From: Francesco Ruggiero Sent: Thursday, March 25, 2004 8:43 PM To: Bruno Zotter; Daniel Schulte; Elias Metral; Frank Schmidt; Frank Zimmermann; Gianluigi Arduini; Helmut Burkhardt; Hiroshi Tsutsui; John Jowett; rlg@physics.umd.edu; Stephane Fartoukh; Werner Herr; Luc Vos; Maxim Korostelev; Alexander Koschik; Eric D'Amico; Walter Wittmer; Gregory Penn; lronen@techunix.technion.ac.il; Elena Benedetto; Elmar Vogel; Javier Resta Lopez; Peder Eliasson Cc: Jean-Pierre Riunaud; Karlheinz Schindl; Louis Rinolfi; Michel Martini; Oliver Bruning; Charles Hill Subject: Minutes of LHC Collective Effects team meeting 19/03/2004 Minutes of the ABP-LCE team meeting on 19.03.04 ------------------------------------------------------------------------- present: EB, WH, AK, EM, FR, DS, EV excused: FZ ------------------------------------------------------------------------- (1) Follow up on action items ----------------------------- OLD ACTIONS: concerning LHC collimator impedance => clarify geometry of trapped modes (DS) -> ongoing Also trapped modes with two nearby collimators are being studied. => practical advice for collimator design (DS) -> ongoing DS attended LHC collimation meeting on 18 March and suggested to add sliding contacts to the outside tank and among collimator jaws. SS to C connections are confirmed. Spring contacts are failure prone and, following a discussion with Lars Thorndahl, DS recommended flexible connections that however require more space and may turn out to be incompatible with collimator tilt. => perform full simulation for the SPS (AK) -> pending ACTION => compare results and mathematica notebooks (FZ, EM, AK) -> partly done? => check epsilon and mu of collimator material for impedance calculation (mu of the graphite was checked by EM and it is close to 1) -> pending ACTION for epsilon (EM) Meanwhile Fritz Caspers suggests to use graphite monocrystals, having ~30 times more conductivity than normal graphite in one plane. => what can be measured in the SPS? -> pending ACTION (AK,EM,FR,DS,FZ) => WH, DS, and FZ will assess specific CPU needs/plans for LHC beam-beam tracking and e-cloud simulation campaigns -> partly done? => FR will talk to J.-P. Delahaye to see if the CLIC parallel cluster can be used (with low priority) also for GdfidL simulations for the LHC, possibly by contributing 1-2 dual processors paid by ABP. -> done. ABP will contribute 3 more nodes (11 kCHF). In addition he will propose to provide DS with a fast desktop machine for e-cloud and possibly HFSS calculations. -> pending ACTION => WH will organize a meeting to discuss beam-beam tracking studies -> done => WH will discuss MAD name extensions with TR -> done => EM attended a meeting with B. Goddard nd F. Caspers on TCDS+TCDQ coatings. Old recommendation was 20-30 micron of Cu coating. New recommendation is 1 micron coating, i.e. same as collimators. (2) Proposed measurements during the SPS scrubbing run (DS) ----------------------------------------------------------- DS presented slides (enclosed) for the APC meeting in the afternoon discussing ecloud measurements required to benchmark simulation results. He recommends parameter scans rather than point measurements, in particular ecloud build-up versus B-field, bunch trains spacing, vacuum chamber height, bunch spacing, bunch intensity, and residual vacuum pressure. He is ready to launch a new simulation campaign to model detectors, surface effects, and required degree of bunch equalization. (3) e-cloud: Effect of Q' on the emittance growth (EB) ------------------------------------------------------ EB presented HEADTAIL simulations results (enclosed) for LHC at injection to investigate the effect of chromaticity on the emittance growth caused by the ecloud. For Q'=2, she scanned the ecloud density in the range 3x10^11-3^10^12 and even for the lowest density she found an emittance growth of 20% after only ~60 msec. By extrapolation, the maximum density compatible with a reasonably small emittance growth of 2.3% after 30 min of machine operation is 3x10^10 m^-3, i.e. about 20 times smaller than the value expected for nominal LHC beam conditions. For the expected ecloud density of ~6x10^11 m^-3, she performed a Q' scan and found a drastic reduction of emittance growth when Q'>15. A chromaticity Q'=20 would be compatible with a small emittance growth of 2.3% after 30 min. These results are in qualitative agreement with those of a broad-band impedance resonator model. ACTION => EB will repeat LHC simulations for different bunch intensities and different number of kicks per turn. She will then extend the simulations to SPS conditions. (4) Stabilization of longitudinal modes by RF AM in the LHC (EV) ---------------------------------------------------------------- EM considered longitudinal mode stabilization by Landau damping associated with RF AM for the LHC (see enclosed slides). Assuming a modulation strength of 16% and an RF voltage of 14 MV, to leave some margin for beam loading, the bunch-by-bunch synchrotron frequency spread is of 8%, or ~0.5 Hz, and the corresponding stability limit can be increased by a factor two. This technique requires an additional RF generator and measurements are needed to check the technical feasibility with existing LHC cavities. The upper peak of the RF amplitude modulation should be located at the abort gap to avoid unstable bunches. There is very little impact on the bunch length. Attachments: DS's slides (presented at last APC meeting on SPS scrubbing run), EB's slides, EV's slides -- E-mail: Francesco.Ruggiero@cern.ch, Location: Building 9/1-008 Address: CERN, A&B Department, CH-1211 Geneva 23, Switzerland Telephone: +41 (22) 767 3726 or 767 5272, TeleFax: +41 (22) 783 0552 WWW: http://wwwslap.cern.ch/~rgo/