Following suggestions in the minutes of last weeks ATF2 meeting that there could be significant beam size jitter at the IP, possibly due to current jitter powering the final doublet, I'd like to draw your attention to the attached file. The FFS power supply system already monitors the current put out to the magnets to very high precision. For example, in the attached file I have included an analysis of this data performed by Briant Lam in 2008 where he shows the power supplies regulate to 1-2 parts per million rms. QD0 is the magnet for which field fluctuations matter most to beamsize. You can see the details in the attached file, but the measured fluctuations lead to IP beam size fluctuations less than 0.5nm (assuming beam at waist). The other magnets contribute far less. Hence, it is unlikely noticeable beam size fluctuations are being caused by the powering of the FFS magnets.
Also, in beam running in Feb & March, continuous measurements of emittance in EXT over timescales of ~30 mins were made (again see attached file), the incoming beamsize jitter into the FFS from this is about 0.3nm on this timescale.
Hence, I'd suggest the measurement data we have in hand mean that there is no significant actual beam size jitter at the IP in the vertical plane.
Regards,
Glen, 14 June 2011