Minutes of the ICE section
8th meeting on Wednesday 15/09/2010 (08:40-10:30, 6/2-004)
ICE members: Benoit Salvant (BS), Christian Hansen (CH), Carlo Zannini (CZ), Diego Quatraro (DQ), 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, DQ, HD, EB, EHM, EM, EW, FS, GR, JLN, KL, MCAP, NB, NM, OB, TP, WH, Rama Calaga, Emanuele Laface, Eirini Koukovini Platia.
1) Newcomers / visitors
- Eirini Koukovini Platia was introduced by GR: she is a technical student working in CC3 for the CLIC DRs (Damper Rings). She is welcome to join the team whenever she wants!
2) Comments on the minutes of the 7th meeting + Actions
- No particular comment from anyone.
- List of Actions => GR gave a short talk to close his Action 5 on the synchrotron periods at the PSB instability timings:
- Reminder: The first instability has a rise-time of few ms, the second few tens of ms and the third again few ms.
- The (small amplitudes) synchrotron periods are: 1/1740 ~ 0.6 ms for the 1st instability and 1/1290 ~ 0.8 ms for the 2nd.
3) General infos
- No particular comment from anyone.
- No SL meeting last week.
- News on the LHC:
- Aperture measurements at injection revealed that we are in very good shape thanks to the orbit which is very well controlled, the mechanical tolerances which are better than foreseen and the optics which is about as foreseen with for instance a beta-beating of ~ 15%, i.e. within the limit set to 20%.
- WH gave a talk on Tuesday at the LHCCWG on bunch trains and crossing angles at injection (http://lhc-commissioning.web.cern.ch/lhc-commissioning/meetings/20100914/Werner_beam-beam_140910.pdf): LR (Long-Range) beam-beam effects were observed with the 150 ns bunch spacing when a scan in crossing angles was performed. Seems difficult to scale from 150 ns to 25 ns case.
- On Tuesday it was reported at the 08:30 meeting that a bad B1 lifetime was observed (together with the usual Christmas tree) and this was traced back to the absence of octupoles.
- Reminder of the info I sent:
- In the context of CERN's participation in the Eiroforum (collaboration between EMBL, ESO, ILL, ESRF, CERN), it has been agreed to run the following joint courses for young researchers (post doc and fellows - 3-5 years experience) at the organisations concerned:
1) Convincing Scientific Presentations course that will be run at EMBL, Heidelberg on 19-20 October 2010.
2) Project Engineering course - that will be hosted by CERN, on 18 - 19 November 2010.
- BS organized yesterday the first GSI/TUD/CERN impedance web meeting with in particular talks from CZ and HD. The idea of these "impedance meetings" is to discuss only impedance (or wake-field) related issues (in particular simulations and measurements, but also theory). In case of discussions of the effect on the beam, as it is of general interest for the whole ICE team, it is proposed in this case to organize an ICE meeting dedicated to this. This will be discussed in particular with Oliver Boine -Frankenheim at the next HB2010.
- OB is checking the aperture of the ZS in LSS6 and the SLAC collimator in LSS5.
- NM will give a talk at the next HB2010 on the news on the 2D resistive-wall theory and he could make a presentation of this at some of the next ICE meetings.
- During the "Review of Proposed LHC Collimation Work in DS for 2012" (http://indico.cern.ch/conferenceDisplay.py?confId=100156), Stefano mentioned our results for the transverse coupled-bunch instabilities (see page 28 of Stefano's talk: http://indico.cern.ch/getFile.py/access?contribId=2&resId=0&materialId=slides&confId=100156) and it was also said that Head-tail simulations will address the single-bunch instabilities. BS and NM did it (we are still checking our results) but it seems that the single-bunch TMCI intensity threshold is 1.7E11 p/b, i.e. the ultimate intensity. Knowing that we can only be optimistic in our predictions we think we have a problem here with the current collimators' settings. The most critical collimators were identified by NM and discussed at the LHC Collimation Working Group meeting held on 05/07/10 (https://impedance.web.cern.ch/impedance/documents/LHC_multibunch_modes_phase1_phase2_IR3MBC_followup.pdf). We will try and see what happens when removing the most critical collimator(s).
