----------------------------------------------------------------------- Minutes of the ABP-RLC section meeting of 04.03.05 present: EB, EM, TP, FR, EV, FZ excused: WH, DS web site: http://ab-abp-rlc.web.cern.ch/ab%2Dabp%2Drlc/ ----------------------------------------------------------------------- (1) Minutes of last meeting and pending actions ----------------------------------------------- EB had sent a comment on the minutes. She pointed out that it was EM who noticed a peculiarity in the PS emittances quoted A. Blas. Her comment was already included in the web minutes. => 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, in contact with G. Arduini, S. Redaelli, et al. Detailed and complete information was obtained from M. Glasior. F. Roncarolo still needs to be contacted for emittance values. FR recommended a joint meeting with all actors at a later time. => ACTION => detailed comparison with old version of the code (FZ) ECLOUD run for the same input file (DS) DONE (see FZ's report below) => ACTION => Maintaining IP knobs of W. Wittmer, check with optics team and with W. Wittmer (WH, FZ) FR discussed this matter with O. Bruning, according to whom the optics team does not maintain the LHC IP beta knobs. FZ mentioned that WH (absent) had been in contact with WW. FR thought that WH will perhaps take care of these knobs. (2) Results from ECLOUD Debugging (FZ) -------------------------------------- FZ presented simulations for an LHC arc dipole with delta_max=1.3 performed with the latest version of the ECLOUD code, with the same code but a wrong theta-dependence of epsilon_max, and with a version from May 2004. The result of the new version with intentional wrong theta-dependence is almost identical with the result from the May-2004 version. The correct new version gives about 40% less heat load for this set of parameters. FZ will repeat this comparison for quadrupoles and drifts. The results for the heat load in dipoles before/after correction are the same as obtained by DS (see previous meeting). DS reported by email that he found nearly the same heat load with FZ's and his input file. FR recommended that any future publication should refer to the new data. He asked for a repetition of all LHC arc ECLOUD simulations and suggested a later presentation to the LTC. FZ also compared estimates of the cooling capacities available for electron cloud (or gas scattering) at 4.6-20 K. These are computed by subtracting unavoidable heat loads (static heat inleaks, heat from image currents, synchrotron radiation) from the total cryogenics capacity. There is up to 2 W/m spare capacity for the nominal LHC, which decreases to about 1 W/m for the Piwinski scheme and to roughly zero for the baseline upgrade. On the other hand, the highest heat load from electron cloud is expected in the last case. It was discussed that raising the beam-screen temperature may enlarge the margin. (3) Progress report on HEADTAIL simulations ------------------------------------------- EB showed results of HEADTAIL simulations with a frozen potential. This type of simulation takes into account only the incoherent effect. The simulations do show a long-term emittance growth, which decreases with the number of electron-beam interaction points. The growth pattern is similar to that of the long-term emittance growth observed in the dynamic simulations, though a direct comparison could not be made, since the electron density was above the threshold of fast instability in the dynamic case. It was suggested to repeat the simulations for an electron-cloud density below the strong-instability threshold and compare the long-term emittance growth for the two types of simulations. FR mentioned that there should be an energy conservation law, which may allow for an analytical solution. FZ pointed out that this simulation does not include longitudinal electric fields, hence might not be symplectic. EB referred to the localization of the rf cavity and the synchrotron motion. EB calculated the potential for different longitudinal positions along the bunch as a function of the transverse coordinate. The potential appeared to follow almost a straight line on either side of the center, which would correspond to a step function in the field. The grid size uses was 4 points per rms beam size. It was proposed to refine the mesh size. EB showed that the dependence on the total extent of the grid is weak. One of EB's plans is to compute the incoherent tune shift as a function of transverse and longitudinal coordinates, which could be used as an input to the model developed with G. Franchetti. FR speculated that the incoherent emittance growth may be minimized by shaping the longitudinal bunch profile. (4) Resistive wall impedance for different velocities of beam and wave (EM) ---------------------------------------------------------------------------- EM generalized B Zotter's formula for the resistive wall impedance to the case of unequal velocities of beam and wake. He considered the lowest wave velocity beta_wave=0.004 (10 times smaller than quoted by BZ in previous meeting). Up to 100 MHz the impedance is the same as for the equal-beta case, then it sharply drops towards zero. FR suggested that this drop might relax requirements on the transverse feedback system, and he recommended a careful interpretation in terms of the actual effect on the beam. => ACTION => EM will discuss with BZ to check the result (5) RLC contributions to the LHC commissioning (FR) ---------------------------------------------------- FR discussed the proposed involvement of RLC members in the LHC commissioning, reproduced below. -> Comments/suggestions to the draft document prepared by Roger Bailey RLC candidates as Commissioner In Charge -> E. Metral and F. Zimmermann -> Accelerator systems responsibilities "Vacuum" should include "Electron Cloud" RLC link -> F. Zimmermann Beam instrumentation: "BPM, trajectory and orbit correction" RLC link -> W. Herr "Luminosity monitors" RLC link -> F. Zimmermann "RF" should include "Longitudinal Beam Dynamics" (bunch length measurement and control, abort gap cleaning, IBS, etc.) RLC link -> F. Ruggiero Longitudinal feedback -> there is NO longitudinal feedback! Transverse feedback RLC link -> E. Metral Experiment solenoids and compensations RLC link -> W. Herr Experiment equipment (Roman pots, velo) RLC link -> W. Herr No Schottky monitor is mentioned: we would strongly recommed to develop one (possibly within US-LARP) in view of the experience at existing hadron colliders and in particular at the Tevatron. RLC links -> E. Metral F. Zimmermann -> Beam based systems responsibilities Injection and first turn RLC link -> W. Herr Clean Beam Extraction B. Goddard is not an ABP group member Longitudinal plane RLC link -> F. Ruggiero Optics and RLC links -> W. Herr dynamic-aperture F. Zimmermann Lifetimes (including beam-gas, IBS, ecloud) RLC link -> F. Zimmermann Collimation (ions) RLC link -> H. Braun Crossing angle schemes and luminosity RLC links -> W. Herr F. Zimmermann Impedance and collective instabilities RLC links -> F. Ruggiero E. Metral F. Zimmermann (6) AOB ------- EM reported from his attendance of the OP days. He mentioned as the most interesting item a presentation by G. Robin explaining that 7 SPS magnets had suffered from a water leak, four of which were of the same type and manufactured by the same company. A design flaw lies at the origin of these leaks. Overall 170 magnets of the SPS may suffer from the same problem. EM noted that the total number of magnets in the SPS is 960, with 260 quadrupoles, and the rest shared almost equally between MBA and MBB dipoles. Posted on the web: Slides by EM, FZ Web site: http://ab-abp-rlc.web.cern.ch/ab%2Dabp%2Drlc/