is betatron tune ωξ = Qβ ω0ξη ξ is chromaticity
Formfactor F(x) for water-bag bunch F(0)=1 (the worst mode m=0) F(x gt 05) asymp 05
Dec 9 2013 126th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Instability growth rate (22)bull Growth rate ~ 1E rarr maximum at low energy
bull At 450 GeV for nominal intensity and ZT =1 MOhmm (ξ=0)
minimum τinst = 015 [s]
bull Condition τinst gt τd gives
ZT [MOhmm] lt 015 (1-x16)τd x = (fr - fξ)τ lt 08
ZT [MOhmm] lt 03 (05+x)τd x gt 08
τd [s] is the damping time by transverse damper
τ is the bunch length typically 10 ns lt τ lt 15 ns
bull For ultimate intensity factor 23
Dec 9 2013 136th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Fortunately the situation looks better at the fundamental because bull we can control the cavity tune vs betatron bandbull we can act on the beam induced voltage via active RF
feedback
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Machine Impedance bull The HighLumi LHC will accelerate 11 A DC current per beam (compared
to 035 A DC in 2012)bull The Crab Cavities will introduce a series of Narrow-Band resonators in the
machine (fundamental plus HOMs)bull Control of the fundamental is the responsibility of the LLRF bull At the fundamental one cavity presents a transverse impedance around
25 GWm
bull At the fundamental frequency the effective impedance can be reduced by an active RF feedback
bull But what is the max impedance at the fundamental
Dec 9 2013 6th Crab Cavity workshop (CC13)
LRR QQc
11
Instability growth rate (12)bull A resonant transverse impedance with resistive part ZT [Ohmm] at resonant
frequency ωr=2πfr will drive coupled-bunch mode (n m) fr
=(n+pM+Qβ)f0+mfs with the growth rate
f0 and fs are revolution and synchrotron frequency
M is number of (symmetric) bunches
Qβ is betatron tune ωξ = Qβ ω0ξη ξ is chromaticity
Formfactor F(x) for water-bag bunch F(0)=1 (the worst mode m=0) F(x gt 05) asymp 05
Dec 9 2013 126th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Instability growth rate (22)bull Growth rate ~ 1E rarr maximum at low energy
bull At 450 GeV for nominal intensity and ZT =1 MOhmm (ξ=0)
minimum τinst = 015 [s]
bull Condition τinst gt τd gives
ZT [MOhmm] lt 015 (1-x16)τd x = (fr - fξ)τ lt 08
ZT [MOhmm] lt 03 (05+x)τd x gt 08
τd [s] is the damping time by transverse damper
τ is the bunch length typically 10 ns lt τ lt 15 ns
bull For ultimate intensity factor 23
Dec 9 2013 136th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Fortunately the situation looks better at the fundamental because bull we can control the cavity tune vs betatron bandbull we can act on the beam induced voltage via active RF
feedback
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Instability growth rate (12)bull A resonant transverse impedance with resistive part ZT [Ohmm] at resonant
frequency ωr=2πfr will drive coupled-bunch mode (n m) fr
=(n+pM+Qβ)f0+mfs with the growth rate
f0 and fs are revolution and synchrotron frequency
M is number of (symmetric) bunches
Qβ is betatron tune ωξ = Qβ ω0ξη ξ is chromaticity
Formfactor F(x) for water-bag bunch F(0)=1 (the worst mode m=0) F(x gt 05) asymp 05
Dec 9 2013 126th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Instability growth rate (22)bull Growth rate ~ 1E rarr maximum at low energy
bull At 450 GeV for nominal intensity and ZT =1 MOhmm (ξ=0)
minimum τinst = 015 [s]
bull Condition τinst gt τd gives
ZT [MOhmm] lt 015 (1-x16)τd x = (fr - fξ)τ lt 08
ZT [MOhmm] lt 03 (05+x)τd x gt 08
τd [s] is the damping time by transverse damper
τ is the bunch length typically 10 ns lt τ lt 15 ns
bull For ultimate intensity factor 23
Dec 9 2013 136th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Fortunately the situation looks better at the fundamental because bull we can control the cavity tune vs betatron bandbull we can act on the beam induced voltage via active RF
feedback
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Instability growth rate (22)bull Growth rate ~ 1E rarr maximum at low energy
bull At 450 GeV for nominal intensity and ZT =1 MOhmm (ξ=0)
minimum τinst = 015 [s]
bull Condition τinst gt τd gives
ZT [MOhmm] lt 015 (1-x16)τd x = (fr - fξ)τ lt 08
ZT [MOhmm] lt 03 (05+x)τd x gt 08
τd [s] is the damping time by transverse damper
τ is the bunch length typically 10 ns lt τ lt 15 ns
bull For ultimate intensity factor 23
