Minutes of the ICE section
78th meeting on Wednesday 03/04/2013 (08:40-10:30, 6/2-004)
ICE members: Benoit Salvant (BS), Carlo Zannini (CZ), Danilo Banfi (DB), Daria Astapovych (DA), 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), Serena Persichelli (SP), Sergio Rioja Fuentelsaz (SRF), Tatiana Pieloni (TP), Werner Herr (WH), Xavier Buffat (XB).
Present/Excused: BS, CZ, DB, DA, EM, EW, FS, GI, GR, JLN, KL, NB, NM, OB, SP, SRF, TP, WH, XB, Alexey Burov (from FNAL), Mauro Migliorati (from La Sapienza), Letizia Ventura, Guido Sterbini, Michael Bodendorfer.
1) Newcomers / visitors
- Welcome to Javier Barranco who started as EPFL fellow (working on beam-beam) on April, 1st!
2) Comments on the minutes of the previous 77th meeting + Actions
- Comment from JohnJ on the Action 9 of the list of actions (http://emetral.web.cern.ch/emetral/ICEsection/ListOfActions_2013.htm):
- During the beam-beam workshop, JohnJ mentioned the luminosity-levelling proposal (Andreas Morsch, many years ago, he called it “beta*-tuning”) of varying beta* during Pb-Pb collisions but it is not linked to beam stability. As discussed many times over the years, the motivation has nothing to do with that. There were always objections that it would be difficult to keep the orbit under control but these are no longer such a concern. The point of JohnJ's remark in the workshop was simply that the necessary operational procedure may (or may not …) be implemented in the 2015-17 Pb-Pb runs and that this could be useful preparation for p-p later. The 2003 LHC Design Report, Chapter 21, and the paper at EPAC2004 http://accelconf.web.cern.ch/AccelConf/e04/PAPERS/MOPLT020.PDF identified the “beam-beam issues” in Pb-Pb operation. Some of the effects are very different from the beam-beam effects in p-p. They have been central to the work programme of the heavy-ion team for the last 10 years and this will continue.
3) General infos
- No particular comment from anyone.
- SL meeting:
- LMC follow-up => Impact on screen orientation on IR8. Beyond 3 microm it will not work. Spectrometer would need to be ramped but this could reduce the lifetime of the spectrometer itself and noise issue etc.
- Discussion about fellows.
- TSC requests to be done and deadline was 02/04/13 to DelphineR.
- Group barbecue meeting on 31/05/2013. EM will not be here and there are current discussions to move it to 19/06 => Please reserve these 2 dates for the moment.
- From the LMC => Dedicated beam-gas vertex detector as 3D bunch shape monitor => Should go ahead with that and see the dynamic range => One should check that there will be no impedance issue (Action 1).
- Safety: Incident in LHC point 4 => A person got burnt. Reminder: Near capacitors even if the mains are OFF they can be charged if there are not well short-circuited!
- Project-related BEMB:
- SBL facility => ~ 100 MCHFas a first cost estimate.
- GIS = Geographic Information System: https://gis.cern.ch/gisportal/machine.htm.
- SP and OleksiyK will both go to the coming CST conference => Please contact them in case you have some questions about CST.
- SC meeting last week: discussion about recent issues discovered with PTC-ORBIT and continuation of the preparation of the coming SC workshop => Scientific secretaries not yet informed but will/should come soon (from both GSI and CERN). Reminder: There will be no summary session but a discussion session => So please don't hesitate to contact FS to list all the questions we have. There are 17 contributed talks in the afternoon sessions, with several talks from our space charge team.
4) Head-tail instability mode numbers (2) in time and frequency domain (EM): pdf
- We had no time to discuss this during the last meeting.
- In the frame of the current discussion with BI for the observation of transverse instabilities in the LHC, EM briefly reviewed how the 2 head-tail mode numbers could be identified in time and/or frequency domain.
- There were some discussions about the analytical power spectrum (which is centered at the chromatic frequency) and the one measured by RalphS => Is the latter symmetric with respect to 0 or is it shifted, meaning that the chromatic frequency shift can also be measured (and thus the chromaticity)? To be followed up (Action 2).
5) Discussion about NM's talk at the coming TLEP workshop: pdf
- NM will give the talk "TLEP: effect of cavity impedance for operation at high current and low energy" on Friday.
