----------------------------------------------------------------------- Minutes of the ABP-LCE team meeting of 06.08.04 present: JJ, EM, TP, FR, DS, EV, FZ excused: EB, AK, WH ----------------------------------------------------------------------- (1) Previous Minutes & Open Actions ----------------------------------- Check how contact resistance of collimator jaws relates to the usual beam impedance and budget ==> ACTION EM & FR? STATUS: EM talked with Sergio Calatroni and with Fritz Caspers. The measured dc resistance at the end of each jaw for the collimator prototype is 30 mOhm. SC expects that this impedance is constant up to ~100 MHz. Still a conversion into transverse impedance needs to be done. Means are available by which the contact resistance for the LHC collimators could be reduced. FC was pleased that this problem is being addressed. After the LCE meeting FR discussed with Sergio Calatroni and left open the possibility to have sliding RF contacts on the outer side of the graphite collimator jaws, i.e. opposite to the beam, provided the contact resistance can be kept well below 1 mOhm. Sergio said this solution would indeed guarantee a better contact force and thus a lower contact resistance. The drawback is that the beam image currents may see a step of 2-3 mm at the jaw ends. On the other hand the transverse impedance decreases for a larger radial distance of the RF contacts from the beam axis. With the "nominal" optimum design this radial distance is given by the distance of the jaw from the beam (say 2 mm in the worst case) plus ~15 mm, while with the proposed "outer RF contacts" the distance would be increased by 2-3 mm. The effect of the geometric graphite step remains to be estimated. ==> Compare LHC e-cloud simulation for quadrupole (FZ and DS) STATUS: pending - quadrupole simulations done by FZ; discussion with DS still needed ==> ACTION: Compute energy loss in the LHC kickers due to higher-order modes STATUS: pending, unclear who would follow this up. FR informed the team that the LCE jobs on the CLIC computer cluster have been stopped by Ian Wilson, who asks for a concrete plan of LHC computations to be done over one year. ==> ACTION FR, DS, EM? Make computation plan for the next year. FZ suggested that access to parallel processing would be useful for many other studies. Examples of parallel codes that presently cannot be run at CERN include the electron-cloud code QUICKPIC, the Fermilab weak-strong beam-beam code BBSIM, and Werner Herr's strong-strong code BEAMX. DR pointed out that 2-processor parallel computer would already be extremely helpful for development work and to gain experience with parallel codes. The full-scale jobs could then be launched via the grid on computers in Germany e.g, via the 'grid'. FR mentioned that an LCE web site is under development by A. Schultheis. The web site will contain an interactive page, by which emails can be sent to the LCE team, which will automatically be stored into a web archive. FR announced that a beam physics note has been published by A Hofmann. The result is similar to the previous finding by EM. The final conclusion was not entirely clear. JJ inquired about the procedure for obtaining beam physics notes. There is no web archive. Juliette seeds out an email with abstract, when a note is published. Hard copies are then obtained directly from her. ==> ACTION (DS & FZ): create new standard version of ECLOUD STATUS: pending; this will be done after LINAC04 conference. ==> ACTION (DS & FZ): create sample input files for debugging of new code versions STATUS: pending; this will be prepared together with the new code version ==> ACTION (BZ) presentation of result for res.-wall collimator impedance in an LCE meeting STATUS: pending (2) Report on PS Adiabatic Capture MD (EM) ------------------------------------------- EM described recent progress in PS MDs on adiabatic capture. 6x10^12 protons were captures without loss, into well separated islands. The only outstanding points are (1) extraction, which would require new hardware, and (2) the equalization of the islands with the center bunch. At the moment the population of each of the 4 islands is about 14-15% of the initial number of protons. For equality, this fraction should be 20%. Last year a kicker was used to blow up the emittance and 18% per island could be reached in this way. The same procedure was not yet successful in 2004. The hardware is different this year, as a new octupole was installed in a location with large horizontal beta function. This octupole has improved the losses. (3) Longitudinal Loss Factors of Collimators and TCDQ (EM) ---------------------------------------------------------- Responding to a request by E. Shaposhnikova, EM has computed the loss factors for the two elements mentioned above. In collision, the total collimator loss factor is 0.2 V/pC and the TCDQ loss factor 0.03 V/pC without Cu coating. Coating would reduce the losses by a factor 10-100. For a final number EM waits for an updated collimator list from Stefano and Guillaume. A rough estimate during the meeting suggests that the total loss (2-3 kW) is about 10% of the energy losses due to the electron cloud at the cryogenic limit (30 kW). (4) Ion Luminosity Lifetime (JJ) --------------------------------- JJ reported work done by summer student Amy Nicholson, which will shortly be published as a beam physics note. Taking into account burn-off at the IP, gas scattering, IBS, and synchrotron radiation, the evolution of luminosity, emittances, and intensity were computed for various types of beams (e.g., lead ions or protons) with a variable number of IPs. Various modified and newly computed cross sections were taken into account. The results for the protons are preliminary and will be redone for a more precise IBS model. The results so far are thought to be roughly consistent with those calculated by FZ for the LHC design report. (5) BBLR Experiments Last Week (FZ) ---------------------------------- Two experiments on BBLR compensation were performed on 29.08. and 30.08. Participants included Gerard Burtin, Jean-Pierre Koutchouk, Federico Roncarolo, Jorg Wenninger and FZ from CERN, Tanaji Sen and Vladimir Shiltsev from FNAL, and Y. Papaphilippou from ESRF. Support during the MD was provided by several member of the BDI group and by EM and the PS crew. For the first time, an attempt was made to compensate the effect of one BBLR by another. The transverse position of the second BBLR and its excitation current were scanned to determine the optimum and the sensitivity. Tune scans were performed around the standard SPS tune for LHC beams, and around the LHC tune. Three conditions were compared: no BBLR, 1 BBLR and 2 BBLRs compensating each other. The compensation efficiency was found to vary substantially with tune. Near the 1/3 resonance the performance with 2 compensating BBLRs is the same as without any BBLR. However for 0.5 lower vertical tunes, the compensation improves the beam lifetime only by a factor 2 and stays well below the values reached without any BBLR. Recorded regularly during the MD were the orbits, 1000-turn data after applying horizontal and vertical kicks, wire scans at different times in the cycle, photo-multiplier signals of beam loss, and time dependent beam intensity (BCT). Attempts to use the scraper for diffusion measurements were not successful, partly due to problems with either the scraper software or the stepping motor and partly due to lack of time. Another BBLR experiment will take place in week 35. (6) Correct Modeling of Wake Effect by a Single Kick (EV) --------------------------------------------------------- For injection or extraction studies, a correct model of instabilities for a small number of turns is important. Rather than to apply a deflection once per turn, proportional to the beam offset at a given location, one can take into account the integrated effect of the resistive-wall wake field at different betatron phases around the ring, by multiplying the complex phase space vector with a complex factor whose real and imaginary parts are proportional to the offset of the driving bunch in position and slope. There was a concern that spurious damping occurs in this treatment, but after the LCE meeting a bug was found, and the approach now yields reasonable results. Attached: Slides by FZ