Dec 9 2013 136th Crab Cavity workshop (CC13)
E Shaposhnikova CC2010
Fortunately the situation looks better at the fundamental because bull we can control the cavity tune vs betatron bandbull we can act on the beam induced voltage via active RF
feedback
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
6th Crab Cavity workshop (CC13)
NB resonator transversebull Transverse impedance of a transverse mode
bull The damping rate and tune shift of coupled-bunch mode l (rigid dipole only) can be computed from the cavity impedance
bull With w=(p M+l) wrev+wb
1
sr
r
r
RZ
jQ
0
2l lpb rev
c q Ij Z
E T
Dec 9 2013 14
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
6th Crab Cavity workshop (CC13)
bull With 25 ns bunch spacing (M=3564) for a resonance around 400 MHz with a BW much below 40 MHz the infinite sum reduces to the two terms (p= plusmn10 -gt p M wrev = plusmn wRF)
bull The damping rate is computed from the difference between real impedance on the two plusmn(l wrev+wb) sidebands of the wRF
bull For example with Qb=643 the damping rate of mode l=-64 is computed from the difference between the real part of the impedance at plusmn03 wrev
bull Negative damping rate -gt instability
NB resonator transverse
0
0
0
2
recalling that
2
Re Re2
l l RF rev b RF rev bb rev
l l RF rev b RF rev bb rev
l RF rev b RF rev bb rev
c q Ij Z l Z l
E T
Z Z
c q Ij Z l Z l
E T
c q IZ l Z l
E T
Dec 9 2013 15
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
During injection and filling cavity detunedbull With a positive non-integer tune (Qh=643 wbwrev above an integer)
the cavity should be tuned above the RF frequency to make the mode l=-64 stabilizing
Dec 9 2013 6th Crab Cavity workshop (CC13) 16
+- + -
0 Re Re2l RF rev b RF rev b
b rev
c q IZ l Z l
E T
Real part of the cavity impedance with 15 kHz detuning (log scale)bull Left mode l=-64 The damping rate is computed from the difference in Real[Z]
evaluated at +03 Frev and -03 Frev STABLEbull Right mode l=-65 The damping rate is computed from the difference in Real[Z]
evaluated at -07 Frev and +07 Frev UNSTABLE but very low growth rate
L=-64 stable L=-65 unstable
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
bull If we can keep the cavity properly detuned the impedance at the fundamental is not a serious problem for stability
bull The detuning amplitude should be set to keep the beam induced kicks (for an off-centered trajectory) within reasonable bounds
Dec 9 2013 6th Crab Cavity workshop (CC13) 17
Growth rate (per cavity) with cavity parked idling at 15 kHz detuning Assuming beta function at location of crabbing equal to average
detuning (kHz)growth rate (s-1) mode index
-15 17 -64-1 85 -64
-05 35 -640 0
05 028 -651 058 -65
15 09 -652 13 -653 24 -65
35 3 -654 4 -655 8 -656 20 -657 85 -658 1200 -659 52 -6510 15 -65
most unstable mode
Operation with idling cavities seems feasible if they are properly detuned
L=-64 large
damping rate
L=-65 small growth rate
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
While crabbingDec 9 2013 6th Crab Cavity workshop (CC13)
bull In physics with the crabbing on we must have an active RF feedback for precise control of the cavity field
bull The RF feedback reduces the peak cavity impedance and transforms the high Q resonator into an effective impedance that covers several revolution frequency lines
bull The actual cavity tune has no big importance for stability anymorebull The growth rates and damping rates are much reduced and we have no more dominant mode
Comparison of the modulus of the cavity impedance without and with RF feedback
18
Real part of the effective impedance with RF feedback Reduced from 25 GWm to 6 MWm
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Dec 9 2013 6th Crab Cavity workshop (CC13) 19
Growth rate (per cavity) in physics with RF feedbackbull Left cavity on tunebull Right cavity detuned by -100 HzAssuming beta function at location of crabbing equal to average
Note RF feedback could also be used during filling and ramping
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Tentative conclusion (impedance at the fundamental)bull Filling and ramping
bull The cavity must be tuned above the RF frequencybull For stability the detuning should be very small But this may lead to
excessive beam induced voltage if the beam is off-centered resulting in crabbing oscillations over the full turn Acceptable
