From: "Francesco Ruggiero" To: "Daniel Schulte" ; "Elias Metral" ; "Frank Schmidt" ; "Frank Zimmermann" ; "Gianluigi Arduini" ; "Gilbert Guignard" ; "Helmut Burkhardt" ; "Jacques Gareyte" ; "John Jowett" ; "Werner Herr" ; "Luc Vos" ; "Maxim Korostelev" ; "Alex Koschik" ; "Bruno Muratori" ; "Eric D'Amico" ; "Walter Wittmer" ; "Gregory Penn" ; "Lifshitz Ronen" ; "Elena Benedetto" Cc: "Hans Braun" ; "Jean-Pierre Riunaud" ; "Karlheinz Schindl" ; "Louis Rinolfi" ; "Michel Martini" ; "Oliver Bruning" ; "Roberto Cappi" ; "Charles Hill" Subject: Minutes of LHC Collective Effects team meeting 28/03/2003 Date: Friday, March 28, 2003 5:08 PM ---------------------------------------------------------------- Minutes of the ABP-LCE team meeting of 28.03.03 present: TdA, WH, JJ, EM, BM, FR (secretary), LR, FS, DS, LV excused: FZ ---------------------------------------------------------------- The joint ABP Forum/CLIC meeting was too long and several people left the meeting room (or did not show-up). ACTION FR -> discuss with Hans Braun to optimize the schedule/length of future joint ABP Forum/CLIC meetings ACTION LCE team (especially students) -> attend LCE team meetings or propose alternative dates The LCE team meeting was largely devoted to discuss LHC collimator impedance and beam stability. 1) Report from last LHC Collimation meeting ------------------------------------------- LV reported about the LHC Collimation meeting of last Friday 21 March, that he attended with EM. Berillium seems to be ruled out for mechanical stress reasons, at least with the current assumptions on the dump kicker failure scenarios. The idea of leaving the collimators open, i.e. with injection settings, during the whole ramp was questioned. If they are closed at some intermediate energy the implications for impedance and beam stability may be serious. Bernard Jeanneret thinks that this will not be necessary. It was agreed that collimator impedance studies should be somewhat decoupled from LHC Collimation meetings: LCE team representatives will report relevant new results on impedance and beam stability when they become available. On the other hand, collimator impedance has top priority for the LCE team. JJ recalled that the LHC Machine Advisory Committee had suggested to consider graphite with some carbon fibers for the collimators. Fritz Caspers has recently proposed a new graphite foam. LV remarked that in his beam deflection measurements Mark Seidel had used a rather high value of the resistivity for graphite. FR speculated that this may be due to baking at 400 C. 2) Follow-up of last LTC meeting -------------------------------- FR summarized the actions and future studies on collimator impedance following the last LTC meeting (see enclosed list). LV remarked that the frequency dependence of his transverse impedance model Z(omega) depends on the "inductive bypass" effect (compatible but not tested by Seidel's measurements), which in turns depends on collimator material and aperture. After some discussion it was agreed to use the baseline collimator apertures and lengths provided by Ralph Assmann. ACTION LV -> ask the LHC Collimation team to give an official value for the resistivity of graphite collimators. Use baseline graphite collimator design for injection and squeezed optics and transmit information on collimator transverse impedance Z(omega) (plots/tables and Mathcad file) to the RF group and LCE team. ACTION FR -> compute coherent tune shifts of higher order head-tail modes and compare to stability curve with LHC octupoles. 3) Detuning due to Long Range Beam-Beam Collisions -------------------------------------------------- WH has computed particle distributions in tune diagram Qx,Qy induced by long range beam-beam collisions, assuming pseudo-parabolic beam distribution in x-y betatron amplitudes (see enclosed ps file). ACTION LV -> use WH's results to estimate stability limits (possibly with some help by FR). 4) MAFIA and/or GDFIDL simulations ---------------------------------- It was recognised that 1-2 young members of the LCE team should start learning MAFIA asap. On a shorter time scale, external experts will be asked to perform MAFIA simulations in time domain to confirm analytic estimates of resistive and geometric collimator impedance. FR said it would be already interesting to check resistive wall impedance for a short high-resistivity section (say 1 m long) in a smooth beam pipe. This would decouple resistive and geometric wakes, and confirm that resistive low-frequency wakes do not travel with the beam. This has significant implications for the collimator impedance, since a multi-bunch instability requires at least coupling from one bunch to the next: if next bunch interacts with the wake of previous bunch a few metres away from collimator, the radius changes by a factor 10 and the deflecting kick is 3 orders of magnitude smaller! ACTION DS -> ask Igor Syratchev to launch some MAFIA runs, if compatible with his CLIC duties. 5) References on Localized Resistive Wall Impedance --------------------------------------------------- FR listed some relevant references on "localized resistive wall impedance" (other references may be found by a simple Google web search) and suggested that they be distributed among LCE team members, who will report on them and follow-up their implications for the LHC. ACTION AK -> Read paper by G. Duncan on "Localized multi-bunch modes in accelerators" (PRST-AB 2, 094401 (1999). Report at the next LCE team meeting. Use simulation code to confirm localized multi-bunch modes predicted by G. Duncan with localized resistive wall impedance and sufficient octupole detuning. - EPAC02 paper by V. Danilov et al on "Closed orbit instability" was already discussed by FZ. Conclusions for LHC, especially with large collimator impedance, are still missing. - EPAC00 paper by M.M. Karliner et al on "Calculation of transverse resistive impedance for vacuum chambers with arbitrary cross sections" highlights that unstable slow wave has a phase velocity 100 times slower than the speed of light and suggests how the impedance should be estimated. No changes for thick wall, but significant differences for thin resistive wall (see BINP codes under Collective web page). - DESY report 94-068 by A. Piwinski on "Impedance in Lossy Elliptical Vacuum Chambers" is available on the web. It is quoted and used by M. Seidel to estimate resistive impedance for the collimator body. 6) AOB ------ - Electron Cloud simulations are in standby. EB has some preliminary results on emittance growth. ACTION EB -> report at the next LCE team meeting. - FS asked for a volunteer to follow-up the MAD-X Plotting Module. - Next week EM will attend a workshop on Space Charge simulations. He will report in a forthcoming Forum. -- E-mail: Francesco.Ruggiero@cern.ch, Location: Building 9/1-008 Address: CERN, A&B Division, CH-1211 Geneva 23, Switzerland Telephone: +41 (22) 767 3726 or 767 5272, TeleFax: +41 (22) 783 0552 WWW: http://wwwslap.cern.ch/~rgo/