Minutes of the ICE section
20th meeting on Wednesday 16/03/2011 (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), 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), Maria Carmen Alabau Pons (MCAP), Nicolo Biancacci (NB), Nicolas Mounet (NM), Olav Ejner Berrig (OB), Tatiana Pieloni (TP), Werner Herr (WH).
Present/Excused: BS, CH, CZ, HD, EB, EHM, EM, EW, FS, GR, JLN, KL, MCAP, NB, NM, OB, TP, WH, Alessandro Vivoli, Chandrashekhara Bhat (Fermilab), Hannes Bartosik, Miriam Fitterer, Rama Calaga, Vittorio Vaccaro (Naples).
1) Newcomers / visitors
- None.
2) Comments on the minutes of the 19th meeting + Actions
- No comment.
- EM explained how to deduce the head-tail instability rise time in (time and) frequency domain (see slides linked to the previous ICE meeting, but which were not discussed: http://emetral.web.cern.ch/emetral/ICEsection/2011-03-02/InstabilityRiseTime_EM_02-03-11.pdf). Action1: BS could check it with the HEADTAIL simulations he did in the past for the LHC head-tail instability observed last year.
- List of Actions. => It is important/urgent to try and answer to the Action 26 asap (i.e. ~ end of next week). This is followed up by HD and the impedance team.
3) General infos
- No particular comment from anyone.
- SL meeting last week but I did not attend it.
- Deadline today for the LHC MD requests submission => Many MD requests from the beam-beam, ecloud and impedance teams. Please do not hesitate for those who would like to join these machine studies. Interesting idea from StephaneF to test a new beam called micro25ns (for micro-batch), which has the following characteristics: 1) 24 bunches (25 ns spacing) + 24 empty buckets + 24 bunches (25 ns spacing) => This simply means (if I am not mistaken) that instead of injecting in the PS 6 bunches from the PSB we should inject only the first 2 and the last 2 (i.e. not injecting the PSB bunches number 3 and 4) => This should be done in the PSB by injecting 0 turns there. 2) The spacing between the batches in the SPS should not be 8 empty buckets (as it is the case for the nominal case) but 24 empty buckets => This has to be adjusted in the SPS (easy). What would require more time in the idea from Stephane is, if I understood well, to have the number 24 replaced by any number x => This is followed up by Simone and Giovanni is informed as MD coordinator to follow/steer this activity if approved by the MSWG. Once this beam is checked in the injectors, it could be envisaged to use it also in the LHC.
- You can find already the web links to 5 (out of 6) of the ICE working groups at http://emetral.web.cern.ch/emetral/ICEsection/ICE.htm. The space charge will come soon... Please do not hesitate to follow the activities, participate or if you need some help on some of these activities.
- BS and NM went to the CCC the last but one week-end to participate to the collimation set-up and make possible impedance measurements in parasitic. It seems that there is a noise line which is very large compared to the tune. A comparison was made of two noise tune peak lines (one close to 0.28 and one also analyzed last year by RiccardoDM during the ion run at 0.4) for B2H during the Sunday collimator setup. The two lines are related (not equal though, they are reversed and a factor 2 in the amplitude has to be applied), and interestingly one line stops when the beam is dumped but the other one continues (1st peak, 2nd peak and bothpeaks). Discussions ongoing with GianluigiA and RiccardoDM.
- News on the LHC from the 08:30 meeting on MO 14/03/11:
- > 80% of machine availability.
- 1st stable beam declared on SU 13/03/11 at 18:05 (physics restarted!).
- They pushed up to 8 bunches (75 ns) without problems.
- The last bucket which can be filled for the moment is 31181 (due to abort gap). Access needed to extend it to 31241.
- Beta-beat in collision reduced to record limit of ~ 5%, i.e. limit of noise. Final beta* values from K-modulation => Errors ~ 4-10% and very close to 1.5 m.
- Collimation set up completed Friday late afternoon.
- NEXT: Final MPS checks + increase number of bunches towards 200 in the next 2 weeks.
4) Highlights from the CERN-GSI ecloud workshop held at CERN on 7-8/03/2011 (GR): pptx
- 30 registered participants, half coming from CERN.
- The workshop was proposed by O. Boine-Frankenheim (GSI) as a starting point to discuss electron cloud modeling with the CERN experts and it was organized by GR, FrankZ and OliverBF under the EuCARD/ACCNET sponsorship.
- Ecloud effects were observed in the LHC in 2010 with the 150 ns, 75 ns and 50 ns beams: the smaller the bunch spacing the more critical the situation. Pressure rise measured at several types of gauges => Its occurrence depends strongly on the local electron flux!
- The pressure rise observed in the LHC last year was the most critical in the cold-warm transitions (with an ion pump), then in the warm-warm transitions (with NEG on both sides) and then in the warm-warm transitions (with NEG on both sides and with an ion pump). Reminder: the pressure is proportional to the desorption yield and the electron flux and inversely proportional to the pumping speed. The pressure rise is determined by electron flux, but also local desorption yields and pumping speeds (the neighboring configuration is important).
- A scrubbing run will reduce the pressure rise by decreasing the 2 terms in the numerator. 1 week of scrubbing with 50 ns beams at injection energy and constant electron cloud activity (i.e. topping up with beam when the pressure is seen to decrease) is expected to ensure safe operation with 75 ns beams.
- Scrubbing, or conditioning, is usually ascribed to a graphitization of the surface under photon, electron or ion bombardment (seems confirmed by surface spectroscopy). Therefore it can be aided by depositing on the surface some monolayers of graphitic C (graphene) in a preventive manner => However, issues of stability in an accelerator environment??
- Dependence of Epsmax during scrubbing: the dependence is non-trivial and probably also depends significantly on the history of the sample.