bull The growth rates are small and correspond to low-frequency modes where the damper gain is maximum
bull The RF feedback could be used but it may not be needed
bull Crabbingbull The cavity will be on-tunebull The RF feedback is needed for precision of the kicks and reduction
of TX noise bull It will reduce the growth rate to values compatible with the damper
The unstable modes correspond to low frequency transverse oscillations
Dec 9 2013 6th Crab Cavity workshop (CC13) 20
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
MACHINE PROTECTIONFast failure of one Crab Cavity
Mechanisms
Can the LLRF help
Dec 9 2013 6th Crab Cavity workshop (CC13) 21
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Mechanism for fast failurebull Case 1
bull The ldquopredictablerdquo case A problem with power hardware supplying one cavity (such as TX trip arcing in waveguide circulator or loadhellip) resulting in switching off the TX The cavity stored energy will flow out through the waveguide and circulator and will be dissipated into the circulatorrsquos load The cavity field will decay to zero with a time constant t
With a QL equal to 106 the time constant is 800 ms Assuming three cavities with a total voltage of 9 MV and a single TX trip the field would drop from 9 MV to 815 MV in the three turns delay (267 ms) until the beam dump is fired
Dec 9 2013 6th Crab Cavity workshop (CC13) 22
2 L
RF
Q
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Mechanism for fast failure (contrsquod)bull Case 2
bull In the case of a quench the stored energy is dissipated in the cavity walls and the main coupling (QL) has no effect on the field evolution Lessons must be learnt from the SM18 and SPS tests
bull Case 3bull In the case of an arc in the main coupler the stored energy will be
dissipated through the arc Again lessons must be learnt from the SM18 and SPS tests
Dec 9 2013 6th Crab Cavity workshop (CC13) 23
Better understanding of the field evolution in case of a quench or arcing needed
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
LLRF mitigation Coupled feedback
bull The LLRF can help minimizing the losses during the 3 turns transient till the beam is dumped The idea is to implement a coupled feedback acting on all CC cavities and keeping crabbing and un-crabbing equal
bull In the case of a crabbing cavity trip it would ramp the un-crabbing voltage down tracking the total crabbing voltage
Dec 9 2013 6th Crab Cavity workshop (CC13) 24
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
25
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Operational scenario
Dec 9 2013 6th Crab Cavity workshop (CC13) 26
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Operational scenariobull Tune control is ON at all time RF feedback is ON during crabbing
and can be ON during fillingramping although it may not be strictly needed
bull During filling ramping or operation with transparent crab cavities we detune the cavity (15 kHz) but keep a small field requested for the active Tuning system As the crabbing kick is provided by three cavities we can use counter-phasing to make the total field invisible to the beam The RF feedback is (probably) not necessary for stability during fillingramping It could be used with the cavity detuned to keep the Beam Induced Voltage zero if the beam is off-centered If so we can use the demanded TX power as a measurement of beam loading to guide the beam centering
Dec 9 2013 6th Crab Cavity workshop (CC13) 27
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Operational scenario (contrsquod)bull ON flat top
bull We reduce the detuning while keeping counter-phasing so that the total voltage is zero We switch the RF feedback ON (if OFF during ramp) to compensate for the beam loading as each cavity moves to resonance The RF feedback also keeps the cavity impedance small (beam stability)
bull Once the cavity detuning has been reduced to zero we drive counter-phasing to zero Any luminosity leveling scheme is possible by synchronously changing the voltage or phase in each crab cavity as desired
Dec 9 2013 6th Crab Cavity workshop (CC13) 28
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
CONCLUSIONS
Dec 9 2013 6th Crab Cavity workshop (CC13) 29
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
