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10/6/2016
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 1Eva Barbara Holzer 1
Eva Barbara HolzerCERN, Geneva, Switzerland
CAS Introductory level course on Accelerator Physics
Beam Instrumentation and DiagnosticsPart 2
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 2
Peter Forck: Lecture on Beam Instrumentation and Diagnostics at the Joint University Accelerator School (JUAS), see also the extended Bibliography http://www-bd.gsi.de/conf/juas/juas.html
M.G. Minty and F. Zimmermann: Measurement and Control of Charged Particle Beams, Springer Verlag2003, (book).
Conference series: IBIC (International Beam Instrumentation Conference), IPAC (International Particle Accelerator Conference), historic: DIPAC (Workshop on Beam Diagnostics and Instrumentation for Particle Accelerators), BIW (Beam Instrumentation Workshop)
CERN Accelerator Schools (CAS):http://cas.web.cern.ch/cas/CAS%20Welcome/Previous%20Schools.htm andhttp://cas.web.cern.ch/cas/CAS_Proceedings.html
Rhodri Jones et al.: Introduction to Beam Instrumentation and Diagnostics, CERN-2014-009.
Daniel Brandt (Ed.), 2008 CAS on Beam Diagnostics for Accelerators, Dourdan, CERN-2009-005 (2009).
Heribert Koziol, Beam Diagnostic for Acclerators, Univ. Jyväskylä, Finland, 1992, CERN 94-01, http://cas.web.cern.ch/cas/CAS%20Welcome/Previous%20Schools.htm
Jacques Bosser (Ed.), Beam Instrumentation, CERN-PE-ED 001-92, Rev. 1994
Resources and References
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 3
Intensity Measurement
Derived Quantities:
Trajectory and Orbit
Emittance
Tune
Chromaticity
Betatron phase advance
Beta function
Dispersion
Special challenges for high intensity and high brightness beams
Overview – Part 2
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 4
Some depend on beam optics model
Extensive use on algorithms and computer programs, fitting procedures, iterative procedure to derive desired beam quantity from measured values.
Beam optics values and functions typically corrected after measurement to come closer to design / optics model values
Often also feed-forward and/or feed-back system employed:
e.g. for RHIC the orbit, tune and coupling feedbacks were key to higher luminosities, polarization and integrated luminosity/uptime.
Derived Beam Quantities
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 5Eva Barbara Holzer 5
Intensity
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 6
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 8
4 batches each containing 72 bunches separated by 25 ns
CERN FBCT Readings of LHC Type Beams in the SPS
R. Jones, DIPAC’03
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 9Eva Barbara Holzer 9
Trajectory and Orbit
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 10
Trajectory: The mean positions of the beam during 1 turn
Trajectories must be controlled in linear machine and transfer lines and in circular machines at injection, ejection, and at transition
Orbit: The mean positions over many turns for each of the BPMs
Closed orbits may change during acceleration, RF “gymnastics” and changes of the transverse optics (e.g. in a collider: bringing beams to collide at physics energy, squeezing of beta function, reducing beam crossing angles)
E.g. four pick-ups per tune value separated by about μ ≃ 90o betatron phase advance
Located at large beta function (e.g. next toquadrupoles)
Definitions
beamtrajectory
Focusing elements (e.g. quadrupoles)
BPM Pickups
s
x, y
M. Wendt
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 11
Threading the first beam through the LHC in 2008
One beam at a time, one hour per beam
Collimators used to stop the beam after each sector
Once the trajectory was corrected open the collimator for the next sector
Example of Orbit Measurement
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 12
Beam Screen first two turns of LHC start-up Run2 in 2015
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 13
Example from a measurement of the transfer line optics (beta function)
Transfer Line – Trajectory Response to Dipole Steering Magnet
J. Wenninger, CAS
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 14
≈100 collimators and absorbers
Including special dump and injection protection collimators
Three Stage Collimation System
Cleaning insertion Arc(s) IP
Circulating beam
Arc(s)
Deflection:Primary
collimator
Absorption:Secondarycollimators
Tertiary beam halo
+ hadronic showers
Shower absorbers
Triplet Protection:Tertiary
collimators
SCTriplet
Warm aperture Cold aperture
beam
1.2 m
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 15
Find center and relative size of beam at collimator location using BLM signal
Collimator Set-Up with BLM
Beam
Primary Collimator
Secondary Collimator
BLMBLM
Beam
Primary Collimator
Secondary Collimator
BLMBLM
1.
