Minutes of the ABP-LCE team meeting of 08.08.03 present: AK, WH (chair), EM, DS, EV, FZ (secretary) excused: FR ---------------------------------------------------------------- (1) Outstanding Action Items ---------------------------- ACTION: FZ should get acquainted with correct use of other CVS features. DONE. Discussion with WH solved problem in 'commit' command. ACTION -> LV, EM and FR will plot the stability region and the complex tune shift vector for the June-2003 baseline scheme. EM will discuss with LV next week and report in the next meeting. ACTION DS,FZ: FR asked for an estimate of photon back scattering off proton bunches and possible detector background. DONE. According a pessimistic estimate, there is one scattering event every 5000th bunch crossing. This likely overestimates the real number by a factor 100 or 1000. In addition, the transverse photon momenta are small, so that these photons may not hit the detector. (2) Presentation by EV ---------------------- Work is motivated by heating of LHC extraction kickers in the SPS. There are 2 rf cavities in the SPS, operating at 200 and 800 MHZ. The 800 MHz cavities are used in a bunch-shortening mode to suppress beam instabilities. If instead one can suppress the coupled-bunch instabilities in a different way, this would not be necessary. One approach is a modulation of the rf voltage across the bunch train. This was successfully implemented by EV at HERA, where the instability thresholds could be substantially improved. Differences between SPS and HERA are the higher ramp speed and higher impedance in the SPS. The synchronous phase is about 25 degree and cannot be ignored. EV computed curves relating bucket filling factor, synchronous phase and bunch length. From the SPS ramp profile he extracted the synchrotron frequency spread and bunch length throughout the cycle. The frequency spread at which instability occurs is taken as the magnitude of the required stablizing bunch-to-bunch synchrotron freqency spread. It is 6.5/s for the SPS compared with 1/s in HERA. An SPS experiment is planned in 2 weeks time. (3) Round Table Discussion of Ongoing Activities -------------------------- DS: Pursuing Gsfidl simulations. Some problems with new CPU to be resolved. Studied photon backscattering with FZ. WH: Changes were made to TRAIN program. Arbitrary individual bunches, with distinct orbit and tune, can now be selected for MAD tracking. The tracking set up is considerably simplified. This could be used for next round of LHC tracking campaigns. A prospective user is also WW, who could study the effect of the tuning knobs on orbit and on dyanmic aperture, as well as dynamic effects. FZ: 3 sections for LHC design report in process. LHC simulations with ECLOUD ongoing. Encountered problems with afs. Is in contact with G. Bellodi and G. Rumolo who compare ECLOUD simulations with a round and a rectangular chamber for ISIS, ESS, and the SPS NEG test stand. Some discrepancies still to be resolved. Web page of code comparisons for long proton bunches was set up at RAL. Work on photon backscattering with DS. Rewrote APD work package for ESGARD N4, on behalf of FR, who is absent. EM: Mo and Tu supervisor at the PS. Wed and Th MD on new CT, today MD on LHC beam. Next week discussion with LV. Had preliminary look at tune dependence of effective impedance. (4) Resistive Wall and Inductive Bypass by AK --------------------------------------------- In collaboration with BZ, AK derived the wake fields for a chamber with inductive bypass and compared it with the conventional resistive wall wake. After 1 s in the SPS the wakes are widely different (about 10% difference after 2 turns). The additional memory loss due to the field leakage through the vacuum chamber remains to be understood and included. The physics of the inductive bypass was not perfectly clear. The reference is CERN-AB-2003-005 ABP (courtesy of EM). ACTION (FR): Organize a forum on inductive bypass (by LV)?