Minutes of the ICE section
52nd meeting on Wednesday 04/04/2012 (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), Elias Metral (EM), Elena Wildner (EW), Frank Schmidt (FS), Giovanni Iadarola (GI), Giovanni Rumolo (GR), Jean-Luc Nougaret (JLN), Kevin Shing Bruce Li (KL), Nicolo Biancacci (NB), Nicolas Mounet (NM), Olav Ejner Berrig (OB), Serena Persichelli (SP), Tatiana Pieloni (TP), Werner Herr (WH), Xavier Buffat (XB).
Present/Excused: BS, CH, CZ, HD, EB, EM, EW, FS, GI, GR, JLN, KL, NB, NM, OB, SP, TP, WH, XB, Alexander Molodozhentsev (from KEK), Alexey Burov (from FNAL), Eirini Koukovini Platia, Hannes Bartosik, Raymond Wasef, Mauro Migliorati (from La Sapienza).
1) Newcomers / visitors
- None.
2) Comments on the minutes of the previous 51st meeting + Actions
- None.
3) General infos
- No particular comment from anyone.
- NEW => See Deadlines and important dates for ICE (which I just started and will try and maintain in the future!).
- SL meeting:
- Barbecue = Monday 18/06/2012 => Please reserve this time slot and inform me in case of problem.
- IPAC12: Should be approved by GL on May 7th => I would like to have all the papers for 02/05/12 and I will inform Oliver on the FR 04/05.
- News from LHC:
- Talk by NM at yesterday's LBOC on recent observations of coherent instabilities.
- Any news on SPS scrubbing run? => HannesB and GiovanniR:
- Effective beam time was not that good => Had to stay below 60 deg for the MKE (in the past years the limit was set at 70 deg when physics after or even 90 deg in dedicated MD).
- No scrubbing seen on the SS liner => To be followed up.
- Do we still have ecloud instabilities? At least it seems we could accelerate 4 batches, 25 ns, ~ nominal instabilities with relatively small transverse chromaticities (~ 0.1).
- Any news on PS MDs measuring the resonance driving terms?
- RaymondW => Trying first to reduce the coupling by skew quads but it seems there are some issues (cabling problems?).
- Associates and Fellows Committee: AFC-2012-1 => Deadline: Friday 27/04/2012 for the committee on Tuesday May 15th.
4) Effect of space charge on transverse coherent instabilities (Part 1/5): coasting beams (Alexey Burov): pdf
- Alexey started by discussing in some detail Mohl-Schonauer Equation (MSE), which was proposed in 1974 to describe the coasting beam oscillations in the presence of both impedances and space charge => There are 3 contributions: the lattice, the wake (or impedance) and the space charge, which is the only non-trivial term in the equation. Note that this equation is written in the frequency domain, which explains why we see the impedance term proportional to xbar (in the time domain it is a convolution with the wake field as it should be, answering a question from GR). GR had also another question concerning the MSE and the width of the slices to be used, due to the very small beta for instance in the case of the PSB. Alexey answered that the width should be much smaller than the ? of the mode under study and the width should not be smaller than the aperture.
- AB asked himself: Why is the nonlinear space charge force describe by a linear term? Does this mean that it is only valid for a KV distribution (with a constant density profile)? It is in fact valid for any distribution as the MSE describes the driven oscillations only, so it results from linearization of the original nonlinear space charge term over infinitesimally small coherent motion, and averaging over the betatron phases of the free incoherent oscillations. Reminder: the single-particle motion consists of 2 parts: (1) free oscillation (typically with beam size amplitude, or transverse action variable ~ rms emittance); (2) driven by the coherent offset oscillation (much smaller than the beam size).
- The space charge coefficient Qsc depends on the action variables and it assumed to be time independent which means that one considers a rigid-slice oscillation only (the core of any beam slice moves as a whole, so there is no inner motion in it) => This should be a good approximation when the space charge is strong enough, i.e. |Qsc| >> the incoherent lattice tune spread (note that it should also not be too big...). Otherwise, the beam shape oscillates as well, and the MSE is not necessarily valid. If the above condition is not satisfied then the equation can be correct or not but we don't know a priori. Alexey separated the particles in 2 groups: core (responsible for the slice as it is a rigid oscillation) and tails (responsible for Landau damping).
- EM showed some of his past results with Francesco Ruggiero in 2004 where Landau anti-damping could also be observed in the case of a negative octupolar detuning (see for instance http://emetral.web.cern.ch/emetral/CommentByKingNgFromBNL_RLC_19-09-06.pdf). Several studies have then been performed to study this effect in detail from both theory and simulations.
- Alexey and Valeri Lebedev solved the MSE analytically in 2008. Landau damping is proportional to the phase space density of the resonance particle. Thus, with strong space charge, it is determined by the distribution tails. Instead of using stability diagrams, Alexey and Valeri represented their results by a line (with stability below and instability above) both for the case of a chromatic spread and and an octupolar spread => Could be interesting to compare these results with the stability diagrams obtained previously.
- Alexey then mentioned that these coasting-beam results can also be applied to bunched beams in the following 2 cases:
- For short-wavelength and fast oscillations (much shorter than the bunch length, much faster than the synchrotron frequency) – the so-called microwave instability;
- For multi-bunched beam, at zero chromaticity and long wavelength (much longer than the bunch spacing).
- Finally, Alexey applied these results to the Antiproton instabilities at the Fermilab Recycler Ring:
- Typical instability snapshot shown.
- Instability always at the tail (both H and V, and nothing on the longitudinal profile), frequency at ~ 70 MHz => This is the end of the damper bandwidth and therefore the lowest frequency which cannot be damped by the damper.
- Threshold prediction always within ~ 30% for the several cases studied.
5) Actions to be taken for the next meeting
- Old actions.
6) Miscellaneous
- Deadlines and important dates for ICE.
- The next (53rd) meeting will take place on 11/04/2012 => Agenda:
1) SPS UA9 impedance studies (HD) .
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 08/04/2012.