Minutes of the ICE section
27th meeting on Wednesday 11/05/2011 (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), Xavier Buffat (XB), Werner Herr (WH).
Present/Excused: BS, CH, CZ, HD, EB, EHM, EM, EW, FS, GR, JLN, KL, MCAP, NB, NM, OB, TP, XB, WH, Alessandro Vivoli, Eirini Koukovini Platia, Giulia Papotti, Javier Cardona, John Jowett, Rama Calaga, Raymond Wasef, Roderik Bruce, Tom Mertens, Vittorio Vaccaro.
1) Newcomers / visitors
- None.
2) Comments on the minutes of the previous 26th meeting + Actions
- The minutes were not available yet.
3) General infos
- LHC technical stop this week + Injectors MDs.
- LHC MDs during the last week-end:
- SB tune shift at 3.5 TeV vs. collimators' position.
- Increasing the beam-beam HO tune shift at 450 GeV.
- TCBI at 450 GeV and 3.5 TeV.
- Discussion with beam-beam team on the "beam-beam limits":
- It seems that during last week's MD, with collision at 450 GeV, a (HO only) beam-beam tune shift of ~ 0.017 was reached with 1 IP (which is ~ 5 times the value assumed for the nominal beam, which was ~ 0.0033 based on some considerations explained by WH for instance at the last Evian workshop in December 2010: http://indico.cern.ch/getFile.py/access?contribId=20&sessionId=5&resId=0&materialId=slides&confId=107310) and ~ 0.034 with 2 IPs. These tune shifts were obtained with a working point on the resonance (0.31, 0.31). At (0.31, 0.32), the 10th order resonance might have been seen (being studied).
=> Questions: What are beam-beam limits (considering HO and/or LR, emittance growth & losses permitted, assuming machine nonlinearities or not, on which time scale, etc.)? => Can we define them (as it seems very difficult to define for a hadron machines)? Difference between leptons (where we have equilibrium beam sizes, between cooling and heating processes) and hadrons machines? What are the mechanisms that can influence them? Is it really impossible to predict them (to which level of precision)? What should we do to try and predict them (deciding on the level of precision we want)? etc.
- The 2 main effects seem to be the machine nonlinearities (resonances) and the fact that beam-beam is a localized interaction (contrary for instance to space charge):
- Are there some studies done assuming no machine nonlinearities (i.e. only 2 bunches colliding at one point)? If yes, what is the result? I would imagine that, with only a HO interaction and no machine nonlinearities there is no limit (as it should also be the case for space charge). Is it really the case? The situation is certainly different for the case of LR interactions (leading to the excitation of resonances) but there was none during this MD.
- The strategy for the LHC was to place the tunes between the 3rd order resonance (1/3 = 0.33) and the 10th order (3/10 = 0.30), leading to a maximum total tune shift of 0.03 (in fact less, as the tunes are separated by 0.01) => Total space available in tune diagram ~ 0.015 (if coupling is small).
- May be more discussions (if needed) could take place to try and define better the terms beam-beam limit(s) and space charge limit(s) => See also EM's talk at the first space charge meeting.
- Proposition from OB to buy some vacuum chambers etc. for our impedance measurements:
- Thanks to Jan Hansen we now have a standard PS type vacuum chamber ( 325 mm long, for SS72 and SS74, 73.5 mm) and also a PS type clamp. This is a spare vacuum chamber for the PS, and we need to give it back to Jan if there are problems in the PS. For future measurements, maybe we should have our own vacuum chamber. We will always need one to damp the evanescent waves. It would cost ~ 2000 CHF to manufacture.
- SL meeting:
- Preference for the group meeting: Tuesday, June 28th at 11:00 + barbecue afterwards.
- Info from BEMB: Clementine Lazignac will stepping down from her role as HRA (Human Resources Adviser) for the BE department (on 01/05/11), and will be replaced by M.L. Falipou (in parallel to her role as HRA in another department).
