Minutes of the ICE section
17th meeting on Wednesday 16/09/2010 (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, Chandrashekhara Bhat, Eberhard Keil, Frank Zimmermann, Giuliano Franchetti, Helmut Burkhardt, John Jowett, Octavio Dominguez Sanchez de la Blanca, Xavier Buffat, Yuri Alexahin.
1) Newcomers / visitors
- Giuliano Franchetti (from GSI, from 14/02 to 08/03 with Frank Zimmermann) will give an AP forum next week on "Space charge driven incoherent effects in SIS100 and their mitigation".
2) Comments on the minutes of the 16th meeting + Actions
- No comments.
3) General infos
- No particular comment from anyone.
- SL meeting last week:
- LARP discussions:
- Phase II collimation: CERN needs to be clarified.
- Interest in the injector upgrade.
- Long range beam-beam compensation need to be studied and design must be conceived for the LHC as RHIC beam-beam compensator is not adapted for LHC.
- Interest for studying the effects of radiation on Nb3Sn magnets.
- Preparation for the LHC start-up are ongoing. We proposed with Gianluigi to add the expected evolution of the bunch length and transverse emittances as expected from IBS => Michel Martini was contacted (John Jowett to be also contacted) and Gianluigi already discussed with OP people: We could either do this through the online model or use Mathematica with Rosali (as Michel and John might have used Mathematica. To be checked). To be followed-up.
- Request by FS and Massimo to the MAD-X module keepers to check their modules before the last release to be foreseen later this month. In principle the new version should be released before the LHC start-up. For this, an action is requested to check the modules and exclude the presence of bugs. The contribution of the module keepers is essential for this task and this service is really precious for the community => A clear procedure has been set up by FS and the following modules need testing by their module keeper:
1) CORORBIT, ERROR - Werner Herr
2) DYNAP, IBS - Frank Zimmermann
3) EMIT - Rogelio Tomas
4) MAKETHIN - Helmut Burkhardt
5) MATCH - Emanuele Laface
6) PLOT - Riccardo de Maria
7) PTC_TRACK - Valery Kapin
8) PTC_TRACK_LINE - Piotr Skowronski
9) SURVEY - Frank Tecker
10) THREADER - Thys Risselada
11) TOUSCHEK - Catia Milardi & Frank Zimmermann
12) THINTRACK - Yipeng Sun
- News on the LHC:
- Requests for the different beams:
- 01/03 => 75 ns 1-batch out of SPS available for the LHC and 150 ns also if pb with the 75 ns beam.
- 21/03 => 50 ns 1-batch out of SPS available for the LHC (highest intensity possible to be ready for the scrubbing run starting on 07/04 => Date to be confirmed).
- 04/04 => 50 ns 2-batch out of SPS available for the LHC (highest intensity possible to be ready for the scrubbing run starting on 07/04 => Date to be confirmed).
- 75 ns 2-batch to be decided after the scrubbing run if problem.
- The installation (already done) of snubber capacitors (=> 2 * 8 = 16 in total) parallel to the EE (Energy Extraction) switches for the nQPS will considerably reduce EM transients (see for instance talk at Chamonix2011 by Jens Steckert: http://indico.cern.ch/getFile.py/access?contribId=52&sessionId=3&resId=1&materialId=slides&confId=103957).
- Link to LBOC (LHC Beam Operation Committee): https://lhc-beam-operation-committee.web.cern.ch/lhc-beam-operation-committee/.
- ICE wishes for conferences and workshops in 2011 sent today to OliverB => IPAC2011:
- GR, WH, FS, KL.
- HD (should be paid by Cockcroft/Manchester => To be confirmed)
- NM (should be paid by EPFL => To be confirmed)
4) 2011 goals for ICE + organization (EM): ppt
5) Experience with beam-beam effect at the Tevatron (Yuri Alexahin from FNAL): ppt
- Yuri discussed the following subjects:
- Incoherent beam-beam @ injection/acceleration/squeeze
- Incoherent beam-beam @ collisions:
- Emittance blowup @ initiate collisions
- Lifetime in colliding beams
- Beam-beam compensation
- Observations of coherent beam-beam modes
- Tevatron run II:
- 36 bunches in each beam are grouped in 3 trains by 12 bunches. Each bunch experiences HO (Head-On) collisions at 2 detectors (experiments called B0 or CDF, and D0) and 70 LR (Long-Range) interactions.