- We informed Ralph Assmann and we will present our results at some CWG meetings. From our side we are a bit worried as the impedance can only be worse than our predictions and 50% margin is certainly not enough. We would prefer at least a factor 2. Furthermore, in the presence of many bunches the intensity threshold for the mode coupling could be even lower. We have no quantitative predictions for our impedance model yet, but this was studied in detail in the past by Scott Berg (see for instance http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-6829.pdf and http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-r-478.pdf). If we look for instance at http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-6829.pdf and compare Fig. 1 (single-bunch mode coupling) and 2 (multi-bunch mode coupling), we see that in his particular case the intensity threshold decreased by a factor 2! We will redo simulations with a slightly positive chromaticity (which better describes the reality) and could improve the situation.
4) Review of some beam-beam predictions for LHC (TP): ppt
- Tatiana made a review of the main studies and predictions on beam-beam made in the past, which are discussed in detail in her PHD thesis.
- By definition, the BBI (Beam-Beam Interaction) is strongly nonlinear. As a result the single-particle and whole-bunch motions can be distorted.
- LHC complex case:
- Strong effect (due to the high brightness, i.e. ratio between the number of particles per bunch Nb = 1.15E11 p/b and the transverse rms beam size = 16 microm). Furthermore, there are 4 experimental (collision) points. The number of collisions per second is around 11000.
- Many interactions => Number of bunches = 2808. The nominal bunch spacing is 25 ns (i.e. 7.5 m) between the bunches of a beam and therefore around the IP, in the presence of both beams, the bunch spacing is 12.5 ns (i.e. 3.75 m). The full crossing angle is 285 microrad and the maximum number of BBIs per turn is 124: 4 HO (Head-On) + 120 LR (Long Range).
- The regular bunches are those with the 124 BBIs, which correspond to ~ half the total number of bunches. The bunches which do not have the regular collision pattern (i.e. do not have the same integrated beam-beam effect) have been named PACMAN (if missing LR BBIs) and SUPERPACMAN (if missing HO BBIs) bunches. There are 200 different bunch families.
- A self-consistent treatment is needed for a complete understanding of the BB effects, as we are in a strong-strong regime, i.e. the source of distortion changes a result of the distortion.
- Other (than the LHC) hadron colliders:
- RHIC: 110 bunches, 2 HO. There is no bunch train, the bunches are equally spaced, and no crossing angle, as the distance between the bunches is large enough.
- TEVATRON: 36 bunches (3 trains of 12 bunches), 2 HO + 72 LR (as there is only 1 beam pipe).
- CERN experience with bunch trains:
- SPS => Trains of 6 bunches.
- LEP => Trains of 8-16 bunches.
- The beam-beam effects are usually divided into 2 groups:
- Single-particle stability (leading to reduced beam lifetime and particle losses):
1) Phase space distortion => For instance with the SIMUL code (which is the 1st code we start with when studying beam-beam effects).
2) Detuning with amplitude => We can produce tune footprints bunch by bunch for any optics and collision scheme and filling scheme, using for instance the MADX code (MADX-Footprints).
3) Reduction of dynamic aperture => For instance with the SixTrack code
4) Emittance effects => Emittace variations (which heavily depend on the tunes) have been predicted by simulations but not yet proven. The available codes are BBSS, BEAMX, BEAMBEAM3D, COMBI.
- Collective effects (leading also to reduced beam lifetime and beam loss):
1) Dynamic beta effect => Hand calculations: In the case of LEP this was used to increase the luminosity, as there was a reduction of the betatron functions (focusing effect). For the LHC, the effect can be predicted but it increases (defocuding effect) the betatron functions by a very small amount only.
2) Coherent dipolar oscillations => Predictions can be made with many codes (BBSS, BEAMX, BB3D, COMBI and many other simplified models linear-rigid bunch) but only 1 benchmark is possible (measurement performed in RHIC with only 2 proton bunches colliding).
