Minutes December 16th, 2008
Present: Gianluigi Arduini, Rama Calaga, Elena Chapochnikova, Fritz Caspers, Alexej Grudiev, Elias Métral, Federico Roncarolo, Giovanni Rumolo, Benoit Salvant, Carlo Zannini.
Agenda
- Benoit Salvant
"First analysis of measurements with long bunches in the SPS" (slides, minutes)
Minutes (many thanks to Elias for his notes!)
AOB
o Minutes of last time => Action1: Elias and Benoit will summarize all our findings and make some recommendations to perform FFTs on computed or simulated wake fields. Fritz proposes to see all this also on a spectrum analyzer with the example of the BPM (or any other resonator).
o For the SPS kicker Mike Barnes already got some data => To be continued and once it is finished Mike will come and make a presentation.
o Rama will show his results on the transverse impedance localization at a future meeting.
Presentation by Benoit:
- SPS measurements with long bunches (2 MD sessions):
o With 8E10 p/b, unstable in the vertical plane before the longitudinal one.
o Threshold ~ 3-4E10 p/b.
o Reminder: We have to apply a window and not apply a sharp cut to the raw data, otherwise fake frequencies are introduced.
o Elena had similar ringing problems in the past with the longitudinal pick-up.
o If the frequency near 1.5 GHz (which appears by the way only in the vertical plane and not in the longitudinal one) is due to the PU and not to the interaction with the beam, then it seems that we do not have high frequency (i.e. higher that ~ 200 MHz) components in our unstable spectrum. This seems strange as we would expect some activity exactly near ~ 1.5 GHz from the contribution of all the kickers!
o Comment from Fritz: Maybe the unstable frequency is hidden if we have a notch at that point! (however, the exponential PU is designed to avoid notches) and we need the proper transfer function of the PU+hybrid+pre-amplifier+cables. According to Fritz it is quite tricky to measure and he would like to check what was done by Riccardo and Wolfgang. Action2: Fritz and Benoit will check the transfer function with Wolfgang and Ricardo. As Ricardo will be at CERN only in the 2nd half of January, Elias will organize a meeting at this period (if still needed).
o Elena said the most important point is to make simulations to try and predict something => Action3: Benoit should now redo all his simulations with the known impedance models: For the longitudinal, all the kickers + 200 MHz RF cavities + 800 MHz RF cavities (First put all this under ZBASE and then update the web page, as we have all the necessary info); for the transverse plane, use all the kickers.
o Note that for the longitudinal plane an update of HEADTAIL is needed to be able to read the longitudinal wake field from a table (as was already done in the past for the transverse plane).
o Proposition from Fritz about the ringing after the bunch. It could be the signal of wave-guide modes which are coming in, and we call wake fields!
o Comment from Rama: Could this mode be already in the directional coupler?
o Another comment from Fritz: This coupler was built from pbar and if it is used in the wrong way (as it is confirmed by Benoit), we can obtain a mess…
Author: Benoit Salvant CERN
AB/ABP-LIS