- p total BB tune shift = 0.017.
- pbar total BB tune shift = 0.027.
- Initial separator arrangement and early run II "Comfort Plot" => 12 separators used.
- Between injection and squeeze there are "slow" beam losses which are said to be due to beam-beam.
- "Cogging" used to shift longitudinally the p and pbar bunches, as the relative longitudinal positions are not the same at injection and collisions => Determines IP azimuthal positions.
- Early run II debacle (impact of beam-beam effects on the peak luminosity) => The luminosity increased linearly with the intensity up to a total intensity of ~ 5200E9 p, i.e. ~ 5200 / 36 ~ 1.4E11 p/b. Afterwards, the luminosity decreased!
- Long list of improvements:
- Increase beam separation
- Reduce chromaticity (natural one was ~ 8 units)
- Reduce beam emittance => V. Lebedev devoted in particular a lot of time to reduce the mismatch between the different machines
- Optimize beam cogging (determines IP azimuthal positions)
- Better control tunes & chromaticity
- Remove unused C0 (i.e. at extraction point) Lambertsons (the tightest aperture restriction) and cover F0 (i.e. injection point) Lambertsons with foil to reduce impedance => This also helped a lot to reduce the chromaticity
- Figure of merit for beam separation => Use separation in maximum sigmas (this is also what is done in the LHC, as commented by WH).
- Radial beam separation => helix in 2005 better than in Jan 2002. 5-star helix had to be reduced due to aperture restrictions and was finally replaced with design using 7 separators.
- Losses during the squeeze were considerably reduced.
- Study of losses vs chromaticity @ 150 GeV => With introduction of octupoles in Jan. 2005 the chromaticity for protons and pbars was lowered to 3/0.
- More Comforting Comfort Plot.
- Incoherent beam-beam effects at collision => Issues:
- Not enough room for pbar tunes between 5th and 12th order resonances
- Large BB-induced split in chromaticity: Chpbar - Chproton ~ 7
- Large emittance ratio eproton/epbar (good for pbars, not for protons)
- Insufficient separation at the nearest parasitics
- BB pbar tune shift => Measured with 1.7 GHz Schottky monitor
- PACMAN bunches have much lower tunes => Do not see 5th order
- Tunes go down with time due to proton emittance growth (mostly by IBS)
- Calculations satisfactorily reproduce measurements
- BB chromaticity => Comes from 2 sources: HO interactions owing to beta-function modulation and LR interactions owing mainly to modulation of the beam separation:
- Measurements with 1.7 GHz Schottky monitor => The bb-chromaticity is huge and difficult to compensate since it varies from bunch to bunch and depends on betatron amplitudes.
- Discussions after the meeting with TP + WH => This effects should be small for the LHC.
- "Scallops" in pbar emittances => Clearly the work of 5th order resonance with PACMAN bunches.
- Diffusion due to Beam-Beam Resonances => Diffusion due to 12th order resonances is a puzzle:
- Observations indicate a strong effect of the 12th order resonances on the lifetime and even on pbar emittance. But calculations show that they are too weak to produce dynamical chaos.
- Possible explanation: cooperative effect of resonances and external noise (multiplicative diffusion enhancement - D.Neuffer, A. Riddiford, A. Ruggiero, 1980).
- The mechanism - loss of phase correlation between subsequent crossings of a resonance in the course of synchrotron oscillations - was first discussed by P. Sturrock while at CERN in 1958.
- What Was Attempted to Reduce Pbar Blowup & Losses:
- Increased beam separation with new separators making 4-bumps instead of 3-bumps
- Reduced pbar emittance (e-cooling in RR, matching)
- Compensation of pbar BB tuneshift decrease during a store with feedowns
- Damping of LB (Low-Beta) quad motion to reduce orbit response => Found to be very important!
- Orbit feedback (essential!)
