----------------------------------------------------------- Minutes of the ABP-RLC team meeting of 10.06.2005 present: EB, UD, AG, WH, EM, FR, FZ web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ (1) Minutes of last meeting, pending actions, announcements ----------------------------------------------------------- => ACTION => compare the size of the longitudinal geometric wake with RW longitudinal wake from A. Koschik (FR) STATUS: PENDING, FR will talk to A. Koschik => ACTION => attempt to derive a general nonlinear theory including the inductive bypass (EM) STATUS: PENDING, PhD student being looked for. => ACTION (EM+FZ): compare and understand the different predictions of the Burov-Lebedev theory calculated by FZ and EM. STATUS: A. Koschik has not yet done any comparison with this notebook. FZ found that the generalized Piwinski formula agrees with EM's result in the limit of small transverse emittance. FR has discussed collaboration items with Ingo Hofmann. Ingo plans to organize a mini-workshop on 'trapping and beam loss' due to space charge and also due to electron cloud in November. FR mentioned that there are other workshops such as Coulomb'05 in Senigallia. Ingo will likely attend HHH-APD LHC-LUMI-05 workshop in Arcidosso. Independently of the discussions with Ingo, a bilateral CERN-GSI collaboration agreement is being finalized, which in addition to joint magnet development includes beam dynamics activities. (2) Impedance estimates for TCDS and TCDQ absorbers (AG) -------------------------------------------------------- The TCDS and TCDQ protect septum and quadrupoles at the extraction point. Each consists of two sections of about 3 m length, separated by a tapered region. The total number of tapers is 4 for both TCDS and TCDQ. Graphite blocks are placed between the stored beam and the extracted beam. The side of the graphite facing the stored beam is copper coated. In the TCDS case, a second graphite block is located at the other side of the extracted beam. AG computed the impedance for the un-tapered devices and for two different types of taper. For the TCDQ, taper type 1 has a tapering angle of 5 degree and the graphite block is closer to the beam; taper 2 is shorter, has a tapering angle of 15 degree, and the wall is further away from the beam. Numerical calculation by GdfidL gave a Z/n of 203 microOhm without taper, 146 microOhm for taper 1, and 90 microOhm for taper 2. The total impedance is higher by a factor of 4 as there are 4 tapers. It was decided to recommend the taper 2 with the lowest impedance. For the TCDS, also two options, taper 1 and 2, were considered, but somewhat different from those for TCDQ, and both of the same length. In this case, the Z/n per transition is 82 microOhm without taper, 60 microOhm for taper 1 and 35 microOhm for taper 2. It was decided to accept either of the two taper options, since the impedance is small compared with that of TCDS. Trapped modes were not found to be an issue, as had been feared. Some trapped modes were identified, but these were unrelated to the splitting of the beam pipe into two halves. One additional concern are the slots between individual segments, forming these devices. There are about 8-10 slots, of 1 mm length, which interrupt the image current flow on the side of the graphite (or other material) blocks. => ACTION: EM and/or AG will analytically/numerically estimate the slot impedance (AG has already performed a numerical estimate: ~30 microOhm per slot, these results will be discussed in a meeting with the equipment groups next monday morning) (3) Tune shift with generalized Piwinski formula (FZ) ------------------------------------------------------ FZ showed the tune shift computed from the generalized Piwinski formula for the benchmarking example with Elias. The tune shift varies substantially with emittance. In the limit of low emittance the result is the same as Elias'. For large emittances it can be almost a factor of 2 difference, explaining the collimator measurement in the SPS. So, the two curves included in Helmut's PAC paper were most likely correct. (4) Reports from meetings and ongoing activities ----------------------------------------------- FZ presented an update of electron-cloud simulations at the MAC. The predicted heat loads are more optimistic than earlier, but the uncertainty is at least a factor of two. Observations with COLDEX, at RHIC and in DAFNE are not well understood, and point to a lack of understanding of the scrubbing. The incoherent emittance growth due to electron cloud could be responsible for the poor beam lifetime seen in the SPS with LHC beam. If e-cloud induced tune shifts should be less than e.g. 1e-4, the acceptable e- densities are of order 1e8 m^-3, hence 30000 times lower than the fast-instability threshold or the heat load limit. M. Harrison asked for beam lifetime predictions in the LHC (or SPS) based on these simulations. => ACTION: EB will perform such simulations for a dipole field. FZ contacted Kay Wittenburg for the impedance of the LCH wire scanners (vessel copied from DESY). Kay has not yet responded, but Ferdi Willeke remarked that the impedance had not been calculated and that DESY never encountered any problems. The same wire scanners are also used for the electron beams with much shorter bunch length. In contact with J. Koopman and AG, EM is setting up HFSS calculations of the wire scanner. Last minute: after the RLC meeting Lyn Evans has (urgently) requested a plot of the head-tail growth rate as a function of chromaticity at injection energy for nominal current. EM and FZ will use an updated LHC impedance model to produce this plot, if possible by next monday. FR has already provided some qualitative information based on an impedance model not including the collimators. => ACTION: produce a plot of HT growth rates as a function of chromaticity at injection energy for nominal current (EM and FZ). Posted on the web: Slide by FZ Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/