3) Orbit effects => We can predict bunch by bunch self orbit and tunes but we need single bunch diagnostics to benchmark. Variations up to 1.2 microm are predicted for the nominal LHC beam. The code available is TRAIN (self-consistent but rigid code, which is enough for the orbits and tunes).
4) Bunch coupling.
- We qualitatively understand and can predict several BB effects but difficult to relate to direct beam observations => The idea is to try and:
- Understand the physical picture of BB with many bunches,
- Compare/Benchmark with collider,
- Model and extrapolate for future scenarios.
=> Tools developed: COherent Multi-Bunch Interactions code COMBI.
- 3 approaches are usually used complementary:
- ALM = Analytical Linear Model,
- RBM = Rigid-Bunch model,
- MPM = Multi-Particle model
- ALM results:
- In the case of one bunch colliding HO with another one, 2 modes appear: sigma-mode in phase and Pi-mode out of phase => Model extended to N bunches and LR (Eigen frequencies and modes).
- The LR introduce sidebands (on the right as it is focusing in the present case) to the main HO coherent frequencies.
- RBM results => This enables a direct comparison with single-bunch measurements, neglecting only Landau damping. Results can be compared to ALM.
- MPM results => In addition Landau damping is included which can suppress some intermediate modes. It was observed that there is no Landau damping of the LR modes!
- Experimental verification at RHIC, proton-proton run 06: 2 Experiments (IPs 6 and 8), 111 bunches with 2 different families of bunches => 9 SUPERPACMAN bunches (1 HO collision) and 102 Nominal bunches (2 HO collisions)
- Reminder: In RHIC, over 9h of store, every 20 minutes a BTF is done (using the BBQ system, in fact the LHC BBQ was tested there, changing the phase).
- Using the code COMBI it was discovered that the measurements were done on a SUPERPACMAN and not on a nominal bunch as thought before. The fact that the measurements were done on a SUPERPACMAN was due to the fact that the latter are the ones with the highest intensity.
- Application to the LHC:
- Study Effect of phase advance symmetry,
- Study effect of filling schemes,
- Study effects of HO and LR interactions,
- Study the effects of beam-beam interactions on the beam parameters (transverse size).
- Bunch by Bunch diagnostics is ESSENTIAL to be able to benchmark simulation outputs vs direct observations.
- Outlook for near future:
- Implement Bunch by bunch footprints in OnlineModel of the LHC (easy...)
- Bunch by bunch self consistent orbits and tunes in the OnlineModel (i.e. TRAIN code) to help in machine corrections (respect to nominal or pacman bunches?)
- Beam-Beam spectral lines to compare with tune spectra
- Single bunch spectrum to compare with tune spectra
5) Beam-beam tune shifts measurements in the LHC (Emanuele Laface): pdf
- Emanuele presented 2 sets of measurements, 1 on HO and 1 on LR:
- HO tune shift measurements on May 2-3, 2010
- Conditions: 2 bunches per beam, 1 collision per IP, 1E11 protons per bunch, end of fill measurements.
- 2 frequency lines are observed with B1 in the vertical plane. Changing the collision pattern, 1 line is remaining at the same position while the other is moving: reducing the beam-beam effect makes it move towards the unperturbed line (in good quantitative agreement with theoretical predictions). In particular, without collision only 1 line is observed.
- LT measurements on September 10, 2010
- Conditions: 28 bunches per beam, in 4 trains of 150 ns, 1E11 protons per bunch at injection energy, 1 train of 4 bunches and 3 trains of 8 bunches, 4 to 20 collisions per bunch, parallel separation > 3 sigma (at nominal emittance), initial crossing angle of 170 microrad (~14 sigma).
- There seem to be clear evidence of LR effects, as the different bunches behaved differently. More quantitative analyses require a bunch-by-bunch instrumentation.
6) Actions to be taken for the next meeting
- Old actions.
7) Miscellaneous
- The next (9th) meeting will take place on 22/09/2010 => Agenda:
1) Review of the Beta-Beams activity (EW)
2) Beta-Beams production ring (EB)
3) Collective effects studies for beta-beams in the decay ring (CH)
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 20/09/2010.