2.
Threshold
BLM Signal
Jaw Positions
Time
G. ValentinoD. Wollmann
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 16
Generating losses in H, in V and longitudinal plane separately
Verification of cleaning performance and collimator set-up (jaw position with respect to beam center and size)
Validation of Collimator Set-Up
‘loss map’: losses along the ring normalized to the losses at the primary collimator
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 17
Beam-based setup with BLM signal is time consuming
Tighter tolerances will be required for future LHC operation
BPM integrated in the tapered end of the collimator jaws (10.6mm retraction from jaw surface)
Drastically reduce set-up time
Allow constant monitoring of beam position to jaw position possibility for interlocks
Reduce the margins for orbit changes in the collimator hierarchy allow for smaller β*
Tested in the SPS (D. Wollmann, HB2012) <25 μm difference to BLM setup
- believed to be dominated by the BLM setup method
single pass (transfer line): <90μm rms
no disturbance observed from protons hitting the jaws or from shower particles
New LHC Collimators with Integrated BPMs
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 18
Alignment of 16 tertiary collimators
BLM based alignment in 2012
BPM alignment in 2015
No information on beam size –use model value
Experience LHC BPM Collimator Set-Up
G. Valentino, IPAC 2016
1 hour
20 second
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 19Eva Barbara Holzer 19
Emittance Measurement
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 20
Beam sigma matrix: correspondence of Twiss parameters to second moments of the beam particle distribution ( 0 :
Σ εβα γ
′′ ′
γ ε det Σ
σ
In a storage ring the optical functions (α, β, γ) are periodical. They are completely defined by the machine optical elements (magnets).
In a linear machine (transport line, LINAC) there is no periodic boundary condition. The optical functions depend on the incoming beam distribution as well as on the optical machine elements.
Recap
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 21
Σ εβα γ
′′ ′
Circular machine
In a dispersion free region
In presence of dispersion ∆
Measurement accuracy ≈ 10% (because of uncertainties on optics parameters)
Emittance Measurement – Circular Machine
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 22
Σ εβα γ
′′ ′
Method 1:
Slit-grid device (1D) or Pepper pot (2D)
Several slices of transverse profile ( ) and angular distribution ( ′) at one location
Low energy beams, where beam can be stopped in slit / pepperpot mask (for hadrons Ekin < 100 MeV/u)
Method 2:
a) ‘Three grid’ method: transverse profile at different location and linear transformation
b) Quadrupole variation: transverse profile at one location with different setting of quadrupole
Emittance Measurement – Linear Machine
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 23
A mask (slit) cuts out a small slice of phase space with defined x-position.
A drift space converts the angles into position, which is measured with a profile monitor
Reconstruct the angular distribution at the x-position defined by the slit.
Reconstruct the emittance by successive measurements at different slit positions.
Slit-Grid Method
x
z
U. Raich, USPAS
s
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 24
Example – Low Energy Ion Beam
P. Forck, JUAS
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Mask has small holes instead of slits
Measures horizontal and vertical in a single shot
Pepperpot Method
Example Pepper-pot GSI-LINAC: 15 × 15 holes with Ø 0.1mm on a 50
× 50 mm2 copper plate Distance: pepper-pot-screen: 25 cm Data acquisition: high resolution
CCD
P. Forck, JUAS
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 26
To determine ε, β, α at a reference point in a beamline one needs at least three beam size measurements with different transfer matrices between the reference point and the measurement locations.
Different transfer matrices can be achieved with different profile monitor locations, different focusing magnet settings or combinations of both.
Once β, α at one reference point is determined the values of β, α at every point in the beamline can be calculated.
In practice better results are obtained with more than three measurements.
Method 2 – Based on Beam Size Measurements
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Three Grid Method
Based on V.A. Dimov, HB2016
Rms ellipse
Beam profile
′
?′′ ′
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 2828
Three Grid Method
′
?
′ ′
position 1 position 2 position 3
Measured rmsbeam size
′ ′
′ ′ But ′ is unknown
′
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 29
Phase space
position 1
Three Grid Method
′
position 1 position 2 position 3
′
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 30
Phase space
position 1
Three Grid Method
′ ′
Linear mapping of the measured rms
beam size onto the initial phase space.
position 1 position 2 position 3
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 31
Phase space
position 1
Three Grid Method
′ ′
Linear mapping of the measured rms
beam size onto the initial phase space.
position 1 position 2 position 3
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 32
′
Phase space
position 1
Three Grid Method
′
Linear mapping of the measured rms
beam size onto the initial phase space.
