----------------------------------------------------------------------- Minutes of the ABP-RLC section meeting of 11.03.05 present: EB, WH, EM, FR, DS, FZ web site: http://ab-abp-rlc.web.cern.ch/ab%2Dabp%2Drlc/ ----------------------------------------------------------------------- (1) Minutes of last meeting and pending actions ----------------------------------------------- DS had sent a comment on the minutes, concerning ECLOUD benchmarking, which was included in last week's meeting. => ACTION => compare the size of the longitudinal geometric wake with RW longitudinal wake from A. Koschik (FR) STATUS: PENDING. => ACTION => confirm bunch length, intensity, and collimator gaps during tune-shift measurement (EM, FZ). STATUS: ONGOING (2) Reports from other meetings and CARE-HHH workshops (FR) ----------------------------------------------------------- A CARE-HHH-APD workshop on crystal collimation was held on March 7-8. The outcome of the workshop is a moderate activity, with machine experiments using new crystals planned in the near future at the Tevatron, and the option to install some crystals in the LHC primary-collimator tanks. The crystal collimation at RHIC gave only negative results. A CARE-HHH-AMT workshop on beam-induced heat generation and quench limits for the LHC s.c. magnets was held on March 3-4. The result of the workshop is that not much is known, and quench limits vary from magnet to magnet. The magnet group will prepare a table of quench limits for all magnets, based on existing thermal models. The corresponding thresholds for the BLMs are probably difficult to obtain. An LTC meeting was held this week. M. Benedikt discussed a way to achieve the ultimate intensity by bunch compression in the PS. This beam could be of interest as a scrubbing tool. C. Fischer reviewed scraping studies in the SPS. An interesting conclusion is that the beam tails do no re-populate within 1 s, leaving sufficient time for extraction. The SPS instruments will need to be reshuffled draining BDI resources. M. Lamont presented the progress of the LHC beam commissioning software. Information can found at the web site http://proj-lhc-software-analysis.web.cern.ch/proj-lhc-software-analysis/ Part of the new LHGC software will be tested at LEAR. The software is written in object-oriented language by professional programmers. At the LTC, it was remarked that a post-mortem analysis is missing. (3) Progress Report on HEADTAIL Simulations (EB) ------------------------------------------------- EB compared frozen and dynamic simulations below the threshold of the fast instability and found that the slow emittance growth is nearly identical in the two cases, demonstrating that this emittance growth is an incoherent effect. She repeated the frozen-potential simulations with several different electron cloud densities and various numbers of kicks. The result strongly depends on the number of kicks, if this number is small (change in the 'effective tune'). In general the growth is larger for larger electron densities. As a crosscheck EB verified that the growth is zero for zero electron density. A further important observation is that without synchrotron motion and for a finite electron density, the growth stops after a small number of turns. Therefore, the continuous slow growth only occurs in the presence of synchrotron motion. All these observations are consistent with the adaptation of G. Franchetti's model of space-charge induced emittance growth to the electron cloud. A first attempt to correlate transverse and longitudinal actions gave inconclusive results. It was suggested to verify the definition of the longitudinal action (and that it is constant in time) and to plot the change in transverse action rather than the total value. FR recommended to also change the bunch length and compare simulations with different values of the synchrotron tune. (4) Continued e-cloud benchmarking (drifts and quadrupoles) (FZ) ------------------------------------------------------------------ FZ performed ECLOUD simulations for field-free regions and quadrupoles in the LHC arcs, comparing three different versions of the code (as the in the last meeting): new version of ECLOUD, the new version with an intentionally introduced error in the theta-dependence of epsilon_max, a version from May 2004. The latter two versions give identical results. The new version gives a lower heat load in saturation for the field-free region, and a slower growth (but about equal saturation level) for the quadrupole field. These tests conclude the benchmarking of the new ECLOUD version. => ACTION => volunteer to make an LTC presentation of the new results (FZ) (5) Estimate of reduced LHC beam lifetime from differences in bunch-to-bunch emittances (FZ) ----------------------------------------------------------- A task in FZ's 2004 MAPS form was to estimate the reduced beam lifetime from bunch-to-bunch emittance differences, in order to derive requirements for the LHC injectors. Motivated by this task, FZ surveyed the effect of unequal emittances of the colliding beams on the beam lifetime in HERA, SPS, RHIC, and Tevatron. From the available data for each collider, he made a rough estimate of the emittance ratio resulting in a 100-h beam lifetime due to the 'scraping' of the larger beam. Plotting the estimated limiting ratios as a function of the total beam-beam tune shift, the data for HERA, SPS and Tevatron lie nearly on a straight line. Extrapolating this line to the LHC beam-beam tune shift, one may expect that the maximum tolerable emittance difference is of order 15-20%. More conservatively, one could use only the data from the SPS, whose beam-beam tune shift is closest to that of the LHC, and in this case the tolerance would be 2-3%. WH pointed out that also the long-range beam-beam interaction depends on the emittance and will therefore introduce an additional tolerance. Posted on the web: Slides by EB, FZ Web site: http://ab-abp-rlc.web.cern.ch/ab%2Dabp%2Drlc/