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Beam Current Measurements in the MPS
Valeri Lebedev
March 14, 2017 Updated at Apr. 19 PIP-II meeting FNAL
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 2
Beam Power Density & Time to Switch the Beam off Maximum deceleration rate
determines the maximum power density in material: dE/dxmax=200 MeV/(g/cm2)
beam size does not change during acceleration: ≈2 mm
That yields maximum power density due to direct beam strike: 12 MW/cm3
A limitation of temperature rise by 250 C yields time for the beam switch off – 10 s
It is the worse-case estimate. Actual temperature rises will be much smaller (multiple scattering, fluctuation of path-length, …
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 3
Acceptable Beam Current Loss (Long-term) Worst case for direct beam
heating is when the beam hits a transition from large to smaller aperture The worst energy is about
100 MeV when the stopping length is about 10 mm (i.e. flange thickness)
Convective air cooling of vacuum chamber can remove ≤50 W (T≈100 K)
That yields acceptable current loss of 0.5 A (I/I≈2.5·10-4)
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 4
Requirements to MPS Beam Current Measurement Time response - ≤1 s Relative accuracy ≤0.5% (still ~20 times above desirable level)
Should not depend on beam velocity, bunch length and details of longitudinal distribution
Good reliability Simple and fast signal processing
Absolute calibration Possible choices Time resolution Operation in CW Absolute
accuracy DCCT No OK OK Ring pickup OK OK ? Toroid OK No OK for pulsed Strip-line OK OK ? RWM OK OK ? With DCCT exception other devises do not see a direct current
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 5
Ring Pickup The length of particle field (length of
charge image) is: ≈0.55 a/ It exceeds the rms bunch length at
each BPM (~6 mm in MEBT, ~1.5 mm – for the rest of the linac)
The device is designed so that the capacitance of electrode to ground, C0, can be neglected. => The signal is:
)( 12 IIdtdQU , 50,1
0
C
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 6
Ring Pickup (continue) The signal duration is inversely
proportional to beam velocity Parasitic capacitance significantly
distorts signal for high Ringing appears when the beam is
moving with speed close to c
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 7
Ring Pickup (continue)
2 2
2( exp 1 exp( / v) ,2v
0.55
spickup
s
S eNR i L
a
Beam signals and their Fourier harmonics depends non-trivially on Fourier harmonics It is also correct for the integral
of beam signal Conclusion: The ring pickup does not look as device capable to measure beam current in the entire range of beam velocities
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 8
Strip-line Ring Pickup (BPM)
Due to larger length signals of upstream and downstream ends are separated but it works for high beta-only
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 9
Strip-line Ring Pickup (continue) Comparison of Button and Stripline BPMs signals
RFQ current 5 mA, bunch length 1.2 mm. Aperture 2a=44 mm for both types. Button BPM is scaled
from HWR stile BPM. Stripline BPM: L=8 cm, =2/12. Conclusion: The stripline does not look as device capable to measure beam current in the entire range of beam velocities
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 10
Resistive Wall Monitor Has unipolar signal Its integral over one period is
equal to the bunch charge We excite an oscillator with
beam signal To get accuracy of signal
amplitude better than 0.1% one needs: High frequency band >> 1 GHz (6 GHz for Tevatron) a low frequency band <30 kHz (1000 periods) (3 kHz for Tevatron)
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 11
Resistive Wall Monitor (continue) The output voltage is:
2 2
2( , exp2vsU Z I I eN
In difference to ring pickup there is no signal suppression at the bunch frequency
Amplitudes of first few harmonics have relatively weak dependence on the bunch length
Comparison of two harmonics allows us to take bunch length into account Response at higher harmonics is
not important
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 12
Resistive Wall Monitor (continue) Summing of 2 harmonics looks as an interesting choice
The 1st and 2nd harmonics look better than the 1st and 3rd
Concept:
RWM signal excites local oscillators at the 1st and 2nd harmonics Rectify by mixing and create I and Q components for both harmonics Digitize and compute current Calibration at 162.5 and 325 MHz can be done by inside wires which are
disconnected at one end during operation
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 13
Resistive Wall Monitor (continue) Frequency response is affected
by parasitic capacitance => the effective impedance is:
( )1
RWM
gap RWM
RZi C R
Resistance of the monitor should be sufficiently small to minimize effect of parasitic capacitance. RRWM ≈ 1 looks as a reasonable choice. For 2 mA beam current the ratio of thermal noise voltage at 50
to the beam signal is ≈2.5·10-4 for bandwidth of 1.3 MHz (≈0.12 s)
The frequency response is flat and does not require additional correction (RWM bandwidth ~30 GHz)
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 14
Field distribution on the walls of vacuum chamber
At low energy the fields of nearby bunches are overlapped even for the point-like bunches To minimize the DC component we need to have sufficiently small radius of RWM
20 mm radius looks as a reasonable compromise for the MEBT not a problem for higher energy
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 15
Bunch lengthening of during transport of 800 MeV beam
Bunch lengthening in the course of beam transport is 6mm/100m
(1.4 deg/100m) has to be accounted but does not represent significant problem
Beam current measurements in the MPS, Valeri Lebedev, FNAL, March 14, 2017 16
Request for the MPS Reliably measure current difference of 0.1% for 2 mA beam current
with time resolution 0.1-0.2 s independently on time structure of the beam
Current comparison should take delays into account (~3 s) Delays should be adjustable
Calibrations can be done with DCCT