----------------------------------------------------------------------- Minutes of the ABP-RLC section meeting of 18.03.05 present: EB, HB, AG, WH, EM, FR, DS, EV, 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 the minutes of last week's meeting. => ACTION => Maintaining IP knobs of W. Wittmer, check with optics team and with W. Wittmer (WH, FZ) STATUS: WH discussed this matter with M. Giovannozzi. WH will prepare a working version and the LOC team will take over the maintenance as soon as it is ready (almost done). => 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, EM compared collimator gaps from Stefano with numbers of Marek and found them in close agreement; next he plans to look at intensity and bunch length. => ACTION => volunteer to make an LTC presentation of the new results (FZ) STATUS: Presentation is being scheduled by O. Bruning, probably for next week (2) Effect of ac conductivity on graphite collimator resistive-wall impedance (AG) -------------------------------------------------------------------- AG briefly reviewed the effect of ac conductivity in the resistive-wall wake theory, as developed by Karl Bane and others. The electronic relaxation time determines the frequency at which the effect is relevant. In the Drude model, this can be obtained from conductivity and free electron density. For graphite and carbon the density is 4 orders of magnitude lower than for copper. For Cu the ac conductivity introduces a peak in the impedance at 10 THz, for graphite and/or C-C the peak is larger (increase by 3-4 orders above the nearby values) and at 1 THz. The frequency is still too high to be harmful for the LHC. AG assumed an epsilon=4 for C-C (which he estimated from the average over different directions graphite). FR mentioned that another effect comes into play at high frequencies, where the displacement-current becomes important. This is described in A. Chao's book. (3) Tune shift from nonlinear RW collimator wake (FZ) ----------------------------------------------------- Starting from the Piwinski-Bane-Irwin-Raubenheimer formula for the nonlinear resistive-wall wake field, FZ derived formulae for the coherent and incoherent tune shift induced by a collimator, including the effect of the finite beam size (plus loss in intensity and change of distribution by scraping) Application to the parameters of the SPS LHC-collimator measurement yields a good agreement between theory and measurement when the beam sizes in the two planes are taken into account. Reducing the beam sizes by a factor of 10 reduces the prediction to that for a pencil beam (e.g., Burov-Lebedev impedance). Increasing one of the beam sizes by a factor of 2 also changes the result drastically (hence, a similar measurement technique could be used for monitoring beam sizes as remarked by HB). While the coherent tune shift steeply increases as the collimator gap is closed, the incoherent tune spread is predicted to reach a maximum for a half gap about equal to the rms beam size. The predicted incoherent tune spread is quite large, of the order of 0.002 at the maximum. This may imply that for LHC Landau damping could be restored by the nonlinearity of the wake itself. The Piwinski theory does not include the phenomenon of inductive bypass. => ACTION => Attempt to derive a general nonlinear theory including the inductive bypass (EM) (4) Updated e-cloud simulations: LHC arc heat load & modified cooling capacity (FZ) ----------------------------------------------------------- FZ presented updated simulations of average arc heat load, electron density and electron flux on the wall vs. the bunch intensity. The values shown are averages over an arc cell, including quadrupoles and drifts. The curves were calculated for R=0.5 (a factor 2 smaller than in previous simulations). The latest estimate of the cooling capacity was included. For delta_max=1.5, the intensity limit is about 1e11 protons per bunch, and the nominal intensity can be reached for smaller values of the emission yield. FZ also presented the simulated heat load in the arc dipoles as a function of bunch spacing. The results indicate that a spacing of 12.5 ns can be achieved for delta_max=1.1. DS suggested to repeat the simulations for more than 1 bunch train and also for R=1.0. He explained, how he had obtained R=0.5 by fitting the SPS data, and that higher R and lower delta_max may describe most measurements similarly well. (5) Update on MAD-X Touschek and Dynap modules (FZ) --------------------------------------------------- FZ summarized recent modifications to the MAD-X DYNAP module. Results for Lyapunov and smear are now printed for multiple particles, and the calculations of tune error and Lyapunov coefficient were both improved. FZ also presented the new web documentation and examples for the TOUSCHEK model. He thanked F. Schmidt and C. Milardi for their help in the TOUSCHEK effort, and he showed suggestions for further extensions of TOUSCHEK by C. Milardi. (6) Transverse resistive-wall impedance for 2 layers derived from Zotter formalism (EM) --------------------------------------------------------------------------------------- EM numerically solved the 2-layer impedance in the Zotter theory for the case of a Cu-coated graphite collimator in the LHC. The agreement with the Burov-Lebedev theory is excellent. The differences are smaller than 1% both for coated and uncoated graphite collimators. The only significant difference between these two theories, of about 30%, was noted for the SPS MKE kickers, and this is attributed to the effect of the complex permeability. More precisely the 30% is the remaining difference for the amended Burov-Lebedev theory, which takes a complex permeability into account. FR recommended to obtain physical insight in addition to the mathematical formula, and also to compute impedances for different values of epsilon (in particular for epsilon=4, which appears to be the best estimate for the collimator). FZ remarked that the gain by the coating is significant, at high frequencies up to a factor of 10, and that only one line of the summation should fall into the region where the coating does not help (i.e., below 1 MHz). Even if the total gain is only a factor of 2 it might be worth the effort, since it would allow reaching the ultimate intensity. FR commented that the effect of the coating may be more positive when epsilon=4 is considered. Posted on the web: Slides by AG, EM, FZ (3x) Web site: http://ab-abp-rlc.web.cern.ch/ab%2Dabp%2Drlc/