- He was proposed to review the TMCI intensity threshold for TLEP (Triple LEP => 80 km circumference) as TMCI was then main limitation for LEP (and LEP2). And in particular to look at the possibility to increase the intensity threshold with chromaticity and/or transverse damper (which was found to be quite ineffective at LEP despite several predictions). This is very important to understand why the transverse damper was not very efficient for LEP TMCI as new results from A. Burov et al. for the LHC would suggest that TMCI should not be a problem anymore.
- GR mentioned that the synchrotron radiation damping should be also taken into account. NM mentioned that he will indeed take it into account by assuming a damping if the instability rise-time is longer than the synchrotron radiation damping.
- The impedance model is done by using 2 broad-band resonators (describing the RF cavities and unshielded bellows, as the rest was relatively small => Sabbi1995).
- The TMCI intensity threhsold in LEP was a bit less than ~ 1 mA.
- To study this, NM developed a new code called DELPHI = Discrete Expansion over Laguerre Polynomials and Headtail modes.
- First benchmarks revealed a very agood agreement without damper with the code MOSES1985 (from Y.H. Chin) and also with damper with the results from KarlinerPopov2005 (who assumed a more sophisticated damper model whose exact parameters are unfortunately not available).
- To be continued and applied to TLEP...
- Further benchmarks with other codes (HEADTAIL, NHTVS from A. Burov, etc.) should also be done after the TLEP workshop).
6) Some thoughts about longitudinal impedance (real part) values at low frequencies (FC): ppt
- FC is questioning our impedance model at low frequency for some time because from his point of view the real part of the longitudinal impedance should merge into the Ohmic resistance (like the coax cable transfer imepdance) for low frequencies.
- DC is academic and this is why FC speaks about the revolution frequency, which is for instance 43 kHz in the SPS.
- FC saw very small numbers of the real part of the longitudinal impedance at low frequencies (down to 1E-10 Ohm / m length ) in some presentations, which cannot be true according to him in a real machine => I think we all agreed that the real value of the longitudinal impedance at low frequencies heavily depends on the boundary conditions (for instance on the presence or not of magnets around the vacuum beam pipe, as it is the case in the SPS for most of the circumference, as reminded by GR). This does not necessarily mean that the model is wrong => One should of course apply the correct (i.e. real) boundary conditions.
- Furthermore, there could be also a problem of definition of the impedance, as reminded by MauroM.
- It was then clarified that the picture of slide 4 from CZ was a benchmark between 2 codes for the case of the LHC beam pipe just to check the new model from CZ and that the boundary conditions do not necessarily represent the reality.
- However, the picture of slide 5, also from CZ, and in this case for the SPS, is the correct picture to use (with our current impedance model) and in this case the behaviour expected by FC is revealed: of the order of ~ 10-20 Ohms at the revolution frequecy and therefore about equal to the Ohmic resistance of the 7 km beam pipe, assuming 2.5 mOhm /meter DC resistance => Looks therefore very reasonable and that we don't have any disgreement anymore!
7) Lessons from 2013: ppt or pdf
- GI and GR reminded us that the LHC transverse emittance measurement (BSRT) is a key instrument for ecloud issues and that it would be very useful to have the bunch-by-bunch RF stable phase and heat load available.
- We had to stop and could not discuss in more detail whereas several people had many things to say => To be continued next week.
8) Actions to be taken for the next meeting
- Old actions.
- New actions:
- Action 1 (impedance team): Follow-up of the proposal to use a dedicated beam-gas vertex detector in the LHC as a 3D bunch shape monitor to see if there could be some impedance issues.
- Action 2 (EM): Check with RalphS if with his new measurement method for LHC transverse instabilities he measures a shifted power spectrum (centered at the chromatic frequency, which would mean that the chromaticity could be measured at the time of the instability) or if this information is lost.
9) Miscellaneous
- Deadlines and important dates for ICE.
- The next (79th) meeting will take place on 10/04/2013 => Agenda:
1) Instability observations in the LHC in special high-beta* runs (Helmut Burkhardt),
2) Continuation of the discussion about the follow-up of the fills, data logging, post processing, etc.? FOR BOTH LHC AND ITS INJECTORS.
- See preliminary agendas for the next meetings.
- See List of Actions.
Minutes by E. Metral, 03/04/2013.