Minutes of the ICE section
26th meeting on Wednesday 04/05/2011 (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, Andy Butterworth, Daniel Valuch, Javier Fernando Cardona, John Jowett, Rama Calaga, Raymond Wasef, Riccardo de Maria, Roderik Bruce, Tom Mertens, Wolfgang Hofle, Xavier Buffat.
1) Newcomers / visitors
- Xavier Buffat, DOCT. with TP (working on LHC beam-beam), starting on 01/05/11.
- Raymond Wasef => 5-month internship (from 02/05/11 to 16/09/11) with EM to work on "HEADTAIL simulation studies of Landau damping through octupoles in the LHC".
- Laurent Deniau (staff from TE-MSC-MDA) joined our group the 1st of May 2011 in the LCU section (to work in particular on MADX).
- Etienne Forest and Alexander Molodozhentsev are at CERN from May 3rd to June 2nd, 2011 (to work on PTC+ORBIT code for PSB-PS-SPS).
2) Comments on the minutes of the previous 24&25th meeting + Actions
- No comment.
3) General infos
- No particular comment from anyone. HD and OB are in contact with WilliamA for the impedance measurements of the PS WS => Will be done in the next days/weeks (it seems the tank will be available until end of May). Action 1 for HD and OB.
- After request, I will add Giulia Papotti and Georges Trad (working on the implementation of V. Lebedev's luminosity evolution model => See minutes of the 7th ICE meeting held on 08/09/2010) to the ICE meeting invitation list.
- SL meeting: ABP has been asked to present the status of observations on electron cloud and beam stability at injection at one of the next LMC meetings => GianluigiA and EM will report. EM will also be invited to give an review talk on the transverse coupled-bunch instabilities of the LHC.
- News on the LHC:
- On Sunday afternoon (1st of May), a new record for LHC peak luminosity was reached: ~ 8.5E32 (note that it was recently found that ATLAS overestimated its luminosity by some 7-8%, which was very recently corrected) with 768 bunches and only 700 bunches colliding in 1 and 5. The deduced transverse emittances are close to 2.5 microm.
- It is a very good result, but from time to time there are issues most certainly linked to the TCBI, which is closely followed up with OP:
- New simulations from NM for trains of bunches.
- It seems that we are more critical vs. chromaticity now than we were during the scrubbing run => 3 possible explanations:
a) During the scrubbing run we had much more intensity variations along the batches => This helps as it adds more Landau damping.
b) The transverse emittances were larger meaning that the Landau damping (necessary to damp the higher-order modes) was larger. Look at the emittances we had with 1020 bunches at the end of the scrubbing run (slides 4 and 5 in https://lhc-beam-operation-committee.web.cern.ch/lhc-beam-operation-committee/minutes/Meeting7-12_04_2011/SomeBeamObservationsDuringLHC2011ScrubbingRun.pdf) and the ones we had for instance yesterday on fill #1750 (thanks Federico!), when some emittance blow-up was observed (thought to be linked to TCBI, which could be): picture 1 and 2. Is it really true and it depends also when we look?...
c) Effect of the filling pattern, as suggested by GianluigiA. Recent simulations by NM seem to go in the same direction: With "many" (1020 or ~ 1700 equi-spaced) the most unstable mode has a low-frequency. With less bunches (480 bunches case studied), it seems that higher frequencies are excited (the batches are ~ independent) as a low-frequency pattern is observed on short train, meaning that it has a higher frequency... To be followed-up.
=> In this case, this would mean that with less bunches higher frequencies are excited, and this could be more difficult for the transverse dampers which is may be not yet optimized for the high frequencies (as mentioned by WolfgangH) => See the possible issue with the transverse feedback bandwidth (http://indico.cern.ch/getFile.py/access?contribId=7&resId=1&materialId=slides&confId=55195, page 6).
- It seems also that, according to GianluigiA, once the injection process is done and we start the ramp, we have more margin. To be confirmed / followed-up. If yes, what could be the reasons (discussions with GianluigiA):
- Smaller impedance due to the fact that we retract the TDI + TCLIs before starting the ramp? We also reduce the ADT gain to be able to measure the tunes and lock the tune feedback.
- Less transients?
- When do we lose? After few batches or not? If yes, could this be linked to some electronic modules? etc.
- EM discussed with Jan and Bernhard to try and make a rapid test to inject the 1020 bunches used during the scrubbing run, but after some discussion we will not use time for that as it will/should come naturally soon... We just have to be a bit patient for the full understanding...
- What is the situation for the ecloud simulations? How effective was the scrubbing run? etc.
- LHC TDI => We should have a closer look to it due to some hints that the impedance could be larger than predicted (factor 2-3?) => Action 2 (HD).
- LHC DS collimators => Email from Nicolas Provot: "Dans le cadre du projet collimateur DS, 4 chambres vide pour une longueur totale d'environ 6m viendront s'inserer dans le LHC. Ces chambres seront munies d'un depot de cuivre de 0.5mm d'epaisseur, cela pose-t-il un quelquonque probleme d'impedance vis a vis du faisceau?" => Action 3 (BS and NM).