bull Phase modulation of the ACS voltage is compatible with crabbingbull There seems to be no major issue of transverse coupled-bunch
instability caused by the cavity fundamental resonancebull During filling and ramping operation may be possible with cavities idling at detuned
position The effect of beam loading must be studiedbull During crabbing operation the RF feedback reduces the effective impedance so that
the growth rate is compatible with the damperrsquos performancesbull In both cases the dominant modes are at very low frequencies where the damperrsquos
gain is highest
bull Fast failure (drop in cavity field) can be caused by quench or arcing in the main coupler The dynamics must be studied LLRF mitigations are proposed
bull The operational scenario did not change since last CC2012 except for the possibility to fill and ramp with RF feedback off
Dec 9 2013 6th Crab Cavity workshop (CC13)
Conclusions
30
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Dec 9 2013 6th Crab Cavity workshop (CC13) 31
Thank you for your attention
CRABS CAN ADAPT TO NEW CHALLENGING CONDITIONS
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
BACK UP SLIDES
From the Daresbury HiLumi-LARP workshop Nov 2013
Dec 9 2013 6th Crab Cavity workshop (CC13) 32
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Transverse Betatron Comb filter
bull One-turn delay filter with gain on the betatron bands
bull To keep 10 dB gain margin the gain on the betatron lines is limited to ~ 6 linear (16 dB)
bull 2 N Poles on the betatron frequenciesbull Reduction of noise PSD where the beam
respondsbull Reduction of the effective cavity impedance
thereby improving transverse stability
bull Zeros on the revolution frequency linesbull No power wasted in transient beam loading
compensation with off centered beam
Dec 9 2013 6th Crab Cavity workshop (CC13)
Betatron comb filter response with a=3132 and non-integer Q=03 Observe the high gain and zero phase shift at (n plusmn03) frev
NQiNQi
NN
zeazea
zzzH
22 11
1)(
We can further reduce the growth rate by selectively correcting the effective impedance on the betatron lines
33
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
bull The transverse impedance of a resonator is [Lee]
bull Assume 300 W RQ and QL=106 the shunt impedance is 25 GWm per cavity
bull With strong RF feedback the minimal effective transverse impedance scales with the loop delay T
bull Assume 1ms loop delay we get
Rmin = 63 MWm per cavity
bull The impedance is reduced by 400 linearbull For HOM the budget is only 1 MVm per cavity
RF feedback on Crab CavitiesDec 9 2013 6th Crab Cavity workshop (CC13)
0min 0
RR TQc
0
0
0
1
s
L
RZ
iQ
34
0s L
RR QQ c
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
RF Power vs QL
Dec 9 2013 6th Crab Cavity workshop (CC13)
TX linked to the cavity via a circulator 3 MV RF RQ = 300 W 11 A DC current 1 ns 4s bunch length (18 A RF component of beam current) Cavity on tunebull Yellow trace beam centeredbull Blue trace x=+1 mm offset bull Red trace x=-1 mm offset
2
(Panofsky Wenzel)
1
2 22L
L
zx
x RFg
dVi ep
dx
V IRP Q xQ cR QQ
For 1 mm offset the range of acceptable QL (P lt 30 kW) is
bull 25 105lt QL lt18 106 for RQ=300 W
bull 8 104lt QL lt6 105 for RQ=900 W
Low values preferred for tuning (microphonics)
Candidate TX 50-100 kW tetrode as used in the SPS 3522 MHz system in the 1990rsquos
35
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
PROPOSED LAYOUT
Dec 9 2013 6th Crab Cavity workshop (CC13) 36
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Dec 9 2013 6th Crab Cavity workshop (CC13)
LHC layout Main (ACS)RF system
Crab CavityRF
Crab CavityRF
37
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
TX and LLRF in new service galleries
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull We have ~300 m distance between crabbing and un-crabbing cavities
bull We propose to dig-out a short gallery running parallel to the tunnel on both sides of IP1 and IP5 for the RF power (TX) and LLRF electronics
bull Such a gallery exists in IP4 (ex LEP klystron gallery)
38
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
LLRF CHALLENGES
Dec 9 2013 6th Crab Cavity workshop (CC13) 39
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Challengesbull Fine-phasing of the CC kick with the individual bunch centrebull Reduce the cavity impedance at the fundamental to prevent