′ ′
rms emittance ellipseposition 1 position 2 position 3
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 33
One can linearly map the measured profiles onto the initial phase space and use tomography to reconstruct the distribution of particle density in a phase space.
Phase Space Tomography
′
V.A. Dimov, HB2016
Linear mapping of the measured rms
beam size onto the initial phase space.
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 34
Quadrupole Variation Method
H. Braun, CAS
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Quadrupole Variation Emittance Measurement at CTF3
H. Braun, CAS
horizontal vertical
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 36Eva Barbara Holzer 36
Tune and Chromaticity in a Circular Machine
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 37
The tune Q is the number of betatron oscillations per turn
Measurement gives the non-integer part q; Measure with slightly shifted tune to distinguish q<0.5 from
q>0.5
Caveat: Excitation of hadron beam leads to emittance blow-up
Excite the beam with Single kick (or white noise, or ‘chirp’)
FFT analysis of position reading from one BPM Betatron tune is the frequency with the highest amplitude
response
In the presence of external excitation the method can even work without kick
Example GSI synchrotron
Tune
P. Forck, JUAS
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 38
Tune measurement with a network analyzer Beam exited with sinusoidal wave; frequency is stepped over the expected tune range
Response of the beam (amplitude and frequency) is determined
Beam acts like a harmonic oscillator
Measurement of the phase response in general more precise than amplitude measurement
High precision: up to 10-4 but slow (up to minutes)
Beam Transfer Function Measurement
H. Schmickler, CAS
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 39
cos φ - cos(2ωt+φ)
A voltage controlled oscillator (VCO) excites the beam with a sine wave, measures the beam response and locks the excitation frequency to the 90 degree phase difference. Tracks any tune changes
Continuous reading
Phase Locked Loop Tune Tracking
Example of continuous tune tracking while crossing horizontal and vertical tunes
Closest tune approach is a measure for coupling
H. Schmickler, CAS
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 40
High Sensitivity Tune Measurement by Direct Diode Det.
Marek Gasior,Faraday Cup Award 2012
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 41
Direct Diode Detection (BBQ - Base Band Tune) CERN
CERN LHC
CERN PS
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 42
Example LHC Tune Feed-back System
Hor. spectrum with Tune-FB OFF Hor spectrum with Tune-FB ON
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 43
Chromaticity , or
∆ ∆
ξ ∆
∆
Measure tune for slightly different beam energies (by varying the RF frequency and keeping the magnetic filed constant) and calculate the gradient.
Correct with sextupole magnets
Chromaticity can be tracked continuously by combining RF modulation with PLL tune measurement
Chromaticity Measurement
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 44Eva Barbara Holzer 44
Beta Function and Phase AdvanceDispersion function
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 45
Twiss parameters: Beta function β , α ⁄ , γ
Phase advance, μ(s): phase change of betatron oscillation
The errors on β and μ are frequently referred to as beta-beating and phase-beating
Dispersion function D(s) is the lateral displacement due to a momentum offset:
∆∆
The actual lattice can deviate from the design lattice due to a variety of errors
In general the measurements are followed a by second step: the correction of the measured lattice errors. This is frequently an iterative process that is repeated until the lattice parameters are judged to be satisfactory
Recap
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 46
∆∆
Measure the orbit offset for different beam momenta
beam momentum e.g. by changing the RF frequency, :
∆α
∆γ, α … relativistic gamma, momentum compaction factor
Measure ∆ around the ring (pick-up)
Dispersion function
Example: Horizontal dispersion in the CERN SPS (protons 14 to 450 GeV/c); 6 long straight sections with low horizontal dispersion.
Measurement of Dispersion Function in a Circular Machine
J. Wenninger, CAS
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 47
Can be defined similar to ring. But the dispersion defined in this way depends on the initial condition at the start of the line and also on the location where the energy offset is introduced.
Example: Dispersion measurement in the transfer line from the SPS to the CERN Neutrino to Gran Sasso target (400 GeV/c high intensity protons).
The dispersion is obtained by varying the RF frequency in the SPS ring and measuring the trajectory for different SPS RF frequency settings.
The transfer line bends both horizontally and vertically.
