Minutes of the ICE section
39th meeting on Wednesday 21/09/2011 (08:40-10:30, 6/2-004)
ICE members: Benoit Salvant (BS), Christian Hansen (CH), Carlo Zannini (CZ), Hugo Alistair Day (HD), Elena Benedetto (EB), Elias Metral (EM), Elena Wildner (EW), Frank Schmidt (FS), Giovanni Iadarola (GI), Giovanni Rumolo (GR), Jean-Luc Nougaret (JLN), Kevin Shing Bruce Li (KL), Nicolo Biancacci (NB), Nicolas Mounet (NM), Olav Ejner Berrig (OB), Tatiana Pieloni (TP), Werner Herr (WH), Xavier Buffat (XB).
Present/Excused: BS, CH, CZ, HD, EB, EM, EW, FS, GI, GR, JLN, KL, NB, NM, OB, TP, WH, XB, Alexey Burov, Georges Trad, Hannes Bartosik, Lajos Bojtar, Miriam Fitterer, Simon White.
1) Newcomers / visitors
- None.
2) Comments on the minutes of the previous 38th meeting + Actions
- None.
3) General infos
- SL meeting:
- Still discussions on fellows.
- LMC: Talk by R. Losito => Main message (from EM and impedance team, confirmed by SteveM and OliverB) is that we should not compromise the machine performance (impedance) by putting large(r) impedances.
- Discussions ongoing on Chamonix and Evian, talks to be proposed, taking into account that the peak lumi will be difficult to improve next year. How can we increase the performance?
- News on the LHC:
- It seems that we start to see temperature increases in the primary collimators in point 7 during the fill (reported at the LHC 08:30 meeting of last Monday). The collimator temperature interlock was raised from 50 deg C to 55 deg C on skew primary collimators in IR7. Furthermore, the cooling capacity was increased to maximum for primary collimators and first two secondary collimators in IR7 (O. Aberle).
- PTC 1/2 day being organised for the end of October to review the CERN (ABP) experience over the last 10 years.
- EM sent some info about the ICFA Beam Dynamics Panel and the latest Newsletter.
- To be discussed (and followed up) with the beam-beam team => Conclusions of the paper "Tune distributions and effective tune spread from beam-beam interactions and the consequences for Landau damping in the LHC" by W. Herr and L. Vos (LHC Project Note 316, 2003).
- The LR effect is ~ 3 times less than what the octupoles can do at 7 TeV/c for nominal beam intensity. It is also said in the LHC Design Report (Chapter 5, Section 5.4.3) that: The tune spread associated with long range beam-beam encounters provides an additional (modest) stabilizing effect [reference to the above paper].
- To collide head-on all the time in order to increase the tune spread implies too many side effects and additional operational difficulties and should not be considered as a possible alternative to get larger tune spreads.
- Beam induced heating on LHC beam screens - Arcs, DSs and ITs (Laurent Tavian) foreseen for next week's ICE meeting => Cancelled. Laurent will give the same talk the day before at the LBOC => The impedance team should try and attend it.
- A "space-charge" cycle (for the PS) is being prepared in the PSB these days by AlanF et al. (for the 3-batch scheme discussed in the past) => Could be very useful to study space charge effects in the PS.
- The transverse emittances are being measured vs. intensity in the LHC injector chain as there seems to be some blow-up somewhere between PSB exit and LHC collision.
- Follow-up from AlexeyB on his studies on LCBI:
- Measurements in Tevatron:
- At injection, there is no growth-time measurements as the pbar is stable and p is unstable and already oscillating at injection and it is limited by nonlinearities.
- At top energy, there are measurements:
- Growth time ~ 30-60 s for pbar. Reminder: Npbar ~ 3 times less than Np.
- Growth time ~ 10 times faster for p. Why, as we have only 3 times more p than pbar? Is it the uncertainty of the measurements or is there something else?
- Computing the growth rates (in the approximation of a rigid-bunch, which is expected to be a good approximation for the rise-time) is ~ 10 times shorter than measured. First possible explanations: factor 2-3 from the rigid-bunch approximation and maybe factor 2-3 from Q factor degradation => To be confirmed.
- Possible experiments to disentangle that (suggested by AlexeyB. It was added to the list of Fermilab's MDs but it has to be done before the stop of the Tevatron on 30/09/11): Measure the growth time with 2 bunches only and with 36 bunches. If the Q factor is so big the ratio between the rise-times should be 36 / 2 = 18 (because in this case this is the total current which matters). To be followed up.
4) Transverse instability in the AD? (Lajos Bojtar): pptx
- Reminder: injection at 3.5 GeV/c, then stochastic cooling and electron cooling. Observations of coherent oscillations at 100 MeV/c.
- The transverse damper is there but not used.
- Seems to be stronger on the upper side-band that on the lower one, but it should be the opposite as the impedance destabilises below and stabilises above => To be understood.
- When the ecooling is OFF there are no coherent oscillations => Either it is induced by the ecooling or it is a consequence of it because the density increases.
- The higher the harmonic the later the start of the instability, which is expected.
- Tunes: 5.45 / 5.42.
- Tails are measured. Were always measured also in the past but larger now. Scraper coming from the right-hand-side and there is a scintillator counting the losses.
- Reminder from AlexeyB: Ecooling provide non-Gaussian distribution because the particles in the centre are cooled more than the tails. But at 300 MeV/c, there is ecooling and the distribution is almost Gaussian according to LajosB.
- Reminder: We have to compare with IBS. If IBS dominates then we can expect that it should be Gaussian. To be followed up.
- Parameters: dp/p ~ 3E-4 whereas it should be and was in the past ~ 1E-4 => There is a pb with the ecooler.
- Definition of chroma: ?, to be checked.
- Transition energy = 4.7 GeV => We are always below transition.
- Very difficult to make rise-time measurements as with cooling we come from stability to instability (experiments from AlexeyB).
- Next steps:
- Make analytical estimate from resistive-wall and taking into account space charge.
- Do an MD in the AD with LajosB, changing the chromaticity.
5) Summary of Fermilab's MDs (FS) => Postponed to the next meeting
6) Actions to be taken for the next meeting
- Old actions.
7) Miscellaneous
- The next (40th) meeting will take place on 28/09/2011 => Agenda:
1) TDI heating and TDI8R observations since the last Technical Stop with added thermocouples (Vincent Baglin),
2) Transverse constant wake field contribution (KL).
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 02/10/2011.