Minutes of the ABP-LCE team meeting on 03.10.03 present: EB, TdA, WH, AK, EM, FR, DS, EV, FZ ------------------------------------------------------------ (1) Actions and Comments on Minutes ----------------------------------- EV had some comments on the last minutes, which may already have been partly included in the meeting announcement sent out by Francesco. He pointed out that there is no strong connection between his approach and the feedback at the B factories. old ACTION: EM will further check the space charge detuning. The previous solution was not self-consistent, the stability diagram was computed using a quasi-parabolic distribution and the incoherent space charge tune shift assuming a parabolic distribution in the transverse spatial coordinates. Following a recommendation by FR, EM has redone the space-charge derivation by computing the projection of the quasi-parabolic distribution onto the two spatial coordinates. From this, he then derived the incoherent tune shift and tune spread. There will be some slight, but not decisive difference to the previous result. 10 double integrals are still left to compute, which can possibly be done in one afternoon. ACTION -> compute double integrals (EM) It was debated how to compute the coherent tune shift vector, which can be superimposed on the stability diagram with space charge. EM suggested that as a first guess one could take the tune shift at the center. FR recalled that the standard way to compute the tune shift is to solve the Vlasov equation, in this case with space charge, similar to what Y. Alexahin did for the beam-beam interaction, and to derive the relevant 'Yokoya factors'. EM emphasizes that space charge is not always stabilizing, even if the good sign of the octupoles is chosen. ACTION -> Automatic Excel spreadsheet for collimator impedance (EM+LV) Still to be done. (2) Early status report on LHC resonator impedance, kickers, BPM's, and cold-warm transitions (FZ) --------------------------------------------------- The status and literature for the various components were reviewed: 200-MHz cavities - no data on damped dipole modes 400-MHz cavities - R and R/Q values in D. Angal's paper without source; otherwise data for damped and undamped monopole and dipole modes transverse damper - dipole mode data missing experimental chambers - MAFIA calculations by Y. Luo to be recovered, data only for CMS; trapped mode at Y chamber injection kickers - formulae by G. Lambertson; ceramic-coating theory & experiments? dump kickers - no information found BPM - paper by L. Vos and A. Wagner, complete! cold-warm transitions - resistive part with coating & inductive bypass, requires to know Luc's theory; also geometric impedances (tapers, rf junction - references?) quadrupolar impedance - geometric wake from Stupakov, resistive wake from Yokoya FZ concluded that for roughly 50% of the elements a complete description of the impedance is available; for some others he has found little information so far. FR suggested to contact B. Spataro and Y. Luo for their MAFIA calculations and related input/output files. Also H. Tsutsui, B. Zotter, and R. Gluckstern may have computed kicker impedances. (3) Tensor Transformation (FR) ------------------------------- FR derived the tensor transformation needed, e.g., to compute the impedance for a flat collimator rotated by 45 degree. The impedance must be weighted with the correct product of square roots of beta functions and multiplied with coefficients 3/4 (same plane) or 1/4 (orthogonal plane). (4) Collimator Wake Calculations using GdfidL (DS) --------------------------------------------------- DS reports that computations with resistivity do not give the expected equality of real and imaginary impedance, the imaginary impedance decreasing much more steeply for increasing frequency. The geometric wake was computed for two example structures. The first case was for a round geometry with entrance and exit taper. The longitudinal wake is the same as that obtained by MAFIA. The second structure was a 'brick' collimator inside a round chamber, with a tapered transition at 10 degree. The wake looks like the shape of the bunch, and not inductive. At longer times, behind the exciting bunch the computed wake grows continuously. Is this the sign of a trapped mode, or an artifact of the code? The real impedance for this case changes sign at some frequency, which does not look physical and could be an edge effect. ACTION -> check sign of real part of impedance (DS) FR suggested to define a criterion for concluding this study. FZ proposed that quadrupole wakes also be calculated for the collimators (with offset test particle).