mps current measurements - fermilabpxie.fnal.gov/pipiimeetings/mps_current_measurements.pdf · beam...

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, …


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)


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


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


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


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


Strip-line Ring Pickup (BPM)

Due to larger length signals of upstream and downstream ends are separated but it works for high beta-only


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


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)


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


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


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)


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


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


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

mps current measurements - fermilabpxie.fnal.gov/pipiimeetings/mps_current_measurements.pdf · beam...

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