Minutes of the ABP-LCE team meeting of 22.08.03 present: TdA, WH, AK, EM, FR, DS, EV, LV, FZ ------------------------------------------------------------ (1) Comments to last meeting and minutes ----------------------------------------- There was a translation problem in the slides of AK distributed after the last meeting. Corrected slides will be attached to this email. FR suggested that results on photon scattering be published as an ABP note and possibly sent to contact persons at the experiments. He also mentioned a second study by DA and FZ for a QCD explorer based on colliding the CLIC beam with LHC superbunches. Estimated luminosity is 1000 times higher than the minimum needed. This study could become a combined CLIC-LHC note. (2) Outstanding Action Items ---------------------------- A) Status of Gdfidl (DS) It was found that the mesh size must be chosen much finer than the skin depth in order to get the correct impedance. Recent results for the longitudinal impedance look promising and consistent with HT’s paper. Typical CPU time is 0.5-1 day for one calculation. Further simulations will be run with Even finer mesh. The geometric impedance will be looked at afterwards. It might be difficult to compute the resistive wall impedance for the taper. ACTION -- > DS, continue great work with Gdfidl. B) Tune Dependence of CB effective impedance (EM) Using Sacherer approach the tune dependence was computed with and w/o inductive bypass. W/o bypass a strong dependence was found in agreement with the classical res.wall theory. With inductive bypass, the growth rates and frequency shifts are independent of the fractional tune, which is ascribed to the broadband character of the impedance. C) Landau damping from octupoles (EM+LV) Using the Berg-Ruggiero formula, EM computed the stability diagram for a 2-m long effective graphite collimator. The bunch distribution was considered pseudo- parabolic. The rms tune spread was estimated to be about 1.8e-4 in both planes assuming present values for the maximum octupole strengths and beta functions. The tune spread differs by a factor of 2 from earlier calculations based on D. Mohl’s paper. This factor of 2 had been discovered by S. Berg. EM's calculation shows that mode=0 is not stabilized by the octupoles, but all higher-order modes are. However, the collimator model was pessimistic by about a factor 3. Using a more realistic model summing over all actual collimators, but considering a 1-dim. Situation and a Gaussian distribution, LV showed that also the m=0 mode is stable, both with and without the copper coating. The coating moves the LHC complex tune shift vector further inside the stability boundary, for a Gaussian distribution, and it is thus recommended, especially, in case the real distribution is not Gaussian. LV points out that the tune shift induced by octupoles warrants a second look, and he referred to an old note. ACTION -> Verify factor 2 of SB&EM (LV)? D) Inductive bypass impedance (EM) EM distinguished between inductive and non-inductive regimes and presented approximate impedance formulae for the various cases. E) Space-charge tune spread versus Landa-damping of HT modes (EM). EM presented data from the PS, where HT modes of different orders were clearly observed in agreement with Sacherer approach and resistive-wall theory. Turning on octupoles could stabilize the beam. The corresponding tune spread was a factor 1000 smaller than the space-charge tune spread. Therefore, the space charge tune spread Is not very effective in providing Landau damping. This is consistent with Mohl&Schonauer (1974) and Mohl (1995), who predicted a small effect only when magnet nonlinearities are also present. The factors 2 from S. Berg and a factor 2 potential enhancement by space could hint at a discrepancy by a factor 5—6 between the measured and theoretical instability threshold in the PS (LV,FZ). It was suggested to repeat the experiment, possibly at two different energies. (ACTION?) LV rcalled data by HB at high intensity in the SPS, which showed no sign of decoherence and no instability. F) RF amplitude modulation & CB transverse feedback (EV) A frequency spread of 6.5/s is required, which corresponds to 20% rf amplitude modulation of the 200-MHz system. The 800 MHz system Will be switched off for preliminary tests, done together with T. Bohl (MD next week?). Improvements for the transverse CB feedback are under study. Possibilities include a reduction of the amplifier noise using an integrator, and a modal analysis utilizing information from two successive turns. G) Electron-cloud status (FZ) Three sections for the LHC design report were completed, on electron cloud, sybchrotron radiaton, and gas scattering. Response from the vacuum group concerning gas desorption is still missing. In the last APC meeting several interesting talks were held on electron-cloud measurements in the SPS, by members of the vacuum group. These measurements shall be benchmarked against simulations. Preliminary studies indicate that the code does not reproduce the (sometimes) observed 2-stripe structure for a field-free region. Also, strong multipacting is simulated (by GR) for a maximum yield as low as 1.0 for a rectangular chamber as used by the NEG test stand. This could be related to the assumed angular dependence of the yield, for which we only have old data for copper, but not for the NEG coating. Recently the SPS LHc beam experienced a poor lifetime of 20 minutes, Which could be due to electron cloud. M. Furman had asked for electron-cloud working topics as part of the US LHC collaboration, and FZ has sent him a few items, mostly related to the SPS studies. ACTION-- > See if ALS could measure photon backscattering on sawtooth (FZ)? H) Multi-bunch simulations (AK) AK presented two movies illustrating the meaning of the different multi-bunch mode numbers. Several examples showed how one can extract the amplitudes of differerent modes from simulated pick-up data. The next step will be to apply a sliding window and to extract growth rates of different modes. (30 AOB (FZ) ------------ Yesterday’s LRBB MD was successful, clearly revealing diffusion on the wire scans. There was a request from FNAL to participate in a review of their BBLR project. It was declined and instead suggested that sombody from FNAL come to CERN and join SPS BBLR experiment.