- IPAC2011 => I sent you already the info that Rogelio Tomas will have a prize (congratulations again Rogelio!).
- EM gave a talk at the last LBOC on the LHC TCBI, to review our past and present predictions, discussing all the mechanisms involved and making some recommendations for the future intensity increases in the LHC.
4) Comparison between simulations and measurements in the LHC with heavy ions (Tom Mertens): pptx
- Goal to simulate Ion runs in 2010 during physics.
- Different IBS models available:
- Piwinski Smoot,
- Piwinski Lattic,
- Piwinski Modified Lattice,
- Interpolation,
- Bane,
- Nagaitsev.
- Tom compared the results of the different models and decided to use Nagaitsev (based on Carlsons Elliptic Integral), as it is fast. Note that it does not include vertical dispersion. John Jowett mentioned that including dispersion can change the growth rate from negative to positive, but it should be still a small effect.
- Tom wrote a Mathematica package to access the logging database which allows him to rapidly see if a fill is useful or not.
- Tome made a correction to the transverse beam sizes (from the BSRT) such that the luminosity from ATLAS and simulated luminosity match
- By the way, RECENT NEWS (reminder): recently ATLAS agreed that they overestimated the luminosity in the past (until few weeks ago) by ~ 7-8% => To be checked and may be re-analyze the fills...
- Furthermore, during the meeting, Giulia Papotti mentioned that the BQM may overestimate (by ~ 10%) the bunch length, compare to what is measured in the luminous region.
- Conclusions and outlook:
- Transverse growth always underestimated in the simulations, while the bunch length growth seems to be overestimated (see comment above that the BQM data give ~ 10% higher values => To be re-checked).
=> Particle Tracking Simulation seems to be missing some effect(s) that makes transverse emittances grow faster than predicted by our IBS models: hump?, particularly in vertical plane...
- Next step: Add the hump in the simulations.
5) Review of Hamiltonian treatment of synchro-betatron motion: Part II/III (KL): pdf
- Reminder: In the part I/III (see ICE meeting held on 30/03/2011), Kevin derived the general electro-magnetic Hamiltonian after having described our universe (particle accelerators) and used in particular the least action principle. Today we discuss the part II/III where the idea it to derive the (practical) transverse Hamiltonian (up to the 3rd order) after a succession of canonical transformations. The synchro-betatron Hamiltonian will be derived in the last part III/III next week.
- We need to use canonical transformations, as an arbitrary transformation would not preserve the symplectic structure, would violate the principle of least action, and would lead to a description of dynamics which is non-physical => We require a simultaneous transformation of positions and momenta, i.e. a transformation in phase space.
- Discussions about generating functions. Comment from John Jowett: The number of mixed-variable generating functions types for a Hamiltonian system of n degrees of freedom is 2^n. (not 4 as the Goldstein book says).
- Spatial translation and Frenet-Serret coordinates to use our usual variables and system of coordinates.
- Transverse expansion => We develop the square root for small arguments, up to the 3rd order.
- KL reminded us that the word symplecticity comes from the greek word entangled. EM found also in http://www.slac.stanford.edu/cgi-wrap/getdoc/slac-pub-9939.pdf : The term, symplectic, comes from the Greek συμπλεκτικos which, according to the Oxford English Dictionary, means twining, plaited together or copulative.
- Comment from EM: Few years ago, Wolfgang Schnell proposed a simple model of the universe, representing the universe by a cavity and he found the masses of several (many?) particles which were related to the eigen-frequencies of this cavity... (if I remember correctly, to be checked...).
6) Actions to be taken for the next meeting
- Old actions.
7) Miscellaneous
- The next (28th) meeting will take place on 18/05/2011 => Agenda:
1) Review of Hamiltonian treatment of synchro-betatron motion: Part III/III (KL),
2) Review of IBS: analytic and simulation studies (Alessandro Vivoli).
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 15/05/2011.