The dispersion is matched to be zero at the target.
Measurement of Dispersion Function in a Transfer Line
J. Wenninger, CAS
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 48
Excite coherent betatron oscillation (kicker, or periodic excitation e.g. AC dipole)
Turn-by-turn BPM position measurement:
cos 2
⁄ cos 2
i … BPM number k … turn number 0 … reference location at
Ai and µi … amplitude and phase at i th BPM
With an absolute calibration of the BPM position readings measure the beta function with respect to beta function at the reference location ⁄
The beta function can also be determined from the measured phase advance∆
in combination with the phase advance derived from the optics model
∆
Beta Function and Phase Advance from Turn-by-Turn BPM Measurement
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 49
LEP (45 GeV): The largest step in beta-beating occur near the interaction points (IP) at the low beta insertions.
Example Beta-Beating from Phase Advance Measurement
P. Castro, PhD
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 50
A small change, Δk, of the strength of a quadrupole (of length L) in a ring leads to the tune change ΔQ
∆14
∆ ≅∆ ̅
4
Measuring ∆ average beta function at the quadrupole location
Example of the period tune modulation due to the modulation of Δk of a LEP quadrupole (here with a square function).
Beta Function from K-Modulation
O. Berrig et al., DIPAC 2001
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 51Eva Barbara Holzer 51
Special Challenges for High Intensity and High Brightness Beams
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 52
Damage potential:
Stored beam energy / power
Energy stored in superconducting machine components
High energy / power density
Avoid uncontrolled losses:
All systems which are part of machine protection (e.g. BLM system): high dependability (reliability, availability, safety, maintainability), fast response, full coverage of critical scenarios (e.g. loss scenarios)
Collimation and related monitoring
Halo monitoring
Avoid intercepting measurement devices
Quench magnets
Instrument can be destroyed by the beam
Non-invasive monitoring of all relevant machine parameters!
Small beam sizes
Systematic effects dominate the measurement
Special Challenges for High Intensity and High Brightness Beams
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 53
High radiation levels at collimation, interaction points, targets etc.
Can interfere with measurement (e.g. beam loss measurement) and require radiation hard equipment
Monitoring of beam instabilities
Bunch-by-bunch and intra-bunch measurements
High dependability of measurement to be used for feed-back systems
Wakefields and RF heating:
Multi pass machines: strict impedance budget of the instrument, in particular for devices which are numerous (BPMs)
Damage due to RF heating
Special Challenges for High Intensity and High Brightness Beams
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 54
Thin carbon ribbons (25-100 nm thick, 1-10 μm wide, 2.5 cm long)
Scanned through the p beam to measure beam polarization profiles
Frequent target breakage (also without beam contact, even in park position) installation of cameras RF heating at the wire ends without
touching the beam
Add “fins” to deviate the EM field from the wire ends reduces significantly the heating
RHIC p-Carbon Polarimeter Target
H. Huang et al., IBIC 2014
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 55
http://www.youtube.com/watch?v=hQsOAyQ7Kck
Video 2
Courtesy M. Minty
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 56
Overheating pressure rise
or material deformation
Beams induced RF heating – LHC run1
RF contact fingers at magnet interconnectsBeam screen around injection protection jaw
Inje
ctio
n K
icke
r
AT
LA
S A
LF
A
De
tect
or
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 57
Mirror heating correlated to:
Beam intensity
Bunch length
Beam spectrum
Synchrotron Light Extraction Mirror
Failure of mirror holder + blistering of mirror coatingOverheated and broken ferrite absorbers (BSRT)
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 58
EM simulations and lab tests are essential for all equipment which is installed on the beam
Mitigation by e.g.:
Design changes to reduce the build-up of wake fields – or deviate from the sensitive location
Adding ferrites to absorb the RF power given there is sufficient cooling for the ferrites
Multi-mode couplers to extract the power and dissipate it outside of the vacuum
RF Heating, cont.
