Minutes of the ICE section
84th meeting on Wednesday 12/06/2013 (08:40-10:30, 6/2-004)
ICE members: Adrian Oeftiger (AO), 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, Alexey Burov, Guido Sterbini, Mauro Migliorati, Letizia Ventura, Rainer Wanzenberg, Andrea Mostacci, Hannes Bartosik, Steve Hancock, Heiko Damerau, Elena Shaposhnikova, Juan Esteban Muller.
1) Newcomers / visitors
- Andrea Mostacci (from La Sapienza) who will stay with us for 4 weeks (from 10/06 to 05/07) for impedance studies for HL-LHC.
- Uwe Niedermayer will come to CERN with a student to do measurements with ManfredW and FritzC on 13-16 August 2013.
- Rainer Wanzenberg is around for few days and we will discuss his impedance studies for HL-LHC (CMS and ATLAS).
2) Comments on the minutes of the previous 83rd meeting + Actions
- No comment.
3) General infos
- No particular comment from anyone.
- SL meeting:
- Mick Draper and Jean-Jacques Gras are the contacts for the Open Days.
- Seems that ILC in Japan is very serious => Prime minister involed, many cartoons etc. One of the consequences for CERN is that some participation will be required.
- BE retreat last week => OliverB will disucss all this at our ABP group meeting next week, on 19/06.
- Sergio Calatroni asked to check the thickness of the copper coating for the TCDS / TCDQ, which was foreseen at 5 microm => This value was confirmed by NM after some studies.
- TCTP => 2 actions for impedance team (see Action 1 below):
- Perform electromagnetic measurements and impedance simulations on 4E2 ferrite (model for the simulations: TCTP collimator).
- Confirm the RF losses on ferrite used as inputs for the thermal simulations (for the TCTP).
=> My message for the monent: BS and the impedance team started the studies, but the latter need to be finalized once the new version of CST (which has a much better fitting of material properties) is installed. Unfortunately, the implementation of this new version encountered some issues.
- EM was invited to give a talk yesterday during the TOTEM collaboration week => http://emetral.web.cern.ch/emetral/SomeOtherInterestingTalksAndPapers/TOTEM_RP-Imped_%20EM.pdf.
- Nice seminar from Mike Blaskiewicz last week on Stochatic cooling => RHIC luminosity increase with bunched beam stochastic cooling and possibilities for LHC. Discussion with him also on the effect of space charge on transverse coherent instabilities, which is an important subject for the PSB, PS and SPS. Recent paper by him: http://accelconf.web.cern.ch/accelconf/IPAC2012/papers/weppr097.pdf.
- LHC performance review => Invitations sent.
- 9th and 10th HL-LHC WP2 task leaders meeting => Actions for us (can also be found on the Task 2.4 web page => https://espace.cern.ch/HiLumi/WP2/task4/SitePages/Home.aspx):
1) Study the dependence of impedance on octagonal Beam Screen aperture => NM will report about his studies on 02/07/2013 (see Action 2 below).
What about the Y-chamber? Will it be different? Could be something to discuss with Mikhail Zobov once we have a drawing as for the moment there are still discussions on the D2 which has some impact on the Y-chamber.
2) Study the dependence of e-cloud effects on octagonal Beam Screen aperture with counter-rotating beams => Deadline: ~ end of summer, mid September (FrankZ and GiovanniR) => See Action 3 below.
3) Check the effect on transverse beam stability of the longitudinal beam distribution in bunch lengthening mode with a possible 800 MHz HHRF system => KL with a deadline for ~ end of summer (see Action 4 below).
Reminder: The effects on beam-induced RF heating of a 2nd or 3rd order harmonic RF system has been already studied and presented at a LMC meeting (ElenaS, LMC, 10/10/2012). The effect on the longitudinal stability will be studied by ElenaS.
4) There was also a discussion about the value of 7.5 cm rms for the bunch length, or 1 ns total, as it becomes necessary for the small beta* => I think we cannot say more, for the moment, that what was said in the talk at Evian given by BS (see http://indico.cern.ch/getFile.py/access?contribId=20&sessionId=10&resId=1&materialId=slides&confId=211614):
1) See slide 33 => The test confirmed that shorter bunch length increases heating for most monitored devices We stayed 1h between 1 ns and 1.1 ns with 1380 bunches at 1.5 1011 p/b without major issue.
