Minutes of the ICE section
51st meeting on Wednesday 28/03/2012 (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, Mauro Migliorati.
1) Newcomers / visitors
- None.
2) Comments on the minutes of the previous 50th meeting + Actions
- No comments.
3) General infos
- No particular comment from anyone.
- Ski outing last Wednesday => Nice pictures at the bottom of http://emetral.web.cern.ch/emetral/ICEsection/ICE.htm.
- SL meeting:
- Forthcoming TSC/Deadline 16 April 2012 => Form to be sent to DelphineR by WE April 4th.
- News from LHC:
- Instability observed last Saturday 24/03/12 and beam dumped at 23:07 (too high losses at IP6 and instability seen on B2H). Nothing seen with tight collimator settings at the end of the ramp => the problem occurred during the alignment, with several secondaries below 5 nominal sigmas => To be followed-up (NM).
- Discussion with Mario Deile and Federico Ravotti to discuss RF heating in the TOTEM Roman Pots => They are finishing a note.
- WernerH and beam-beam team proposes to have a beam-beam workshop at CERN next year during the 2nd or 3rd week of April => Could be under ICFA => I will ask.
- We should also start to organise the impedance workshop next year => Could be also under ICFA => I will ask.
- TCTP impedance => Email from Alessandro Bertarelli:
- Already received a first batch of TT2-111R ferrites by Skyworks, enough to assemble one full TCTP collimator => Strongly recommend to rapidly carry out all the tests which are needed to qualify Skyworks ferrite, including UHV compliance, RF/magnetic properties, heat treatment compliance etc. => Followed up by BS and CZ.
- In parallel, they would like to finalize their simulations to assess ferrite temperature inside the tank and hence know how far we are from critical temperatures. To do so, they need an estimation of the thermal load => An upper limit would suffice. I said 10 W => To be confirmed by impedance team (BS and HD).
- FOM (BS):
- PSB instability perturbing the injection in the PS => Transverse instability in the PSB confirmed by GR being followed up.
- SPS scrubbing => Seems late compared to schedule due to some problems and seems that there is electron cloud but no scrubbing effect. MKE situation better with the 2 serigraphed ones this year (revealing no heating) but one remaining without serigraphy which limits the scrubbing run.
- LSWG (NM) => 3 MDs might be interested for us:
- Tight collimators' MD.
- Longitudinal resistive impedance measurements => Depending on frequency it seems there is a discrepancy and not in the same direction.
- WolfgangH on the ADT => Switch it OFF on 1 bunch to measure the tune.
4) Past collective effects studies for DAFNE (Mauro Migliorati from Roma, La Sapienza): pdf
- DAFNE layout => 3 accelerators: linac, accumulator and DAFNE.
- Linac: e- up to 800 MeV and e+ (produced from e- hitting a target) up to 550 MeV.
- Accumulator ring: ~ 1 / 3 of DAFNE circumference, 510 MeV constant energy, used to stabilize the beam from the linac. The 2nd reason is to obtain the single-bunch current which requires many shots to have 1 (big) bunch.
- DAFNE ring where the beam is injected in the 2 rings => Circumference is ~ 100 m (and it works well above transition), up 120 bunches, same energy for e+ and e- which is 510 MeV and sigmab ~ 2-3 cm (after the instability etc. see after) and it is always a Gaussian bunch.
- They worked mainly on the longitudinal beam dynamics: SB (single-bunch) and MB (multi-bunch):
- SB:
- KSB criterion required a Zl/n of ~ 0.01 Ohm => Had to work in this instability regime => They followed 2 ways:
1) Design the impedance as low as possible:
- resistive-wall with form factor for ellipticity.
- Transverse feedback kicker (2 striplines to damp vertical RW instability).
- Vacuum port screen.
- Antechamber slots for the synchrotron radiation where we put the pumps to absorb the gas etc. => in the bending magnets.
- BPM (button type).
- Tapers for all the components of the machine. Comment form OB: vacuum don't like tapers as it increases the pressure.
- scrapers.
- bellows.
- other inductive elements.
=> To summarize: Im[Z/n] = 0.53 Ohm => Was evaluated some years before the start-up (in 1995). DAFNE commissioning started in 1997.
2) Develop a simulation code to study in detail the beam dynamics in this regime to be sure that they are able to control the beam:
- The simulation code takes into account: RF + wake-field + radiation damping + quantum excitation.
- Comparison with theory: potential well distortion (below microwave instability threshold) => The longitudinal distribution is described by the Haissinksi (integral) equation. Once we have the wake-field we can deduce the distribution by solving this equation.
- Particular solutions of Hassinski equation:
- Pure resistive impedance => We can have a direct equation.
- Pure inductive impedance => There is an implicit equation.
- Broad-band resonator => Has to be solved numerically.
- MauroM re-launched his code and re-checked that it worked very well in the case of a broad-band impedance.
- Results:
- Very good agreement between simulations and measurements!
- Page 23 => bunch length is stable due to SR damping even if very low threshold of microwave instability (~ 6 mA).
- In the accumulator there is a small radius which leads to a large SR and therefore damps everything.
- DAFNE accumulator ring impedance model => Fitted with an inductance and a resistance to reproduce the measurements.
- Simulation code has been used also to study the option of a 3rd harmonic cavity to lengthen the bunch for DAFNE => Simulation made, cavity prototype made and measured.
- Simulation code was used also for 2 other rings: SUPER ACO (in Orsay, France) and ELETTRA (synchrotron light source, Trieste, Italy).
- MB:
- HOM evaluation:
- RF cavity:
- HOM waveguides to damp the HOMs.
- Longitudinal feedback system.
=> Decided to develop a longitudinal multi-bunch code including the feedback with finite bandwidth, noise etc. It is worth mentioning that MauroM developed these 2 simulation codes, SB and MB (considering the bunch as a macro-particle) => No names given to these codes => MauroM's LSB and LMB codes.
- CZ made a comment on the analytical formula for the stripline kicker. CZ benchmarked it last year with CST simulations and reminded us that Zs is the characteristics impedance of the common mode (as not usually well explained). Furthermore it seems to work quite well in the low frequency regime, i.e. until a certain frequency.
- BS => In LHC we have also stripline kickers.
5) Beam impedance measurements => Can we make a wire-measurement, without using matching boxes (OB): pptx
- Started from Vaccaros' paper "Coupling impedance measurements: an improved wire measurement" (INFN/TC-94/023, 1994), where he explained how to measure the impedance of a DUT (Device Under Test), measuring it and comparing this measurement with the DUT replaced by a reference pipe.
- However this is not the way we measure today => we use matching boxes (succo boxes, to match the 50 Ω of the Network Analyzer to the characteristic impedance of the vacuum chamber) => question: Can we get rid of them?
- The matching boxes are difficult to calculate and difficult to make => Idea: use the Network Analyzer to calibrate away the mismatch (the calibration is a special 2-port calibration, idea of F.Caspers, where each of the wires is compensated).
- Conclusion and next step: The method seems to work but the 2-port matching should be repeated and the method should be tried on a real vacuum chamber.
=> So we would have 3 methods: (1) Vacarro (with formulae compensating the mismatch, measuring 2 times), (2) with matching boxes, and (3) with a VNA calibration.
6) Actions to be taken for the next meeting
- Old actions.
7) Miscellaneous
- The next (52th) meeting will take place on 04/04/2012 => Agenda:
1) Effect of space charge on transverse coherent instabilities (Part 1/4): coasting beams (Alexey Burov).
- See preliminary agendas for the next meetings.
Minutes by E. Metral, 28/03/2012.