OLD Extraction MirrorNEW Extraction Mirror
Sol
utio
n fo
r R
un I
I w
ith lo
w R
F
‘foot
prin
t’ an
d sh
ield
ed c
aviti
es
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 59Eva Barbara Holzer 59
(Quasi) Non-Invasive Beam Size Measurement
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 60
Beam imaging with vertex reconstruction of beam gas interactions Reconstruct the tracks coming from inelastic beam-gas interactions
Determine the position of the interaction (vertex)
Accumulate vertices to measure beam position, angle, width and relative bunch populations
Main requirements Sufficient beam-gas rate → controlled pressure bump
Good vertex resolution → precise detectors and optimized geometry
Beam Gas Vertex Monitor
Plamen Hopchev
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 61
Goal: develop a transverse profile monitor for (HL) LHC
Overcome the limitations and complement the existing devices
Demonstrate the potential of this technique by installing a prototype BGV system on one beam at the LHC
Commissioning planned for 2015
BGV Demonstrator
Detector Scintillating
fibres read out with SiPMs Same
technology as for the LHCbupgrade
Plamen Hopchev
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 62
Only for electrons & very high energy protons/ions (LHC)
For linear machine: difficult to separate the light from the beam
Difficult to get absolute calibration: Image correction factors typically bigger than the beam size
Dynamic range 200 (105 by changing the attenuation)
Accuracy 30%
Spatial resolution 50μm
Synchrotron Light Monitor
LHC: transverse blow-up of individual bunches
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 63
Residual Gas Ionisation
Dynamic range: up to 103
≈ 10 times more sensitive than luminescence
Image broadening due to space charge
More complicated to build High voltage
Guiding magnetic field
Compensation magnets for the beam
IPM (Ionization Profile Monitors)
M.Schwickert, P.Forck, F.Becker, GSI
T. Giacomini et al., GSI
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 64
Beam Induced Fluorescence (BIF)
Insensitive to electric and magnetic fields (e.g. beam space charge)
Sensitive to radiation leading to background
Low signal yield gas injection (e.g. N2, H2)
Dynamic range: ≈ 103
Luminescence Profile Monitor
N2 injection
To signalprocessing
CCD
I [MCP]
Beam
400 l/s 400 l/s
Lens, Image-Intensifierand CCD FireWire-Camera
N2-fluorescent gas
equally distributed
M.Schwickert, P.Forck, F.Becker, GSI
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 65
Profile Collected every 20ms Local Pressure at ≈510-7 Torr
2DSide view
3DImage
Beam Size
Tim
e
Injection
Beam size shrinks asbeam is accelerated
Fast extraction
Slow extraction
Luminescence Profile Monitor – Example CERN SPS
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 66
For H- and electrons
Electron is stripped from the H-, deflected and measured, e.g. with a Faraday cup(inverse Compton scattering for electron beams)
Can measure down to µm level
dynamic range: up to 103
Laser wire scanner
A. Alexandrov
1 MW beam powerY. Liu, SNS, PAC’11
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 67
Electron beam scanner (SNS, PAC’11, HB2012, W. Blokland)
Electrons are deflected by proton beam and measured on a fluorescent screen
Electron Beam Scanner
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 68Eva Barbara Holzer 68
Halo Measurements
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 69
Beam tails (10-2 –10-3) are just within range of standard profile measurements
Halo (< 10-4): need very high dynamic range >105
Not one of the ‘common’ measurements
For high intensity machines it is mandatory to limit beam losses good understanding of the beam tail and halo very helpful
Halo typically not well understood (simulation codes do not reproduce halo populations very well)
Linear machines: Wire scanners shown to work well for halo measurements
Often beam steered to minimize losses: steering on the beam halo rather than on the beam core
Halo can re-populate along the machine after being scraped
Synchrotron: collimator or scraper or wire close to the beam Measure (precise relative measurement) and destroy the halo at the same time
Halo Measurements
Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 70
Signal: SEM (secondary emission current) for low beam energy or by scintillator for high beam energy or ‘vibrating wire’ method (measured quantity: resonance frequency change due to wire heating – very precise for few interactions)
Use of special techniques to improve S/N ratio and enhance the dynamic range
Wire static in the halo for considerable time suitable for linear machines and transfer lines
A Browman et al. PAC 2003:measurement by SEM current readout at the extraction line of the Los Alamos Proton Storage Ring:
Each position averaging over several beam pulses
dynamic range of 105
Wire Scanner for Halo Measurement
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Eva Barbara HolzerCAS intr. level course on accelerator physics October, 2016 71
Counting with the wire at constant position in the halo can be combined with fast wire scan in the beam core, e.g.:
A.P Freyberger, Jefferson Lab, PAC’03 DIPAC’05: coincident counting (for background subtraction); combining multiple wires with different diameter (also using a plate):
huge dynamic range: 108
Wire Scanner cont.