2) See slide 34 => Bunch length test summary:
- The items that heated more are planned to be upgraded (ALFA, MKI8C, TDI, BSRT) or removed (TCTVB).
- No hard showstopper unveiled by this test to run at lower bunch length
=> Next question: How does this scale for the HL-LHC parameters? => One should build a table with all the components and see what is the power deposition for the HL-LHC nominal scenarios (using table in the PLC web page: https://espace.cern.ch/HiLumi/PLC/_layouts/xlviewer.aspx?id=/HiLumi/PLC/SiteAssets/Parameter%20Table.xlsx&Source=https%3A%2F%2Fespace%2Ecern%2Ech%2FHiLumi%2FPLC%2FSiteAssets%2FForms%2FAllItems%2Easpx&DefaultItemOpen=1). We could start with the table that BS presented for the 2012 beam parameters and make the computations for HL-LHC parameters assuming the 2012 impedances and considering 1 ns => Action: BS for ~ end of Summer (see Action 5 below).
- Reminder on our plan / goal:
1) First estimate of the impedance model by 01/11/2013 => i.e. hopefully our best guess of the impedances of the main equipments.
2) Then, study of the beam limitations => i.e. table of power loss / equipment and effects of these impedances on the beam stability => First estimate needed / planned for 01/05/2014.
- Rhodri Jones accepted to coordinate the efforts for Task 2.7 on “Intensity limitations from existing LHC HW” => Strong interaction with Task 2.4.
- Impedance meeting on Monday => Talk in particular of Nicola Minafra for his studies for TOTEM RPs.
- The LIU HL-LHC workshop will take place in October, 8-10. One week after the experiments will do the same kind of exercise.
- Mauro Migliorati => SASS contract extended by 1 year => Congratulations Mauro and thanks for all your help!
- EM mentioned a possibly interesting paper he found in the archives of Dieter Mohl about "A New Bench Method to Simulate Electromagnetic Fields of Slow Beams" by F. Caspers and D. Mohl, June 1990 => BS made a scan and the paper should be available soon on the impedance web page.
- IR8 and beam-beam constraints => 1 number (beam-beam separation in sigmas) is not enough for the beam-beam team (1 number can be enough sometimes but it depends on several assumptions) => MassimoG and LCU informed. As such cases happen also for instance when deadling with impedances (as sometimes 1 number was given in the past), it would be good to have one example case revealing that 1 number is not enough (Action 6 below).
4) Simulations of the longitudinal coupled-bunch instability in the PS (Mauro Migliorati): pdf
- Reminder on LCBI: eigenvalue system to be solved and nice analogy with coupled harmonic oscillators through springs even if, as Alexey Burov mentioned, this is a Hamiltonian system, which should be always stable.
- Reminder on the LCBC = Longitudinal Coupled Bunch simulation Code developped 20 years ago ("A Time Domain Simulation Code of the Longitudinal Multibunch Instabilities" by Bassetti et al., June 23, 1993, Frascati, Note: G-19):
- Considers bunches as point charges (no internal structure).
- Time domain
- o = out and i = in.
- Goal was to investigate the effect of a bunch-by-bunch feedback system.
- Was benchmarked with analytical formulae.
- g = feedback gain in V / rad and few 100 V / rad is a reasonable value.
- Application to the PS case => Very good agreement (for the instability rise-time) with analytical formula for 7 bunches on h = 7.
- Application to the PS case: Frequency-domain FB:
- Mode analysis to be done from the bunches motion and then damping of a particular mode.
- Benchmarks done by exciting only 1 mode (without FB), by damping with the required FB gain without excitation of the mode, and by finally using the requierd FB gain in the presence of the mode (damping rate equal to growth rate).
- Application to the PS case: External excitation
- Example with a mode spectrum of 21 bunches in h = 21, excited at the upper sideband of 19 frev => Nice agreement between measurements and simulations. Alexey Burov mentioned that this case can be fully solved analytically.