transverse Coupled-Bunch instabilitiesbull Prevent transverse emittance blow-up caused by RF noisebull Manage beam loss following a klystron trip or cavity quench
Dec 9 2013 6th Crab Cavity workshop (CC13) 40
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
The Crab Cavity systems (IP1 and IP5)Dec 9 2013 6th Crab Cavity workshop (CC13)
A slower loop ( 5 ms response time) combines the antenna signals f rom the 3 crabbing and 3 un-crabbing cavities and acts on the individual set-points to minimize the non-closure of crabbing gymnastics
Phase PU
MIMO feedback6-In 6-Out spider
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
IP1 or IP5One beam
Phase PU
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
TX
Ant
Crab
Cavity Controller
The 400 MHz RF and Frev Bx references received on Fiber Optic links f rom SR4
For each cavity a f ast local loop (lt 1 ms response time) keeps the voltage at the desired set-point f or each bunch A local Phase PU would improve bunch per bunch accuracy
300 m
41
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
RF NOISE
Dec 9 2013 6th Crab Cavity workshop (CC13) 42
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Cavity RF Noise
Dec 9 2013 6th Crab Cavity workshop (CC13)
RF feedback noise sources The RF reference noise nref
The demodulator noise (measurement noise) nmeas
The TX (driver) noise ndr (process noise) It includes also the LLRF noise not related to the demodulator
The Beam Loading Ib Dx
We get
0
1 1
sZ
cav set ref meas b drs s RQLQ
K G e Z s Z sV V n n x I n
K G e Z s K G e Z s
Closed Loop response CL(s) Equal to ~1 in the CL BW Increase of K increases the BW Within the BW reference noise and
measurement noise are reproduced in the cavity field
Beam Loading response = effective cavity impedance Zeff(s)
Equal to ~1KG in the CL BW Increase of K decreases Zeff within the CL BW Within the CL BW TX noise and beam loading
are reduced by the Open Loop gain KG
0
1 2
with
L
L
R QQZ s
sc Q
s j
Main coupler
43
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Scaling the ACS noise to CCsDec 9 2013 6th Crab Cavity workshop (CC13)
In the 10kHz-400 kHz range the noise level is defined by the demodulation of the antenna signal The noise BW increases with the loop gain
In the transverse plane the first betatron band is at 3 kHz Reference noise is not an issue The performances will be defined by TX noise and measurement noise
Noise in the 10Hz-1kHz range is not an issue as the first betatron band is around 3 kHz
s
radfS
fL 210
)(
in102 )(
Hz
dBcfL in)(
44
First betatron band
TX noise is important in the band extending to 20 kHz It is reduced by loop gain and scales as 1Sqrt[QL] Tetrodes are less noisy than klystrons
bull ACS SSB phase noise Power Spectral Density in dBcHz
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull The beam will sample the noise spectrum in all betatron side-bands bull Assume a SSB phase noise of L= -135 dBcHz or S= 63E-14 rad2Hz
then summing the noise PSD from DC to + 300 kHz over all betatron bands we get 30011 x 2 x 63E-14 rad2Hz =34E-12 rad2Hz from DC to the revolution frequency
bull A ldquocopyrdquo of the LHC ACS design (300 kW klystron ) would generate 37E-8 rad2 white noise that is 2E-4 rad rms or 11E-2 deg rms phase noise 400 MHz
Scaling the ACS to crab
45
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Dec 9 2013 6th Crab Cavity workshop (CC13)
bull Simulations are numerous In the longitudinal plane I have used
bull For the CC I use an early analysis indicating that 2 nm rms white noise would lead to 1 emittance growth per hour(R Calaga et al small angle crab compensation for LHC IR upgrade PAC07)
bull The 2E-4 rad rms correspond to 10 nm rmsbull Scaling emittance growth with the noise power leads to a predicted 10
emittance growth per hour from a ldquocopyrdquo of the LHC ACS design bull Improvements are expected because
bull The damper gain can be increased compared to the PAC07 figuresbull The CC TX (50 kW tetrode) will be less noisy than the ACS klystronbull And an additional reduction (16 dB) will come from the betatron comb
A simplified analysis is needed to pilot the LLRF developments such as a very low-noise demodulator Measurements of TX noise are also urgent
Resulting transverse emittance growth
46
)(4
22
fSdt
ds
