Minutes of the ABP-LCE team meeting of 01.08.03 present: RA, AK, WH, EM, FR, DS, EV, LV, FZ ---------------------------------------------------------------- (1) Outstanding Action Items ---------------------------- ACTION: FR will discuss issue of Z base young users with EM, DS, and FZ and then inform TdA. FR named Em, DS, FZ as young users for Zbase. TdA can explain its usage. Young users should learn how to add and extract impedance information from Zbase. This is considered a background activity. ACTION: AK will now include a model of the feedback system and apply his code to study injection oscillations in the LHC. Feedback has not yet been included. FFT approach with resistive wall does not give exactly the same result as original wake-field calculation. The differences are small, of order 10%, and could be partly explained by the difficulty of extracting rise times from the simulation. FR recommends that AK looks at a single mode when computing rise time. FZ recalled that Helmut Burkhardt uses a sliding window filtering to infer mode rise times for SPS experiments. AK has included the resistive wall inductive bypass, but the impedance approach causes problems on later turns. Perhaps it would be better to first convert back into time domain, and then apply the wake field. AK reports that BZ computed a time-domain wake for the bypass, but this wake ahs an imaginary component. ACTION: FZ will implement the suggestions by TdA or report back on possible outstanding problems. TdA's solution worked fine, and ECLOUD is now handled under CVS. The version on the web still needs to be updated in parallel. NEW ACTION -> FZ should get acquainted with correct use of other CVS features. ACTION: LV will further clarify the differences of his approach compared to the standard approach by Sacherer, Chao, and HT. A bug was found in the calculation of the effective impedance. As a result the resistive part of the effective impedance was reduced by a factor 2-5, while the imaginary part increased 2-2.5 times. In consequence the efforts to reduce the transverse impedance should not be relaxed. (2) Ongoing Activities ---------------------- a- collimator impedance Results of LV, HT, and EM are now all consistent. A thin Cu coating reduces the imaginary of the impedance, but increases the resistive part. Probably a 1 micron coating is the best compromise. Calculations are conservative as the Be resistivity was assumed instead of that of Cu, to take into account surface impurities for a thin layer. FZ asked about the tune dependence of the effective impedance with bypass. RA pointed out that the ion collimators may be different from the proton collimators. For ions, a getter coating could be appropriate. There are ongoing tests by E. Mahner on desorption yields. EM reported on higher-order head-tail modes following up on a previous meeting. Thanks to the inductive bypass there should be no problem for chromaticities between 0 and 0.1 (xi units). The Yokoya factor for non-cylindrical symmetry was confirmed by H. Tsutsui and included in the estimates. RA mentioned that L-shaped collimators at a location with equal horizontal and vertical beta functions are an option currently under study. For the moment, though, he suggested to stay with the baseline scheme for the impedance studies. FR pointed out that feedback together with octupoles result in emittance growth. The m=0 tune shift computed by EM appeared marginal without feedback. NEW ACTION -> LV, EM and FR will plot the stability region and the complex tune shift vector for the June-2003 baseline scheme. The octupoles give a tune shift of a few 1e-4 at 1 sigma, which might correspond to severa 1e-4 in the stability diagram (FR). DS is running GdfidL to compute the geometric part of the collimator wake field. Possibly he can also study the effect of a coating and/or the quadrupole wake. FR reminded the team that other actions in progress for the collimation are bench measurements by F. Caspers and beam measurements at SLAC. b - collimator electron cloud FZ recalled that the electron cloud at the collimators should still be investigated. Presently it is second priority, postponed to after finalizing contributions to the LHC design report. Based on A. Ferrari's FLUKA simulations, a significant flux of electrons is to be expected. Possible countermeasures include solenoids, coatings, and clearing electrodes. The last item could increase the impedance and is controversial. A voltage across the collimator jaws would efficiently suppress the electron build up. This requires isolated jaws, which would be compatible with a scheme of impedance reduction proposed by F. Caspers. However, the jaws are only a small part of the concerned region, and loner clearing electrodes could introduce an undesired impedance. FZ mentions that, as far as he knows, the LHC baseline does not foresee to coat the LHC cleaning insertions with getter. But this would certainly be worthwhile. c - beam-beam studies Last week there was joint meeting with the optics team. Tracking with beam-beam is still to be discussed. WH has written a small tracking routine for MAD (which possibly could also be used as input to SIXTRACK), that allows the tracking for an arbitrary bunch including head-on and long-range collisions. WH presently analyzes the effect of missing head-on collisions. In the near term, activities will shift to CAS preparation. P. Grafstrom has asked for an ABP presentation on the handling of the ALICE spectrometer magnet and its compensation at the 8 September ALICE installation review. WH will be the ABP representative. ACTION DS,FZ -> FR asked for an estimate of photon back scattering off proton bunches and possible detector background. d - electron cloud SR study or LHC insertions was concluded. Several improvements were added to the ECLOUD code. CVS is working thanks to TdA's help. ECLOUD/POSINST code comparison is ongoing for ESS&ISIS by G. Bellodi. Various discrepancies found. Contributions to 3 sections of the LHC design report are in preparation. Some simulations were done on the effect of changing the elastic reflectivity to conform with the measurements of Cimino/Collins. The heat load for LHC dipoles increases by about a factor of 2 compared with the old parametrization of elastic reflection. S. Heifets made some comments on the simple q.m. model of elastic reflectivity. T. Katsouleas would like to reinforce our collaboration with USC group, and asked if we can send a student for a couple of weeks. (3) AOB ------- FR will be absent for 2 weeks. He encourages the team to continue with weekly short Friday meetings and write minutes.