- Lower chromaticity (possible due to reduced impedance)
- Landau damping octupoles @ collisions abandoned
- New working point (close to 2/3 or 1/2) abandoned
- Run II final separator arrangement => 3 more separators added, i.e. 15 in total.
- Collision helix upgrade:
- Beta* was reduced from 35 cm (initial design) to a new design value of 28 cm.
- More than 10% increase in separation at the nearest parasitics.
- BB tune shifts evolution => After the commissioning of e-cooling in RR the problem with the proton lifetime became dominant. The cures implemented were:
- pbar jacking to increase emittance
- Correction of chromatic beta-beat (A. Valishev) beneficial by itself, but also reducing 2nd order chromaticty (and therefore the total tune spread)
- Effect of 2nd order chromaticity compensation => Important as it determines the tune spread due to momentum spread => Decrease chromatic beta-beating.
- BB compensation (BBC) with Electron Lenses (TELs) => Both TEL1 and TEL2 improve p lifetime significantly better than just lattice tune change (i.e. the effect is very strong and more than anticipated): Nonlinear resonance compensation?
- Yuri doesn't think so, there is much simpler and effective (linear) mechanism: beta-beat. It happened so that TELs reduce beta-functions at IPs making the ratio of p/pbar sizes smaller.
- No study of the beta-beat due to TELs performed.
- Coherent oscillations => Run Ib observations:
- At collisions, the chromaticities are quite small and may even be slightly negative. (They are probably between about -5 and 5 units in both planes.) This seems to be a requirement for good particle and luminosity lifetimes. It has also been observed that when the beams are colliding they can tolerate chromaticities that would make a single beam unstable.
- Nobody of the Tevatron people could recollect working with negative chromaticities nor were able to find any evidence in logbooks!
- 36 * 36 spectra at collision => At initiate collisions nice quasi-Pi and Sigma mode excited (there is large difference in lattice tunes and intensities: Na=2232e9, Np=7855e9) but are quickly damped at nominal chromaticity.
- Dedicated experiments on colliding beams stability => When the chromaticity was lowered, the BB tune spread failed to provide Landau damping => Beams went unstable (despite the huge BB tune spread!). As usual, if the tune spread is huge but the coherent line is outside the beam will be unstable => The value of the tune spread is not enough to conclude on the beam stability, we need t know also where the coherent line is (as with space charge etc.)!
- Lessons learned:
- HO tune shifts as high as 0.025 can be tolerated in hadron colliders (i.e. without the help of synchrotron radiation)
- Separation > 6 sigma, for a short time as low as 3 sigma
- Low chromaticity (smaller than 4) is essential for good lifetime
- Multiple crossing of even weak high-order resonances (12th in the Tevatron case) can affect particles in the beam core (the mechanism first discussed by P. Sturrock in 1958 while at CERN)
- There can be a non-trivial interplay between beam-beam and lattice nonlinearities (as testified by detrimental effect of octupoles in collision)
- BB tune spread may be insufficient for colliding beam stability contrary to Run I indications and simplified theoretical predictions
- Beneficial effect of electron lense exceeded expectations (but the reason is not completely understood)
- Info sent by FrankZ (after the meeting) about Peter Sturrock who was mentioned by Yuri during his talk and who worked on multiple crossings of resonances in 1958 at CERN: http://exoscience.org/peter-sturrock.html.
6) End of the review of LHC equipments which we used during our ecloud MDs in the LHC in Oct/Nov 2010 (how many equipments? how do they work? Different modes of operation? Data saving? etc.):
1) LHC BLM by Humberto Maury Cuna and Octavio Dominguez Sanchez de la Blanca: ppt or pdf.
2) LHC BBQ by Tatiana Pieloni and Benoit Salvant: ppt or pdf.
=> POSTPONED TO NEXT MEETING.
7) Actions to be taken for the next meeting
- Old actions.
8) Miscellaneous
- The next (18th) meeting will take place on 23/02/2010 => Agenda:
1) Transverse coherent tune shifts measurements in the PS (Sandra Aumon).
2) LHC BLM by Humberto Maury Cuna and Octavio Dominguez Sanchez de la Blanca.
3) LHC BBQ by Tatiana Pieloni and Benoit Salvant.
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 16/02/2011.