- TSC => We got our requests:
- DOMNGUEZ SNCHEZ DE LA BLANCA Csar Octavio, DOCT, ABP/LCU, Frank ZIMMERMANN, 01/09/2011, 61811.
- IADAROLA Giovanni, DOCT, ABP/ICE, Giovani RUMOLO, 01/06/2011, 61811/61304.
- KOUKOVINI-PLATIA Eirini, DOCT, ABP/ICE, Giovani RUMOLO, 01/07/2011, 61720.
- RIJOFF Tatiana Libera, TECH, ABP/LCU, Frank ZIMMERMANN, 01/06/2011-31/05/2012, 12 months, 61811.
- Follow-up of the ecloud simulations and the recent heat loads measured in the LHC => Action 4 (Ecloud team).
- Power loss from the resistive wall measured by Laurent Tavian in different half cells => Action 5 (EM).
- ZSTF in LSS6 (from Bruno Bahlan).
- Talk from Hugo at the last Collimation meeting (on Monday): Impedance studies of new collimator designs.
- Discussion with MassimoG on subjects to be followed up => Action 6 (EM and MG):
1) Laslett tune shifts => Where are the poles? Estimations from Francesco Ruggiero (Eq. (8) of http://cdsweb.cern.ch/record/279204/files/p83.pdf): Expected incoherent tune shift to be ~ - 7E-4 in V and ~ + 7E-4 in H for 100 bunches with ~ 1.2E11 p/b (increases linearly with the number of bunches). This has to be compensated in the LHC.
2) Use of octupoles at injection? What is the current we could use for DA consideration and current stability?
- Info from Giulia Papotti linked to the Laslett tune shifts and tune feedback correction:
- See ELogBook entry at 05:26 on Monday 02/05/11: https://ab-dep-op-elogbook.web.cern.ch/ab-dep-op-elogbook/elogbook/eLogbook.php?shiftId=1034671.
- Seems that B1 behaves as expected but not B2 (PictureFromGiuliaPInTheELogBook), why? Seems this is understood and followed up by Jorg and OP.
- At yesterday's LBOC, Jan Uythoven mentioned some heating on the LHC injection kicker (MKI). To which level? Is it in agreement with our predictions? => To be followed by HD (Action 7).
- SPS => Yannis and team observed a kind of LHC hump in the SPS during acceleration. Seems it is there for some time according to Karel, but does not seem to be too harmful. To be followed up.
4) Measurement opportunities with the LHC transverse damper (Daniel Valuch): pptx
- LHC transverse damper = ADT => Used for 2 things:
- Transverse injection errors,
- Damping of coupled bunch instabilities.
- It is composed of:
- 2 PUs => Stripline pickups at point 4, magnets Q7 and Q9.
- Electrostatic deflectors => Bandwidth = 3kHz 20MHz. In fact the gain rolls off after ~ 1 MHz (until 20 MHz).
- Observation modes:
- Circular buffer (constantly recording data),
- Single shot buffer.
- Observation triggers => Various trigger methods allow to obtain (1) Synchronized data for Q7/Q9, (2) Independent data for each pickup.
- Observation data:
- Entire ADT works with 16 bit integer data.
- Some calculations are done in 32 bit precision, selecting the 16 relevant bits for the result. System records 18 bits of data, 16 are available from FESA.
- Bpos = Beam Position Module => Calculates normalized beam position bunch by bunch, independent of intensity:
- There is 1 stack of memory called Observation and 1 stack of memory called Post Mortem.
- As user, we have access to Observations only.
- Comments/questions from RiccardoDM:
- There is always 1 position which is exactly at 0 (confirmed by BS) => Seems it is a bug.
- Cane we see the 1st bunch in circular buffer? Daniel answered yes, it is 1 bit in the standard mode (see slide 7).
- Normalized position calculation:
- The system can saturate (ADC).
- Estimated saturation for 2 mm, 1.2E11 p and 1 ns bunch length. As they are tight in signal to noise ratio, they try and optimize the settings for each operation, and for the moment it is like this. If we have 1.7E11 p, it should saturate (without doing anything)...
- Estimated precision ~ 2 microm rms. Coming from noise => Could be optimized if needed.
- DPSU = Digital Signal Processing Unit:
- Damper feedback loop, Injection + Abort gap cleaning,
- Works bunch by bunch (40 MHz rate).
- Possible improvement => Could make 1 buffer with 2 097 152 points, but need ressources for that.
- Note that if 2 users are accessing the buffers are the same time, this will lead to problems to both (erratic data etc.)!
5) Simulations tools for the time evolution of the heavy ions beam parameters (Roderik Bruce): ppt
- Goal: modeling and understanding of ion luminosity during fill in RHIC. Later: application to the LHC ion runs and possibly protons.
