Minutes of the ICE section

1st meeting on Wednesday 14/07/2010 (09:30-11:00, 874-1-011 = CCC conf. room)

 

Present: Benoit Salvant (BS), Christian Hansen (CH), Carlo Zannini (CZ), Hugo Alistair Day (HD), Elena Benedetto (EB), Ewen Hamish Maclean (EHM), Elias Metral (EM), Elena Wildner (EW), Frank Schmidt (FS), Giovanni Rumolo (GR), Jean-Luc Nougaret (JLN), Kevin Shing Bruce Li (KL), Nicolas Mounet (NM), Olav Ejner Berrig (OB), Werner Herr (WH).

Excused: Diego Quatraro (DQ), Nicolo Biancacci (NB).

  

0) Comments before the meeting

- From FS => Should add Ewen Maclean (student of him) in the ICE section list as he will also look into beam-beam effects.

- From O. Bruning => Dobrin Kaltchev will be at CERN next week and we should take this opportunity to discuss with him some possible collaborations, as done in the past on beam-beam.

 

1) Welcome and introduction of the new ICE section: ICEsection(EM) (pdf)

- EM presented the new structure (already shown by O. Bruning at the last ABP group meeting on June16th) mentioning that some details are being finalized, the mandate of the ICE section, as well as the mandates of the LIS and LCU sections (as we will work in close collaboration with them).

- EM made some propositions to have a weekly meeting etc. (see slides) => All the people should think about it: it will be re-discussed at the next meeting and the final decisions will be taken there. The current possible scenario could be:

 

2) Status of the LHC instabilities (EM) (pdf and movies from BS in separate files: MeasuredData, SimulationData and SimulationDataWithLosses)

- EM made a status report of 2 transverse instabilities observed so far in the LHC:

- With only 1 bunch in a ring (no beam-beam) at high-energy without octupoles.

- Several bunches in stable-beam conditions (i.e. with beam-beam and also octupoles).

- The 1st one has been studied for some time (EM showed many recent HEADTAIL simulations performed by BS) and it is believed to be relatively well understood (studies continues and will continue) => Head-tail instability m = -1, which is Landau damped by putting some octupoles (K3F = K3D = -6 at the moment). We also believe that we can explain the "Christmas tree" often mentioned in the CCC and at different meetings => A similar pattern is observed in our HEADTAIL simulations (thanks to BS) WHEN WE START TO LOSE BEAM. But before losing we predict and measured a pure m = -1 head-tail instability. The final proof could be given by making a small MD: removing the octupoles at top energy and measuring the intra-bunch motion (with the HEADTAIL monitor) turn by turn and superimposing consecutive traces (say ~ 20 turns). A request has been made to the relevant people and we are waiting for some time to make these observations (note that the HEADTAIL monitor is requested and it does not seem to be working yet. To be checked). For more information, a quite detailed talk (on all the observations and on our predictions) was given at the LCU meeting held on 10/06/2010: https://impedance.web.cern.ch/impedance/documents/SBInstabilityStudiesInTheLHCAt3500GeV_LCU.pdf. A comment was made by FS on possible similar Christmas trees (i.e. many lines growing at some point) in other cases: it might be good to investigate in detail these cases to see if we are discussing the same things, and try and fully understand the physics behind => Action 1:  FS will give some references about these observations or simulations.

- The 2nd instability seems to be beam-beam related and it is followed up in detail by WH and EL. EM mentioned in particular the fact that when beam losses appear, a coherent signal is measured at the BPMs: exponential increase with a rise-time of ~ 10 s, which is close to the one measured with only one bunch (1st instability case). Is it the single-bunch instability (from the machine impedance) which is being measured (after a loss of Landau damping, which still has to be investigated in detail, if real), or is it a coherent beam-beam modes? EM asked if we have predictions for the instability rise-times of the coherent beam-beam modes and WH answered that this is a quite difficult (impossible?) task. Action 2: WH will make a detailed review of all the beam-beam predictions (it is already foreseen for the 3rd meeting, i.e. not the next one but the one after). It seems that these beam losses appear during/after some lumi-scans. At the time of writing, the last 2 fills did not suffer from these beam losses in stable-beam conditions, and the 2 main modifications seem to be: gentle manual scans (instead of automatic ones) and increase of the tune split (from 3E-3 to 5E-3) between the 2 beams (see for instance a nice simple paper on the the effect of the tune split on beam-beam in the case of only 2 bunches: http://sl-div.web.cern.ch/sl-div/publications/LHC99BB/PAPERS/hofmann.pdf and another one here: http://sl-div.web.cern.ch/sl-div/publications/LHC99BB/PAPERS/alexahi0.pdf). All this needs to be studied in more detail, accumulating more observations. For more information, 2 presentations have been given by WH at the LHC Beam Commissioning Working Group:

- https://lhc-commissioning.web.cern.ch/lhc-commissioning/meetings/20100706/LHC-BC-WG-Min06July10.pdf.

- https://lhc-commissioning.web.cern.ch/lhc-commissioning/meetings/20100713/LHC-BC-WG-Min13July10.pdf.

 

3) Actions to be taken for the next meeting

- Action 1:  FS will give some references about these observations or simulations (see text above).

- Action 2: WH will make a detailed review of all the beam-beam predictions for the LHC (it is already foreseen for the 3rd meeting, i.e. not the next one but the one after) => Would be perfect to train everybody and try and understand what happens in the LHC in stable-beam conditions!

- Action 3 (everybody): At the next meeting (see point 4)), we will have a round table to present everybody and explain what the status of their activities (linked to our section) is: successes, issues and next steps. I would like everyone to prepare a short text which he could send me (either before the meeting or just after the meeting for the minutes), including links to all the web pages we should have in our ICE section web page. Note: For those who cannot come to the next meeting (or to a meeting where we have a round table), could you please send me the small text before the meeting and I will read it to the others? Many thanks in advance!.

 

4)  Miscellaneous

- The next meeting will take place on Tuesday 20/07/2010 between 15:30 and 17:00 in room 6/2-004 => Agenda:

- 1st part: General info and decisions on ICE organization (room, time, etc).

- 2nd part: Round table to present everybody and explain what the status of their activities (linked to our section) is => See Action 3.

  

Minutes by E. Metral, 15/07/2010.