Minutes of the ICE section
35th meeting on Wednesday 10/08/2011 (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, Alexey Grudiev, Chandrashekhara Bhat, Gabriel Mueller, Rama Calaga, Raymond Wasef, Theodoros Argyropoulos.
1) Newcomers / visitors
- None.
2) Comments on the minutes of the previous 34th meeting + Actions
- No comment.
3) General infos
- No particular comment from anyone.
- SL meeting => Nothing particular.
- News on the LHC:
- Reminder: As discussed with the beam-beam team, it would be good to have the HO and LR tune footprints available at the CCC asap to help the operation of the machine. However, due to several constraints it seems it will not be possible before ~ 1-2 months, which is a pity. This has high priority.
- ~ 0.5 fb^-1 have been accumulated last week! We have sometimes ~ 0.1 fb^-1 / day... => Excellent results these days and the total integrated luminosity exceeded 2 fb^-1 last Friday.
- 1-batch emittance BU on 06-08-11 when the intensity per bunch was increased to ~ 1.25E11 p/b: slides => To be followed up.
- Impedance measurements of a SPS MKE kicker => EM discussed with MikeB and we agreed that we will not perform any measurements now (due to other constraints) and that we start to plan other measurements in ~ October.
- BS mentioned that he will give 2 talks at tomorrow's SPSU meeting: the 1st on the effect of NEG coating on the impedance and the 2nd on update of TMCI for Q26 and Q20.
4) 1st TT20 steering MD (OB): doc
- OB explained what he did during his 1st MD (whose goal was to try and reduce the beam size at the T2 target by a factor 2). There is not yet a final confirmation that the goal was achieved but it seems that the line is relatively well understood and the confirmation should come during the 2nd MD.
- Reminder: ~ 1 mm was predicted in the past and ~ 8 mm were measured. Therefore the line did not seem to be well understood in the past. Now it seems to be well understood and therefore the only way to reduce the size by a factor 2 is to reduce the beta function by a factor 4.
- The measurements were made using a "ruler" to estimate the width of beam profile at the splitters => The results are consistent if we assume that what we measure corresponds to a width at ~ 6 sigmas (+- 3 sigmas).
- Next: Finalize this study during the 2nd MD.
5) Theory, observations and mitigation of dancing bunches in the Tevatron: Part II (AlexeyB): ppt
- AlexeyB gave the 2nd part of the talk as foreseen and applied his formalism to several distribution functions for the beam parameters of the Tevatron at both injection and top energy.
- AlexeyB told us that with this new formalism Landau damping appears as phase mixing of the different eigenvectors (and that Landau damping is not an eigenfunction or eigenvalue of the Vlasov equation).
- After a comment from ElenaS, AlexeyB mentioned that we he did is essentially what Oide-Yokoya did in the past, but trying to solve the problem for any RF (as they did it only for a parabola if he is not mistaken).
- In this formalism, F is the steady-state phase space distribution which is given from the beginning and which does not change.
- On slide 15, AlexeyB summarized his results for 3 distributions and for a constant inductive impedance above transition: H-P (Hofmann & Pedersen) distribution, smooth (Sacherer's) distribution and the particular one of Tevatron with 7 coalesced bunches. His findings are the following:
- For H-P, he finds a threshold which is ~ 3 times smaller than with the previous (Sacherer et al.) formalism.
- For the smooth Sacherer's distribution, he found a threshold which is ~ 1 order of magnitude below (in the usual formlism it is only a factor ~ 2-3 below).
- For the particular case of the Tevatron, Alexey had to use the "exact" phase space distribution to find the measured intensity threshold which was ~ 1 order of magnitude below the smooth's one.
- Based on this analysis, AlexeyB's recommendation to fight this instability is to smoothen the core of the distribution as it is very effective.
- As AlexeyB's mentioned, all his findings are qualitatively in agreement with the previous theory (scalings etc.) but the numerical value can be very different and heavily depends on the evolution of the shape of the longitudinal profile through potential-well distortion (i.e. the rigid-bunch approximation is not sufficient).
- Next steps:
- EM will present in one of the next ICE meetings a comparison between AlexeyB's new formalism and the "usual" one, applying it to the LHC and Tevatron (confirming AlexeyB's predictions...).
- 1 puzzling thing to be checked / followed up: ElenaS deduced the Zl/n from some measurements with some physics beams of loss of landau damping and found a value very close to the estimation, i.e. ~ 0.1 Ohm (see ICE meeting held on 01/09/10: http://emetral.web.cern.ch/emetral/ICEsection/Meeting_01-09-10/Longitudinal%20impedance.pptx).
- What happens for the case of an inductive impedance below transition (or space charge above), i.e. the opposite case as the one discussed here?
6) Impedance of the SLAC phase 2 rotatable collimator (EM, slides and questions from Tom Markiewicz) => 2 ppt presentations:
- Finally not discussed during this meeting.
7) Follow-up of the HEADTAIL simulation studies of Landau damping through octupoles in the LHC (Raymond Wasef): pptx
- Raymond explained what he is doing (see also the last HDWG meeting where he made a presentation).
- Until now Raymond had some issues comparing Sacherer, MOSES and HEADTAIL, which is the first necessary step for his study.
- Reminder: Usually MOSES and HEADTAIL agree well (for all cases) whereas some big disagreement can be sometimes observed with Sacherer => 3 possibilities of "errors":
- Sacherer's formula is only an approximation (with its pros and cons...) and therefore we can expect few tens of % of error with MOSES and HEADTAIL (but may be not few units).
- Bad implementation of Sacherer (summation vs integral, chromatic effect, from - infinity to + infinity etc.)
- Sacherer's formula works only in the head-tail regime where all the modes can be treated independently (i.e. each mode are not influenced by others).
- Everything seems to be ~ OK now and the simulations are running for a current in the octupoles of - 10 A to have tune shifts sufficiently small. Waiting for the results...
8) Actions to be taken for the next meeting
- Old actions.
9) Miscellaneous
- The next (36th) meeting will take place on 17/08/2011 => Agenda:
1) Beta beams implementation at CERN (CH)
2) Outcome of Nufact11 (EW)
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 15/08/2011.