- Reference: R. Bruce, M. Blaskiewicz, W. Fischer, and J. M. Jowett. Phys. Rev. ST Accel. Beams 13, 091001 (2010).
- Time evolution of bunch distribution and intensity given by combined actions of several interdependent physical processes.
- Several possibilities of modeling: ODEs (=> 2nd order, i.e. to look at the emittances), Fokker-Planck equation (to look at the distributions), particle tracking simulation.
- ODE model:
- Assumption: All bunch dimensions stay Gaussian and only their standard deviations vary in time => Sufficient to study transverse and longitudinal emittances.
- Numerical solution implemented in Mathematica
- All processes can be switched on or off
- Needed input: machine data (revolution frequency, β* radiation damping time etc), cross sections (for luminosity and beam-gas), assumption on RF noise, IBS rise times
- IBS evaluated with MAD-X off-line on grid of points in emittance space,
- Interpolated online very fast evaluation.
- Advantages => Very fast: a solution of the ODE system for a 10h store takes much less than 1s on a normal desktop PC.
- Disadvantages => Non-Gaussian bunches and effects making the bunch non-Gaussian can not be treated accurately.
- In RHIC, a strongly non-Gaussian distribution is measured, which means that the ODE model will certainly not be sufficient => Tracking simulation (looping through some physical processes turn by turn). Note that the tracking program was written by Roderik and Mike Blaskiewicz, extending the stochastic cooling program of Mike Blaskiewicz:
- Physical processes included (which can all be switched on or off for flexibility):
- Burn-off from luminosity,
- Radiation damping,
- Betatron and synchrotron motion,
- IBS:
- Existing IBS models assume Gaussian bunches.
- FS reminded us that the MADX IBS module was still under construction until very recently (few weeks ago, when it was modified and released).
- Different models compared: Piwinski (smooth or lattice), Modified Piwinski and Bane approximation, New: Nagaitsev (same as Bjorken-Mtingwa but expressed in Carlsson-integral for fast numerical evaluation) => http://prst-ab.aps.org/abstract/PRSTAB/v8/i6/e064403.
- Longitudinal and transverse aperture checks,
- Physical processes non included:
- Beam-beam (difficult to be added),
- Beam-gas (easy to be added but not important),
- Stochastic cooling (for RHIC),
- RF noise (not well known),
- LHC hump.
- Very important loss mechanism in RHIC, called debunching: Particles are diffusing out in the side buckets through IBS. Outside separatrix particles perform unbound oscillations until impact on collimators => ODE model disagrees (debunching not well modeled).
- Roderik compared his models with the one used at Tevatron (see also ICE meeting held on 08-09-10).
- Predictions for nominal LHC with 2.76 TeV/nucleon Pb ions:
- Intensity loss dominated by burnoff,
- Radiation damping stronger than IBS causing a shrinking emittance,
- (though hump etc. not accounted for).
6) Actions to be taken for the next meeting
- Old actions.
- New actions:
- Action 1 (HD and OB): Measure the (5) impedances of the new PS wire scanner, in May.
- Action 2 (HD): Follow-up of the LHC TDI, due to some hints that the impedance could be larger than predicted (factor 2-3?).
- Action 3 (BS and NM): Follow-up of the LHC DS collimators => Email from Nicolas Provot: "Dans le cadre du projet collimateur DS, 4 chambres vide pour une longueur totale d'environ 6m viendront s'inserer dans le LHC. Ces chambres seront munies d'un depot de cuivre de 0.5mm d'epaisseur, cela pose-t-il un quelquonque probleme d'impedance vis a vis du faisceau?".
- Action 4 (Ecloud team): Follow-up of the ecloud simulations and the recent heat loads measured in the LHC. Can we quantify the scrubbing run?
- Action 5 (EM): Follow-up of the power loss from the resistive wall measured by Laurent Tavian in different half cells.
- Action 6 (EM and MG): Follow-up of:
(1) Laslett tune shifts => Where are the poles? Estimations from Francesco Ruggiero (Eq. (8) of http://cdsweb.cern.ch/record/279204/files/p83.pdf): Expected incoherent tune shift to be ~ - 7E-4 in V and ~ + 7E-4 in H for 100 bunches with ~ 1.2E11 p/b (increases linearly with the number of bunches). This has to be compensated in the LHC.
(2) Use of octupoles (Landau or spool pieces) at injection? What is the current we could use for DA consideration and current stability?
- Action 7 (HD): Jan Uythoven mentioned some heating on the LHC injection kicker (MKI). To which level? Is it in agreement with our predictions? => To be followed-up and compared to predictions.
7) Miscellaneous
- The next (27th) meeting will take place on 11/05/2011 => Agenda:
1) Comparison between simulations and measurements in the LHC with heavy ions (Tom Mertens),
2) Review of Hamiltonian treatment of synchro-betatron motion: Part II/III (KL).
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 15/05/2011.
