Minutes of the ABP-LCE team meeting of 07.05.04 present: AK, WH, FJ, EM, FR, EV, WW (partially), FZ excused: EB, DS ----------------------------------------------------------------------- (1) Minutes & Follow-up of pending actions ------------------------------------------ FZ asked for the conclusion on the correct formula of the synchronous phase shift from the last meeting, discussed by EM. FR responded that he thinks only the formula from A. Hofmann is correct. Concerning the 370% margin for the damper with a slightly reduced number of bunches in the CNGS beam, EV clarified that this number refers to the inverse ratio of the damping time to the time interval between the 1st and second extraction. ACTION => EB will extrapolate HEADTAIL results for 30 min operation and extend the simulations to SPS conditions (with feedback and dipole field) -> partly done. ACTION => EM will update the LHC collimators impedance according to the latest layout provided by R. Assmann. -> still active? Other actions are mentioned in the individual reports below. (2) Beam-Beam Studies & Visit by Fred Jones (FJ) ------------------------------------------------ FJ is visiting AB/ABP until 20 May. He is working with WH on almost-3D self-consistent strong-strong beam-beam simulations. Currently the program runs on 10 processors and turn-around-time is 1-2 days. The code is only almost 3D since longitudinally a slicing is applied. The longitudinal field is not fully self-consistent yet. Additional computing resources have become available at UBC and Alberta. In addition FJ has a long-standing interest in space-charge simulations for the PS booster, where his contact is M. Martini. FR pointed out that Lihui Jin in collaboration with Jack Shi recently published a paper on beam-beam simulations in Phys. Rev. E 69, 036503 (2004), and that there is a paper on the Tevatron beam-beam effects in this month's PRST-AB issue. FZ mentioned a similar program development by Andreas Kabel at SLAC, which could serve for comparisons and cross checks. He referred to A. Kabel's and other talks on the web site of the FNAL beam-beam meeting in March 2004: http://www-ap.fnal.gov/~tsen/TEV/beambeam_march04.html (3) Update on Collimator Res.-Wall Simulations (AK) --------------------------------------------------- OLD ACTION: perform full simulation for the SPS AK used his program to compute tune shifts and rise times from resistive wall, for the bare SPS machine and with an additional collimator closed to a half gap of 2 mm. He considered parameters for the SPS collimator test (288 bunches in 4 batches at 270 GeV/c), and compared results for with and without inductive bypass. Without inductive bypass the collimator reduces the growth time from about 410 turns to 116 turns, with inductive bypass the growth rate is hardly unchanged. With inductive bypass the collimator creates a tune shift of 5e-4, without the bypass the tune shift is 1e-3. Especially the growth rate is sensitive to the impedance model, and the difference in the tune shift could be visible. FZ asked how the growth rate for 288 bunches would be measured and whether this has ever been tried? FR suggested to use the feedback for this type of measurement, and possible input from EV. ACTION => Clarify why B. Zotter's results are an order of magnitude larger than the results from Vos, Burov&Lebedev BZ will give a presentation in one of the next LCE meetings. (4) Update on LHC Impedance (EM) -------------------------------- EM discussed impedance calculations for 3 items: (1) beam screen, (2) warm pipe, (3) TCDS For the beam screen a 3 layer model is used consisting of 50 micron Cu, 1 mm Stainless Steel, and vacuum. The magnetoresistance of Cu is taken into account. The computed effective impedances agree with the results of Luc Vos in the LHC design report to within 10 or 20%, the only exception being the imaginary part at top energy, where EM's value is almost twice as large. Also the heat load at top energy agrees with LV's number. FR suggested to consider 75 micron Cu layer (and drop the RRR to the correct value) and to include the cold bore on the outside of the pipe. Also for the warm pipe, LV's values were confirmed. The TCDS is 6 m long. Its first part is made from carbon and carbon composites, which is followed by the isolator AlN, and then by blocks of Ti and Stainless Steel etc. EM assumed that only the AlN will be coated with a Cu layer of 5 micron. He finds that the impedance is about 1% of the total LHC impedance and the heating 185 W, whereas LV had calculated 260 W and 1 kW cooling is foreseen. FZ asked if the resistivity of the carbon changes with temperature. (This question would also apply to the collimators.) The impedance values that EM calculated differ from the usually defined impedances by a factor sb/(2 sqrt(pi) sigmaZ) which is 16 at injection and 28 at top energy. (5) Update on RF AM and Transverse Feedback (EV) ------------------------------------------------- EV discussed the bunch length variation as a result of AM modulation observed in the SPS experiments. The bunch length change measured from the start to the end of a batch was 6% in good agreement with theoretical expectation. After reaching top energy the difference dropped to 3-4%. This drop occurred at about the time when the rf voltage was raised to match the beam for (future) extraction into the LHC. Also, the absolute value of the bunch length (without modulation) appears to increase near top energy compared with the expectation, presumably as a result of collective instabilities. EV added 12-bit feedback quantization to his modeling code. The quantization noise has a small impact on the short-term emittance growth when damping an oscillation. FR and FZ suggested that the quantization noise could lead to long-term emittance growth. EV, FR and FZ mentioned literature by Daniel Boussard and Luc Vos on this topic. Possibly a model for the long-term orbit motion should be part of the long-term assessment. (6) BBLR Impedance (FZ) ----------------------- The BBLR assembly is nearing completion. The first new device will be installed in LSS5 on May 26. An impedance estimate had earlier been suggested by G. Arduini. DS agreed to such calculation, but no result is available yet. FR informed the team that following an agreement with the CLIC Steering Committee, in the future all Gdfidl simulations required for LHC will be done by E. Jensen and his coworkers in the RF group. (7) Follow up from ECLOUD04 and Future Plans (FZ) ------------------------------------------------- FZ presented some highlights and inspirations from the ECLOUD'04 workshops. There are lots of new ideas related to upgrades and better understanding of the codes ECLOUD and HEADTAIL. See the attached power point file for details. Two highlight presentations were the calculation of the electron cloud build up using fitted 3rd order polynominal maps and the search for phase transitions by U. Iriso, and the theoretical and experimental study of grooved surfaces with reduced secondary emission yield by G. Stupakov and M. Pivi at SLAC. (8) Report from PEP-II MAC (FZ) ------------------------------- FZ summarized presentations by S. Ecklund, M. Sullivan, A. Novoktahsky and A. Kulikov on HOM heating and electron cloud. HOM heating is somewhat higher than expected and a strong limit to present operation. Bellows and pumps are particularly affected. NEG pumps were found to be heated up to almost 1000 degree F, by HOM power entering through the pumping screens. The HOM power increases linearly with rf voltage. One of the suscpected sources are collimators. Crosstalk between the two beams was observed. Electron cloud is not a limitation at the moment, after doubling the strength of the solenoids and changing the working point. When swtiching off solenoids locally there is still a large multipacting and a high electron flux. The flux does not decrease with time (no conditioning?). A field of a few Gauss is sufficient to suppress the multipacting. Again, see attached slides for details. PEP-II luminosity per month was increased by about 50% in 6 months. PEP-II is thus on track for achieving the director's goal of increasing the luminosity by a factor 3 every 18 months. Attached: slides by AK, EM, and FZ