Minutes of the ABP-LCE team meeting on 07.11.03   present: EB, WH, AK, EM, FR, DS, EV, LV excused: FZ ------------------------------------------------------------------------- (1) Minutes of Last LCE Meeting and Pending Actions --------------------------------------------------- Concerning the minutes of the last meeting, EV remarks that the special chips have been already removed to operate the FPGA module at 80 MHz. Further comments by W. Hofle can be found in the attached email. (1a) Follow-up of LHC collimator impedance ------------------------------------------ ACTION -> EM will clarify ambiguity in collimator orientation. DONE: Excel tables for collimators layout provided by RA give angle between the x-axis and the direction perpendicular to the jaws (see enclosed slides by EM). ACTION -> EM will check possible longitudinal 'Yokoya factor' relating           round and rectangular cases. ACTION -> Check discrepancy by a factor ~3 on collimator impedance          (EM+LV) and put numbers into ZBASE (EM). Partially DONE: discrepancy traced back to the use of different collimator layouts. LV's estimates are in excellent agrement with EM's automatic collimator impedance computations for "Phase 1" baseline collimator layout (see enclosed slides by EM). Results still need to be put into ZBASE. (1b) Impedance of Roman Pots for SPS test and for the LHC --------------------------------------------------------- ACTION -> DS will cross-check these estimates by numerical simulations           and follow-up the impedance of Roman Pots with Roger Perret. Partially DONE: DS has contacted Roger Perret and will cross-check impedance estimates of Roman Pots. He is also running GDfidL to compute geometric collimator impedance. ACTION -> AK will repeate simulations with a constant injection offset. ACTION -> JJ will explain how to produce eps plots with Mathematica. ACTION -> FR will read and approve the draft note by AK. DONE (2) Specs for Q and Q' meter: implications from Impedance and Beam-Beam ----------------------------------------------------------------------- J.-P. Koutchouk needs input from the LCE team to specify Q and Q' measurement accuracy/functionality at the LHC. In particular, limits on Q' at injection for various beam currents, originally estimated by S. Berg in LHC-PR100, need revision in view of changes of the impedance budget. The tolerance on Q' sharply depends on the beam current, the machine impedance and the assumptions on Landau damping. With the 1997 assumptions based on LHC-PR100, the tolerances were estimated as follows at injection: - Single bunch, up to 10^10 p: no constraint (Q'>-150). - Nominal beam structure, 10% of nominal current: Q'>-15. - Nominal beam: Q'>0 and in practice DQ'=±1 for Q'=2. JPK has also a question concerning transverse spectra: should we expect clean transverse spectra or can the spectra be significantly modified by coupled-bunch modes or the beam-beam interaction, even when the beams are separeted? He argues that if the spectrum could become like a forest, for example following the large impedance increase, we would have to invest in a 40 MHz bunch-by-bunch kicker that is not in the baseline. EV asks whether the feedback system has to be switched off during tune measurements and remarks that even fast kickers will never have a performance comparable to that of the feedback system. The Q-meter in HERA WAS the feedback system itself. WH has written the following draft text for further discussion: We assume that the question concerns the excitation of single or multiple bunches and that the Q measurement is done on a single bunch only. Indeed we must expect contribution from other frequencies in the single bunch spectra due to the beam-beam interaction. However, simulations show that they are heavily suppressed but strongly depend on initial conditions. Furthermore, they should be independent of whether a single bunch or multiple bunches are excited since on the time scale of a tune measurement, the excitation cannot be coupled to the other bunches fast enough. This is true for coupling through beam-beam interactions as well as coupled-bunch modes. In case the simultaneous excitation of multiple bunches becomes a problem, we suggest to evaluate the use of feedback kickers to excite bunch oscillations for the tune measurement. (3) Electron cloud effects with 75 ns bunch spacing --------------------------------------------------- Recent SPS observations with 75 ns bunch spacing and more than 1 bunch train indicate electron cloud activity especially during the ramp. We need to revise simulation results, also to confirm the dependence on bunch length in the SPS and in the LHC, using debugged (and stable?) code. ACTION -> DS will launch new ECLOUD simulations. EB simulations of emittance growth with lower electron density levels and increased number of kicks indicate better convergence. (4) Impedance of NEG coating of all warm sections ------------------------------------------------- The impedance of multi-layered vacuum chambers can be computed using a Transmission Lines formalism developed by LV (see paper to appear soon) and implemented in the Mathcad program available on the web at http://wwwslap.cern.ch/collective/luc/Transmission-Lines.mcd. For a resistive coating on a good conductor, such as the NEG coating now foreseen for all Cu thick chambers in the warm LHC sections, the impedance is given by the surface inductance mu_o*thickness divided (2 pi b). LV finds a surface inductance per unit lenght of 5 pH/m/micron corresponding, for a chamber radius b=40 mm and a 2 micron NEG layer over 4 km of warm sections, to Z/n~2-3 mOhm. For comparison, the pumping slots in the LHC beam screen correspond to a linear inductance of 10 pH/m and for 23 km of beam screen to Z/n=17 mOhm. (5) AOB ------- There are serious afs disk space problems for /afs/cern.ch/eng/lhc/collective  and  /afs/cern.ch/eng/lhc/zbase. Some 13 GB have been freed by FZ and the problem for "zbase" is solved. ACTION -> FR will ask for additional disk space for "collective". FR and the whole LCE team would like to warmly thank LV for his important contributions to the study of collective effects in the LHC and wish him a happy retirement. Attachments: email by W. Hofle, slides shown by EM on Automatic Collimator Impedance Computations and Landau Damping by Octupoles and Space Charge.