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Trade-off
bull A weak coupling (large QL)bull Reduces the effect of TX and LLRF noise Goodbull But we want to keep BW sufficient so that microphonics are located
within the cavity BW
bull A large feedback gainbull Limits the effect of TX and LLRF noise Goodbull Increases the integrated effect of measurement noise by increasing
the BW Bad
bull Trade-off requiredbull Will depend on the TX noise
Dec 9 2013 6th Crab Cavity workshop (CC13) 47
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
LLRF HARDWARE
Dec 9 2013 6th Crab Cavity workshop (CC13) 48
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
LLRF Hardware
bull Very close synergy with the 800 MHz Harmonic cavities in the SPS and the Linac4 3522 MHz system
bull On-going commissioning of the Linac4 prototype with the RFQbull Commissioning of the 800 MHz version at SPS restart (2nd half 2014)bull Hardware must be tuned for 400 MHz and firmware must be
developed to have prototypes ready for the SM18 test by 2015 in advance for the planned 2016 installation
Dec 9 2013 6th Crab Cavity workshop (CC13) 49
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
For each Crab Cavity we have a Cavity Controller includingbull An RF Feedback Loop for noise and beam loading controlbull A TX Polar Loop to reduce the TX noise and stabilize its gainphase shiftbull A Tuner Loop to shift the cavity to a detuned position during filling and ramping Then smoothly bring the
cavity on-tune with beam for physicsbull A field Set Point for precise control of the cavity field
Tuner Processor
Dir Coupler
Fwd
Rev
DA
C
Ic fwd
Ic rev
TUNER LOOP
CAVITY LOOPS
TX
Circ
Ig fwd
TX Polar Loop (not needed)
Feed-forward
Tuner Control
Ic fwd
CONDITIONING DDS
SWITCH amp LIMIT
SWITCH
Ana
log IQ
Mo
dulator
IQ Rotator ampGain Control
LO
Var G
ain RF
A
mpifier
DDS AM Chopper
Main Coupler Vacuum
FAST LIMIT
RF Drive permitted
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Fwd
Ant
CRAB CAVITY
Voltage functions
I0Q0
DIGITAL IQ DEMOD
DIGITAL IQ DEMOD
Ant
SUM
Version 20111111
AFF for Beam Loading
compensation
SinCos CORDIC
Gain amp Phase
IC revFrom Tuner
Loop
Gain Set
IC rev
Crab Cavity Servo Controller Simplified Block Diagram
Technology DSP
CPLD or FPGA
Analog RF
SignalsDigital
Analog baseband
Digital IQ pair
Analog IQ pair
RF 3522 MHz
Ant (from paired cavities)
DIGITAL IQ DEMOD
PU
Digital RF feedback
with Cavity Coupling
and Betatron
comb
Dec 9 2013 6th Crab Cavity workshop (CC13)
Field control fully integrated with the rest of the LHC by using standard FGCs
Interconnection with the paired cavity(ies) Coupled feedback
Measurement of bunch phase modulation
Gain increased on the betatron bands
50
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Dec 9 2013 6th Crab Cavity workshop (CC13)
SSB Modulator
IF (I amp Q) asymp25MHz
Dual TxDac 16 bits
RF Demodulator
RFampLO mixing
IF asymp25MHz
14 bits ADC
Fs=4IF asymp100MHz
4x
LO Distribution
2 x Duplex Optical Serial Links
2 in amp 2 out
2Gbitss
(le32Gbitss)
SRAM 2x8 Mbyte for diagnostics
VME P1 backplane for
slow controlsreado
ut
Dedicated backplane (P2)
bull Power Supplybull Clocksbull Interlocksbull hellip
Xilinx Virtex 5 SX
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
51
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Dec 9 2013 6th Crab Cavity workshop (CC13)
2 x Duplex Optical Serial links
2 inputs
2 outputs
Status LED
RF output
RF inputs (4x)
LO input
G Hagmann BE-RF-FBdesigner
SPS 800 MHz TWC prototype
52
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Dec 9 2013 6th Crab Cavity workshop (CC13)
ACS LLRF 1 rack 2 VME crates per cavity in a Faraday Cage in the UX45 cavern
53
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
PLANNING
Dec 9 2013 6th Crab Cavity workshop (CC13) 54
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
Planningbull 2013-2014 Cavity Testingbull 2015-2016 LLRF developmentsbull 2016- SPS tests 2 cavities in one cryostatbull 2015-2017 (LS2) Prototype Cryomodulebull 2018-2020 (LS3) Productionbull In operation in the LHC after LS3
Dec 9 2013 6th Crab Cavity workshop (CC13)
SPS testbench with 2 Crab cavities in the LSS4 The cryomodule can be moved in and out of the beam line (2016-)
Proposed SPS cryomodule with 2 Crab Cavities The cryomodule is designed to accept all three CC makes
Courtesy EN-MME
55
