----------------------------------------------------------- Minutes of the ABP-RLC team meeting of 21.10.2005 present: EB, UD, AG, WH, JJ, EM, TP, FR, GR, RT, DS web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ (1) Minutes of last meeting, pending actions, announcements ----------------------------------------------------------- The minutes of the RLC meeting of 14.10.2005 are approved. => ACTION: EM will clarify the offset at the first turn and the offset at which beam loss occurs. STATUS: DONE (see EM's presentation) => ACTION: the RLC team should provide a proposal for aperture and field requirements. Chromatic performance of all IR solutions should be evaluated. STATUS: PENDING. The revised list of pending actions is posted at http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/Actions/actions.htm. (2) Flat beams for nominal LHC and LHC upgrade (SF) --------------------------------------------------- This presentation will be delayed to the next RLC meeting. (3) LHC collision configurations: status and news (WH) ------------------------------------------------------ WH has studied all possible beam crossing configuration schemes as required by the experiments and is preparing a report in which he shows which can be operated and which can not. This analysis will be presented to the LHC-OP committee in two weeks time. One possible option investigated for LHCb is reducing beta* to 1-2 m (from 10 m), which is not feasible for one polarity of the spectrometer because aperture is not sufficient. The polarity change option is presently under discussion and might be given up by LHCb if the spectrometer can be kept on at injection, thus generating a crossing angle of about 4 mrad and requiring compensation by an external crossing angle to avoid double crossing. Working solutions exist for beta* from 1 m to 10 m if the spectrometer polarity is blocked. Answering a question by FR, originated by Alain Blondel, WH explained that the contribution to the tune footprint from ALICE in proton operation is negligible (see enclosed long range beam-beam tune footprints for nominal LHC, with beta*=10 m and 5 sigma beam separation at IP2). (4) Follow-up of fast instability at SPS injection (EM) ------------------------------------------------------- Elias Metral has reported about the evidence of TMCI in the SPS and the underlying theory. Theories of BBU and TMCI give similar results. Standing wave patterns a la Sacherer/Laclare for dipole single bunch modes are recovered at low beam intensity with any chromaticity and traveling wave patterns at higher beam intensities are obtained (GR's comment after the meeting: can the phase velocity of the traveling wave pattern be used to infer the exact value of chromaticity? Probably yes, this can be investigated and pinned down with HEADTAIL simulations). There is still a discrepancy on the propagating direction of the traveling wave when applying the TMCI theory, whereas with the BBU theory the formalism seems to be fine. Measurements clearly reveal traveling wave patterns along the bunch, compatible with the TMC theory with a proper selection of the two modes that first couple, obtained by the code MOSES. Chromaticity has a stabilizing effect and actually the value 2.04 (in normalized units) seems to be enough to have complete stabilization of the bunch at the considered intensity. Measurements are pretty much in agreement with EM's calculations using a broad band impedance model with R=10 MOhm/m, f_r=1 GHz, Q=1. TMCI was also (probably) observed in the PS at transition crossing. The bunch went vertically unstable and got lost at a specific location along the bunch profile, revealed by a hole in the longitudinal distribution. A high frequency dipole oscillation was also detected at the position where the loss occurred. EM's model using simple analytical formulae and a model of broad band impedance for the PS (R=3 MOhm/m, Q=2, f_r=700 MHz) can reproduce the instability, but somehow head and tail are exchanged. The model could be applied assuming that, even if we are in the accelerating ramp, the instability sets in on a time scale much shorter than the ramping slope. MOSES has also been applied to predict TMCI thresholds and rise times and the agreement with the simplified models used by EM as well as with the experiment seem very good. Use of HEADTAIL to simulate these measurements is planned as future work to try to have a better fit for the transverse impedance of the SPS. FR pointed out that the "deconvolution" of the measured dipole signal from the known longitudinal distribution, discussed at the last RLC meeting, would still help in the analysis. He also tried to clarify the logics adopted in explaining the observations using Laclare's formalism together with MOSES. In a discussion following the meeting, EM and FR agreed that it would be useful to extend MOSES and automatically provide the time evolution of the dipole signal for direct comparison with the measurements. An additional comment by GR after the meeting is that maybe one should check the PS experimental data to be sure of what is the head and what the tail of the bunch, since they can be reversed if the longitudinal signal is reconstructed with undersampling. EM proposed to make new measurements in the PS to obtain the same "movie" as from theory and clearly identify where the instability starts and how it evolves. (5) Tertiary collimators and TCLI's with two beams (FR) ------------------------------------------------------- FR reported a discussion with RA on tertiary LHC collimators and TCLI injection protection absorbers, showing some drawings that require feedback from the RLC team. Tertiary collimators and absorbers will be placed near D1 (in either open or closed position) in regions where both beams are circulating, even though they actually collimate only one of the two beams. Position, material, and possibly side coatings of the jaws should be chosen appropriately. As there are two beams in these regions, the impedance should be calculated taking this into account, which doesn't make it a routinely calculation considering that the two beams will cross-talk via the wakes. The time scale to answer these questions is about a month, but faster feedback would be welcome. We also have to comment on a preliminary SLAC design for Phase II rotating collimators. (6) AOB ------- The RLC team is happy to welcome the two new staff members Giovanni Rumolo (who helped FR preparing these minutes) and Rogelio Tomas. Using the code HEADTAIL, GR will perform a study of TMCI threshold for higher injection energies into the SPS. The goal is to find out whether the higher energy, which in principle would make the threshold for TMCI higher, wouldn't lower the TMCI threshold because of the absence of the stabilizing space charge effect. This is a high-priority task requested by the PAF committee to assess the impact of a Super-PS on the performance of the SPS as LHC injector and FR encourages GR to conclude this study within a few weeks, delaying further parameter scans to 2006. Posted on the web: slides by WH and EM. Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/