- FB damping rate is linear with FB gain.
- FB damping rate is linear vs. bunch intensity.
- FB damping rate is ~ constant with longitudinal emittance for the considered bunch lengths.
- Impedance model and comparison with measurements:
- Main source of impedance for LCBI in the PS is coming from the 10 MHz RF cavities => The coupling impedance of the cavities is not a simple resonator due to the feedback loop and the power amplifier => See paper from H. Damerau et al. (PAC'07): Longitudinal coupled-bunch instabilities in the CERN PS.
- According to the paper above, the growth rates range from ~ 1 s-1 to 5s-1, with a weak dependence on the longitudinal emittance:
- Unstable mode n = 1 => Growth rate = 2.5 s-1.
- Unstable mode n = 2 => Growth rate = 3.0 s-1.
- Unstable mode n = 3 => Growth rate = 1.0 s-1.
- Results from the simulations:
- Unstable mode n = 1 => Growth rate = 1.84 s-1.
- Unstable mode n = 2 => Growth rate = 2.14 s-1.
- Unstable mode n = 3 => Growth rate = 0.52 s-1.
=> Summary: Quite a good agreement.
- Preliminary work done for 18 bunches in h = 21 => The goal is to try and explain the measurements results published in the Proc. of HB2010 (Longitudinal Performance with High-Density Beams for the LHC in the CERN PS by H. Damerau et al.).
- Comment from ElenaS => They started to use also the HEADTAIL code but it is not yet usable for long bunches and it seems there are some issues to be solved. To be continued.
- ElenaS proposed to change the LCBC code and replace the matrix by a Fourier transform in order to avoid the fitting procedure by several resonators (as the code works for the moment only with resonators).
5) Brief reports of the 4 WGs (by the WG leader or replacement) => Plan, activities, deadlines, issues, etc.:
a) BB (WH):
- Follow up beam-beam workshop:
- 60% of papers received, 30% promised to come, 10% will not be written.
- Summaries of workshop have been sent to Beam Dynamics Newletter and CERN Courier.
- Beam-beam meetings:
- Dynamic aperture studies with 6D beam-beam lens have been presented and show sizable effect.
- Crab cavities are being implemented into sixtrack.
- LLWG:
- Following the proposal after Evian, a Luminosity Levelling WG has been set up. So far with members of beam-beam Team (WH, TP, XB) and OP.
- First 3 meetings already held, indico site will be prepared.
- Lumi scan for ATLAS:
- Computations (TP) now in good agreement with measurements, computation with MAD needs a clarification.
b) SC (FS replaced by EM): pptx
- FS is currently studying MAD-X with frozen space charge in the US at BNL to discover more bottlenecks and necessary developments to make it useful for a broader audience.Could one get MAD-X under MPI for speed-up?
- PTC-ORBIT studies are suffering from crashes that are actively pursued in collaboration with RaymondW and HarryR.
- FS have provided on lxplus SLC5 mpich2 that has the potential to be faster but even safer in therms of crashes. This will help HarryR to evaluate the crashes. The SLC6 version is being prepared for the future. This will be summarized for the space charge group in the coming days.
- These mpich2 software is accessible from our space charge web site together with the newest version for SLC5/6 with and without mpich2.
- Major effort in terms of benchmarking both SYNERGIA and ORBIT. Amazingly many issues have already shown up. In particular, the noise issue has been reproduced by Jeff albeit reduced via symmetrized distributions.
- Mitigation effort by Vorobiev for the ORBIT code is continued.
- Both for PSB and the SPS un-physical behavior has been detected and is being pursued.
- Computing resources can be upgraded by a factor of 2 in case of heavy usage. IT will take action in the coming weeks. Large improvements via super computers seem difficult to be feasible in the near future.
- Despite the problems the simulation studies should now go into full gear.
- FS has prepared both SVN for PTC-ORBIT and set-up the framework for the documentation. This should start to be filled after the summer at the latest by the space charge participants with FS's help if needed.
