Minutes of the ABP-LCE team meeting on 30.01.04 present: EB, WH, AK, EM, FR, DS, EV, FZ ---------------------------------------------------------------------- (1) Minutes & Pending Actions ----------------------------- Zbase: O. Bruening's Zbase tutorial was a great success. Young Zbase users are encouraged to intensify their usage of the code and to report incurred error messages to TdA. ABP-RF-BDI meeting on beam-beam diagnostics with feedback: WH is organizing this meeting. A first preparative meeting with W. Hofle and EV is planned. Afterwards BDI will be involved. There is a question from Gerhard Schneider as to whether a 100-micron coating should be applied to the warm BPMs in LHC. The preliminary assessment by FR is ambiguous, as the positive effect is small and resonant modes may be less damped. FZ will try to answer this question with the help of the team. FR suggests that the LCE team should take the initiative and initiate this kind of studies. Answers to requests should be quick. He mentioned that according to FC the impedance of a strip line etc. depends on the outside circuit. (2) Rigid-Bunch Collimation Code (EV) ------------------------------------- EV studies a faster scheme of tracking with collimators, where multi-particle tracking is replaced by a tracking of 3 numbers (rms size, cut-off and intensity). The beam size damps exponentially with a pre-calculated damping time. This simulation scheme could be used for the CNGS feedback simulations. It was remarked that centroid oscillations might complicate the picture (no scraping after the first turn unless the coherent tune shift is significant). Decoherence effects and feedback response times would also be important. FR states that the goal should be to have a valid physical model. (3) Stability for Beam Collimation at 6 Sigma (EM) -------------------------------------------------- Berg-Ruggiero formula underestimates the stability, because it has a cutoff at 3.2 sigma. LV's Gaussian model overestimates the stability. A more reasonable model is a 15th order distribution function, devised together with Andre Verdier, which is consistent for 6 sigma collimation. The stability calculation provides the expected result. With the good setting of the octupoles, the stability boundary is between the other two curves. However, for the bad setting the distribution is more stable than the Gaussian. In the imaginary tune-shift direction the stability is about the same as before, but in the real tune-shift direction a factor 2 is gained with respect to the Berg-Ruggiero distribution. This is important for TOTEM. The 2nd order distribution function is kept as the pessimistic baseline. Real tails could be larger or smaller than estimated by any of these distributions. (4) Collective Limitations for TOTEM Beam (EM) ---------------------------------------------- The angle of observation should extend down to less or equal 10.1 microrad. FR asked if the crossing angle can be controlled with this precision. EM showed that there is a Landau-damping limit on the quantity (Nb/eps_N G^3), where Nb is the bunch population, eps the normalized emittance and G the gap size in units of sigma. The minimum angle that can be provided at pilot-bunch intensity is 5.25 microrad. (5) Electron Cloud in LTC (DS) ------------------------------ DS will present e-cloud simulation results next Wednesday. There is one open question: discrepancy between original and elliptical boundary (solved after the meeting). DA will review the status of benchmarking for the SPS. WAMPAC1 is in good agreement. WAMPAC3 is not in good agreement, but results are invalid according to the vacuum group. Stripe position and response to magnetic fields are consistent. The e-flux on the wall is off by an order of magnitude, but the heat load and mean energy seem to fit. So there is a puzzle to be solved. The electron energy spectrum may disagree in some cases (Private communication by JMJ). The team goal is to reduce the discrepancy. FR will ask Laurent Tavian for an updated official cooling capacity (LT should confirm plot provided by FZ; ACTION FZ: send plot to FR; ACTION FR: contact LT). We could attempt to predict scrubbing time. (6) Emittance Growth from Electron Cloud (EB) --------------------------------------------- Benchmarking of HEADTAIL and QUICKPIC is ongoing. Various simulations were run for LHC at injection, with different boundary conditions, two treatments of the rf voltage, with and without dipole field, and varying number of interaction points up to the continuous limit in QICKPIC. There are discrepancies by a factor of 2 for nominal inentical input between the two codes. The variation of any of the other parameters gives comparable or larger changes. The reason for the discrepancy should be understood. The continuous QUICKPIC simulation shows no emittance growth inside a magnetic field. The 1-kick approximation is not adequate, but after 5-8 kicks the code converges. FR recommended to include simulations for 5-8 kicks in the comparison. Also one should compare simulations at lower density (below the threshold of fast instability). FR and DS will meet on Monday to discuss LTC presentation. (7) Chamonix Follow Up (FR) ---------------------------- MKE impedance modelling is an important issue that we need to pursue. FC will measure the impedance of an LHC kicker prototype as soon as this becomes available. Steve Myers asked for a revision of the overall LHC impedance model. (8) Gdfidl (DS) ---------------- The resistive problem cannot be solved in the time domain. It is better to compute the eigenvalues and afterwards add a small resistive component. FZ mentioned that Gdfidl could also be used for computing the impedance of the BBLR. DS explained that Gsfidl can compute any type of impedance (e.g., instability and detuning terms), depending on the processing by the user. Enclosed: Slides by EM.