- Reminder on instabilities in positron machines: single bunch instability (from meas. in KEKB) shows an upper side band spaced by slightly more than the synchrotron tune (a factor between ~ 1 and ~ 2). PEHTS and HEADTAIL simulations exhibit the same feature in the bunch spectrum. Bunch-by-bunch (wide-band) feedback system can suppress the coherent motion associated to the betatron tune line if it has high enough gain and band. It cannot suppress the sideband!
- Be careful!:cloud density fluctuations in space and time can affect considerably the beam evolution.
- Status of ecloud at GSI & FAIR:
- No electron cloud build up in the current SIS18.
- Anticipated possible problems for future operation with higher intensities:
- Electron cloud build up along trains of bunched beams in both SIS18 and SIS100, but only if bunch length is below 5m.
- Two-stream instabilities with coasting beams during the slow extraction in SIS100.
- Development of tools underway:
- Electron cloud and coasting beam.
- Electron cloud wake fields with a 3D model of cloud pinch.
- Method to simulate the electron cloud wakes with 3D PIC code:
1) Imports the beam pipe geometry prepared in CST Particle Studio.
2) Discretizes the computational domain with cartesian grid.
3) Loads the domain with electrons.
4) Emits beam and tracks both the beam and electrons.
5) Calculates wake fields using a full Maxwell solver.
=> Follow up: use the SPS parameters to calculate transverse and longitudinal wakes.
- PS measurements => No ecloud with clearing electrode (with some voltage) and/or a-C coating.
- SPS measurements => Electron cloud as a function of SEY (from all the years of meas. with different samples etc.): Very good agreement between meas. and GR's simulations.
- LHC measurements (the upper line is for 50 ns and the lower one for 75 ns) => Energy loss due to e-cloud? Interesting thing to be followed up. Can the longitudinal e-field be responsible for the energy loss measured by ElenaS? This could be answered may be by simulations with the 3D PIC code (to be followed up).
- Coherent and incoherent effects studies for the LHC => Injection and top energy (4 TeV, as was discussed before the Chamonix2011 workshop... since then it was decided to stay at 3.5 TeV/c for this year, but 3.5 and 4 TeV cases should be very close), Field-free and dipole regions => Similar results as those obtained in the past by EB and Ohmi + new interesting results in particular in the incoherent tune footprints where 2 lines show up spaced by slightly less than the synchrotron tune. Long term behavior studies started and need to be refined.
- NEXT (to be followed up by the ecloud team):
- Long term behavior of the beam under the action of an electron cloud. Model planned to include elements with detailed pinch => KL, FrankZ, GiulianoF etc.
- 3D self-consistent calculation of e-cloud wake fields: Quantify energy loss and influence of electron magnetic field on transverse wakes => GR, OliverBF, Fatih etc.
- E-cloud build simulations with ECLOUD: In warm-warm transitions with the present pressure rise data (internal to CERN) and benchmark with GSI Petrovs code for SIS18 bunches => Giuseppe, Vincent, Octavio, Fedor etc.
- Questions to be answered:
- Feedback system? Inefficient?
- Electron cloud fluctuations
- Scrubbing: change of Emax? Graphene layers. Beams with high energy electrons.
- Energy loss measured with the synchronous phase shift related to electron cloud?
- Another proof that a-C does not build e-cloud (PS)? Why then pressure rise in SPS?
5) General wall impedance theory for 2D axisymmetric and flat multilayer structures: Part I (NM): ppt
- NM started to review the wall impedance theory he developed (extending the work from B. Zotter) for 2D axisymmetric and flat multilayer structures (both for longitudinal and transverse impedances). The 2nd part will be given next week.
- Assumptions: Linearity, Isotropy and Homogeneity (in fact piecewise homogenous, i.e. homogenous in each layer, as commented by VittorioV). Anomalous Skin Effect and Magneto-Resistance neglected.
- Matrix formalism (as used already by others like Ivanyan2008 and Hahn2010) => Only need to multiply N-1 (relatively) simple 4x4 matrices and invert the final result, to get the constants => # of layers no longer an issue. Comparison with other formalisms ongoing, codes available.
- Only the cylindrical part could be covered during this meeting (in fact the numerical applications still need to be discussed next week). The main results (compared to previous computations) are the following:
- CYLINDRICAL:
- Notion of wall impedance (instead of resistive-wall impedance) => We introduced it at some point in the past as it is not possible to disentangle between the resistive-wall part and the PC (Perfectly Conducting) part in all the frequency regimes, in particular the 1st (low) one.
- New quadrupolar term (usually considered only for asymmetric chambers). Could/should be checked with CST code with beta < 1.
- Many clarifications/demonstrations by NM for the complex dielectric constant, consistency of the field-matching conditions, m, sum over all the modes etc.
- FLAT CHAMBER:
- Transverse quadrupolar terms in general not opposite in sign. Some hints already from BS past CST simualtions.
- Generalized Yokoya factors (material and frequency-dependent). As already seen by BS and CZ during their theoretical and simulation studies on some simple models of kickers. We could check the agreement with Tsutsui's formalism for any kind of material.
6) Actions to be taken for the next meeting
- Old actions.
- New actions:
- Action1: BS could check the frequency method to deduce the head-tail instability rise-time with the HEADTAIL simulations he did in the past for the LHC head-tail instability observed last year.
7) Miscellaneous
- The next (21th) meeting will take place on 23/03/2011 => Agenda:
1) Status of the list of Actions (rapidly...),
2) 1st LHC beam-beam observations in 2011 (TP),
3) General wall impedance theory for 2D axisymmetric and flat multilayer structures: Part II (NM),
4) Review of Hamiltonian treatment of synchro-betatron motion: Part I (KL).
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 16/03/2011.
