Minutes of the ICE section
79th meeting on Wednesday 10/04/2013 (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), Javier Barranco (JB), 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, JB, JLN, KL, NB, NM, OB, SP, SRF, TP, WH, XB, Helmut Burkhardt, Hannes Bartosik, Mauro Migliorati, Letizia Ventura, Guido Sterbini, Michael Bodendorfer.
1) Newcomers / visitors
- Uwe Niedermayer (from GSI) is with us for 4 weeks (from 07/04 to 05/05) to help us for the impedance model of HL-LHC => Many thanks in advance Uwe!
- Javier Barranco started as EPFL fellow for the HL-LHC project on April, 1st, 2013.
2) Comments on the minutes of the previous 78th meeting + Actions
- Comment from FC (by email):
- If that phrase (below) is really true (and accepted by everyone),… I would be very happy +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ 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! ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ But this result was obtained assuming a thick layer of ferrite or iron (kicker model) outside the beampipe. Of course the machine does not have ferrite all the way around … But using ferrite in the outermost layer as a trick to remove the “hidden bypass” effect (since a priori the model does not allow for a longitudinal voltage drop on the outer surface of the beampipe) is a very reasonable approach (in a way it does the same thing as the ferrite in a wall current monitor), but then it should be clearly stated that this is a numerical trick to bring the situation for this kind of model as close as possible to reality (where the is no ferrite but also no ideal conductor [at least for the SPS]) and that we need this kind of trick to let the longitudinal impedance merge to the Ohmic resistance for low frequencies…
And I hope then, that really everyone agrees that there is no more dispute.. (in particular : the real part of the longitudinal impedance should merge to the Ohmic resistance at low frequencies unless there is really some sort of true bypass present (like in the cold part of the LHC)
And one additional remark: If nearly all the images current at low frequencies is in the beampipe, then there is very little magnetic field (only second order) outside and thus it does not matter if ferrite or iron is present outside, since there will be no magnetic field to interact with.
An another one: We seem to agree that the longitudinal beam coupling impedance for frequencies where the skindepth is smaller than the wall thickness is very close to the impedance of a coax line with a lossless inner conductor and the outer conductor = beampipe. Why should the situation then be different for low frequencies?
=> To be followed up.
3) General infos
- No ICE meeting next week due to space charge workshop: http://indico.cern.ch/conferenceDisplay.py?confId=221441.
- SL meeting:
- ABP BBQ has been moved to Wednesday 19/06.
- IPAC13 papers: the internal ABP deadline is set to 19/4. SLs will approve the papers and OliverB will start approving them as of 25/4 (morning).
- Friday 03/05/12: All requests for AFC-2013-1 (Associates and Fellows) sent by OliverB to Jeanette Schueler. Therefore, send the requests to OliverB with cc to EM before. The committee will take place on 24/05/13.
- Writing-up of the Evian papers these days... and then IPAC13...
- Talk of NM last Friday at the TLEP workshop => Disagreement with Karliner-Popov as concerns the possibility to raise the TMCI intensity threshold for LEP with a transverse bunch-by-bunch damper => Very important and interesting result in view also of our predictions for LHC. Why does this seem to work for LHC but not LEP? Is it due to the large Qs for LEP (~ 0.1) compared to LHC (~ 2E-3)? To be followed up.
- Impedance meeting on Monday:
-New BSRT simulations with solutions without ferrite, presented by Manfred Wendt.
- Nicola Minafra started to work on TOTEM one month ago.
- OB => EDMS document.
- Talk on highlights from the Diamond workshop by EM to be given another time.
- Comment from EiriniKP about a disagreement between CST and NM RW's code => To be followed up (there was a factor ~ 4.4 in the past which should have been solved).
- Ecloud meeting on Monday also:
- Some thoughts about saturation of LHC e-cloud scrubbing in 2012 by GR => Trapping in quadrupoles. Discussion about the re-diffused electrons.
- Studies from Octavio about the different filling schemes proposed by StephaneF + studies for (V)HE-LHC.
- GiovanniI on SPS results and studies for a possible scrubbing beam => FrankZ mentioned that the beam with doublets from the SPS could be the good one to start with for scrubbing of the LHC in 2015.
- HumbertoM => Mystery of the missing photoelectrons (in ECLOUD compared to PyECLOUD) seem to be solved and linked to the initialization of the e- close to the wall.
- MSWG yesterday =>Talk also from GI.
- PS LIU meeting yesterday with rehearsal talks for LIU day on Friday => GI again (busy week!).
4) Instability observations in the LHC in special high-beta* runs (Helmut Burkhardt): pdf
- High betas run done for IP1 ATLAS/ALFA and IP5 CMS/TOTEM:
- IP1 and 5 => beta* = 90 m to 1 km, no Xing angle, colliding HO.
- Injection / end of ramp optics in the rest of the machine.
- Small number of bunches.
- No parasitic Xing.
- Very tight collimator settings.
- TCP collimators down to 2sigmas and RP detectors very close to the beam (0.87 mm, 3 sigmas).
- "Fundamental measurements with major consequences" => SigmaInelastic ~ 85 mbarn (not 60!) at 7 TeV.
- 1 km physics run on WE-TH 24-25/10/2012:
- Fill # 3216.
- 3 bunches + 1 pilot in each beam with 1 pair of bunches non colliding and the strongest non-colliding bunch became unstable shortly after moving the primary collimator to 2 sigmas. Note that the Landau octupoles current was at 208 A.
- Other instabilities were also observed later.
- The interpretation is a lack of Landau damping.
- (Possible) next steps:
- Try and identify some coherent activity during the loss to be sure that it is a coherent instability.
- Try and measure the rise-time.
- From the simulation point of view, make HEADTAIL simulations with the corresponding impedance model, putting losses to cut the beam at 2 sigmas, putting the octupoles at 208 A and we will see what happens...
- What about the values of the chromaticities?
- Was the ADT ON? If yes, with which gain?
5) Continuation of the discussion about the follow-up of the fills, data logging, post processing, etc.? FOR BOTH LHC AND ITS INJECTORS (everybody): some slides from BS-XB-TP-RdM for LHC
- BS presented some (first) random ideas for discussion for the LHC, which he collected with Xavier, Tatiana and RiccardodM concerning the:
- Logging / storage,
- Viewers,
- Organization,
- Acquisition.
=> To be continued
- WH reminded us that for the LHC we need to move to the direction of having the bunch-by-bunch data (ideally orbits, tunes and chromaticities) if we really want to understand what happen.
- GR reminded us that for the injectors the situation is normally different as in operation there is usually no pb and that for the MDs, each team has to prepare its MD, contact the experts and store the data needed. NB however mentioned that if we could have the longitudinal bunch profiles for each bunch automatically it would be great.
- BS reminded us that there was this emittance blow-up issue in the injector chain last year and a working group had to be put in place to follow this during the year.
=> To be continued.
6) Actions to be taken for the next meeting
- Old actions.
7) Miscellaneous
- Deadlines and important dates for ICE.
- The next (80th) meeting will take place on 24/04/2013 => Agenda:
1) Future accelerator neutrino activities: EUROnu conclusion, and then? (EW)
- See preliminary agendas for the next meetings.
- See List of Actions.
Minutes by E. Metral, 10/04/2013.