Summary of wakefield session (Jan. 23 afternoon)
A, Comparison of Calculation and Experimental Observation
1, Observed response of downstream orbit change to reference cavity (on mover) position was consistent with calculation.
Snuverink’s slide 24 said 0.3-0.4V/pC/mm from experiment and 0.21 V/pC/mm from calculation. However, the calculation assumed bunch length 10 mm. Assuming reasonable bunch length, about 7 mm, the calculation would give about 0.32 V/pC/mm.
See Kubo’s slide 5 too.
The experiment suggested significant higher order wake (quadrupole field). (See Snuverink’s report)
Dependence on intensity and bunch length looks reasonable. (See Snuverink’s report)
2, Observed response of IP beam size to reference cavity (on mover) position was inconsistent with calculation.
E.g., compare Snuverink’s slide 17 and Kubo’s slide 6: Experiment showed much larger beam size change than calculation.
It was pointed out that the reference cavity position affected background of IPBSM, which could affect apparent beam size if background subtraction was not properly done. This should be checked carefully.
3, Wake of cavity BPM cannot explain observed beam size, strong intensity dependence.
E.g. see White’s slide 6. The expected beam size increase due to cavity BPM wake, based on BPM reading log data is small even for bunch population 1E10 and assuming twice strong wake than calculation.
Also, the effect of the cavity BPM wake should have been compensated by adjusting the reference cavity position (See Kubo’s slide 6). However we could not observed small beam size with high intensity by scanning the reference cavity position.
4, Emittance growth in EXT
Emittance in EXT (measured by OTR system) also depended on intensity but not as strong as IP beam size.
No dependence on DR RF voltage observed (White's slide 7). Betafunction is relatively small in the upstream part of EXT line. So, wakefield in this part is probably not important.
5, Further works
Continue calculation of wake potential of other structures (e.g. vacuum port)
Check effect of IPBPM wake (including re-calculation of wakepotential)
Check IPBSM measurement
- Background dependence
- Response to non-Gaussian beam, etc.
More experiment
- Orbit data with higher intensities
- If possible more bunch length variation
- Use CBPM as beam tilt monitor?
- Normal CBPM on mover for comparison
More study for ILC BDS and RTML
B, Possible mitigation of wakefield effect
Avoid beampipe steps as much as possible
Larger aperture BPMs or remove BPMs at high beta region
Centre BPM wrt (quadrupole) magnetic centre
Second cavity mover at different phase advance (Effective?)
Steer more through BPM-centresor Wakefield free steering?
ILC only:
Wakefield reduced cavity BPM design
All BPM on movers
Orbit bumps to correct wakefield upstream
C, Comparison with ILC BDS and RTML
Relative effect of wake in ILC BDS and RTML(return line) were compared with ATF EXT/FF. It is very rough and there are many ambiguities, then, no conclusion yet.