- LIU planning should be done after summer and FS is planning another consistent EXCEL file as done for the parameters.
c) EC (GR):
- EC progress in the last few months. There were two meetings:
1) EC WG meeting on 8 April 2013 https://project-ecloud-meetings.web.cern.ch/project-ecloud-meetings/meetings2010.htm. Four presentations covering: GR --> possible ideas on why scrubbing has leveled off. Quadrupoles, modeling of the SEY curve, behaviour of scrubbing on cold surfaces GI ---> e-cloud studies in the SPS, doublet beam tested with double batch injection OD --> HL-LHC studies, different configurations studied (also including 2.5 and 1.25 ns spaced doublets) HM --> tracking studies of synchrotron radiation started for LHC.
2) Brainstorming meeting on the two questions: 1- Scrubbing speed: why has the scrubbing process seemingly stopped in the LHC ? 2- Behavior of the heat load/stable phase shift with the beam energy. The meeting took place on 23 May, 2013 with the participation of G. Arduini, V. Baglin, H. Bartosik, S. Claudet, O. Dominguez, J. Esteban-Müller, G. Iadarola, M. Jimenez, H. Maury-Cuna, G. Rumolo, E. Shaposhnikova. Several actions were defined during this meeting, concerning both simulations and VSC studies on future e-cloud diagnostics in the LHC. Everything is summarized in the note circulated after the meeting: LHC-Scrubbing-Minutes.
d) IMP (BS): pptx
- Status discussed on Monday during impedance meeting.
- Nice theoretical current studies by SP and NB.
- Many requests and current worries are the SPS WS.
- Working well:
- Many help from other labs.
- Moving to the PS and PSB.
- Working less well:
- Procedures.
- Licences: 15 to 20.
- Effect of very short bunches.
- ALFA RP and BSRT => With ferrite we have pb.
- Lack of deep knowledge on techniques to damp HOMs => To be developed (possible collaboration with Andrea Mostacci).
6) Actions to be taken for the next meeting
- Old actions.
- New actions:
- Action 1 (Impedance team): 2 actions, in fact, related to TCTP collimator studies => i) Perform electromagnetic measurements and impedance simulations on 4E2 ferrite (model for the simulations: TCTP collimator) and ii) Confirm the RF losses on ferrite used as inputs for the thermal simulations (for the TCTP).
- Action 2 (NM): Study the dependence of impedance on octagonal Beam Screen aperture (in the framework of HL-LHC WP2) and report about the results on 02/07/2013.
- Action 3 (FrankZ and GR): Study the dependence of e-cloud effects on octagonal Beam Screen aperture (in the framework of HL-LHC WP2) with counter-rotating beams => Deadline: end of summer, mid September.
- Action 4 (KL): Check the effect on transverse beam stability of the longitudinal beam distribution in bunch lengthening mode (and I would also suggest bunch shortening mode) with a possible 800 MHz HHRF system (in the framework of HL-LHC WP2) => Deadline: end of summer.
- Action 5 (BS): Scale the LHC beam-induced RF heating estimates to the HL-LHC parameters (using table in the PLC web page: https://espace.cern.ch/HiLumi/PLC/_layouts/xlviewer.aspx?id=/HiLumi/PLC/SiteAssets/Parameter%20Table.xlsx&Source=https%3A%2F%2Fespace%2Ecern%2Ech%2FHiLumi%2FPLC%2FSiteAssets%2FForms%2FAllItems%2Easpx&DefaultItemOpen=1) with the current LHC impedances to have a first idea (it will be then redone properly afterwards with the impedances for HL-LHC) => Deadline: end of Summer.
- Action 6 (Beam-Beam team): Study / show on an example case why 1 number (beam-beam separation in sigmas) is sometimes not enough to approve an optics.
7) Miscellaneous
- Deadlines and important dates for ICE.
- The next (85th) meeting will take place on 26/06/2013 => Agenda:
1) Transverse impedance localization results from BNL (NB).
2) Main 2013 MD results / issues on collective effects in the PSB, PS and SPS (GR).
- See preliminary agendas for the next meetings.
- See List of Actions.
Minutes by E. Metral, 14/06/2013.