----------------------------------------------------------- Minutes of the ABP-RLC team meeting of 14.10.2005 present: EB, EM, FR, DS, FZ web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ (1) Announcements ----------------- FZ was contacted by Tanaji Sen concerning the expected orbit distortion at the IP, which is needed for Mokhov's simulations. FZ estimated it should be at least 10-15 sigma, and he told TS to contact OB or WH. TS is waiting for a response. Following a suggestion by FR, FZ has asked OB, RdM, JPK and TS to provide baseline optics versions in MAD format to be posted on the HHH web site. No file was received so far. Elena Shaposhnikova has asked FZ for considerations on the effect of an increase in SPS injection energy to 40-60 GeV on the electron cloud, for a report to the PAF on 17.10. She assumes that the bunch length remains the same. FZ did not expect a change in the electron density and a reduction in the multibunch growth rate. Single bunch instability thresholds and incoherent tune shifts should stay about the same as at 26 GeV/c. In response to this reply, ES suggested simulations by GR, DS or EB. EM reported on a meeting with Luca Bruno, Jean-Michel Fraigne, AG and himself on TCDD and TDI. Follow up meeting for TCDD will be held on 07/11/05 with results by AG. 4 Questions are to be answered for TDI. Guillaume has sent to EM an update of the LHC collimators parameters. EM will reiterate the LHC impedance and stability calculations with the new parameters. (2) Follow Up of Fast Instability at SPS Injection (EM) -------------------------------------------------------- EM revisited the fast instability in the SPS at injection. In addition to results presented previously, he now showed animated time evolutions of both the measurement and two theories (BBU and TMCI). The frequency of the instability is about 1 GHz in all cases. The instability seems to travel in opposite directions in the theoretical results compared with the measurements. The initial offset should be verified. EM also presented FFTs of the signals which were obtained by adding zero over most of the revolution period, and Fourier transforming the whole data set, which results in a periodicity of 40 MHz. For the TMCI case EM showed results obtained for coupling of modes 3 and 4, as well as for a single mode. The latter does not resemble the measurement. EM also presented curves of HEADTAIL phase shifts for various chromaticities. With nonzero chromaticity there is an apparent traveling instability, which for mode 0 inverts its motion after half a synchrotron period (which happens to be roughly equal to the time when beam loss occurred in the experiment). DS suggested to normalize the signal by the Gaussian bunch shape before applying the FFT. EM concluded that both theories reproduce the measurement if he assumes Q=1, 1 GHz resonant frequency, and a shunt impedance 10 MOhm/m. => ACTION EM will clarify the offset at the first turn and the offset at which beam loss occurs. (3) Impedance of PS Kickers for New CT (EM) ------------------------------------------- Measurements were done by FC and TK as reported earlier. The only theory which describes the type of geometry (combination of ferrite and conducting boundaries) is Hiroshi Tsutsui's. EM almost reproduced trhe curve shown by HT, but not quite (assuming 4A4 ferrite as did HT). The ferrite of the real kickers is 8C11. The theoretical longitudinal impedance agrees well with the measurement. The measured vertical impedance is three times larger than predicted (both real and imaginary part). The horizontal impedance has a sign opposite to the prediction. At high frequency the transverse impedance seems to approach the value of the Burov-Lebedev theory. EM noted that the kicker ferrites are blocks with a period of 24 mm (19 mm ferrite followed by 5 mm of Al), which is not taken into account by the theory. In the worst case the PS impedance will go up by 15%, with consequences for the nTOF beam. Its longitudinal emittance would need to be increased from 2.3 eVs to 2.5 or 3 eVs. Since two 200-MHz cavities have been removed from the PS, it is not clear whether such an emittance is feasible. Recent news from MG is that instead of 4 only 3 new kickers will be installed. (4) First Results with HFSS for the LHC Collimator (EM) ------------------------------------------------------- EM could reproduce most of HT's results. HFSS does not give any impedance below 1 MHz (this might be due to length of model considered), and several high frequency points are off scale. DS suspected that a remedy might b to reduce the mesh size, as he had encountered a similar problem with GdfidL. EM's simulations at the moment consider a wire with radius 0.5 mm or two wires with 2 mm distance. (5) Crab Cavities (FZ) ---------------------- FZ compared diffusion rates simulated by Ohmi with the Sen-Zorzano theory and found a better agreement than reported by Ohmi at HHH-2004. The proper relationship between action and amplitude diffusion needs to be taken into account. FZ derived an analytical formula for the luminosity degradation by a non-zero R22 matrix element between the crab cavity and the IP and illustrated it for three crossing angles. Another formula describes the effect of the crab rf frequency. For a crossing angle of 1 mrad, 400 MHz is acceptable, but 800 MHz is not, unless the bunch length can be reduced, e.g., by decreasing the longitudinal emittance. A formula by Yuri Alexahin describes the emittance growth due to random beam-beam offsets, including decoherence. The tolerance for 1% emittance growth per hour in the LHC is a random offset of 1.5 nm, if a feedback is active, and 0.6 nm without feedback. Presentations at Nanobeam'02 from RHIC and the Tevatron indicated orbit motion much larger than this, and large vibration amplitudes of the low-beta quadrupole cold masses. FZ asked whether such vibrations could be measured on one the LHC low-beta quadrupoles, in order to get a feeling for the magnitude of this effect in the LHC. FZ mentioned that EV is developing a phase stabilization system for the XFEL with a tolerance of 0.02 ps, i.e., tigher than the SuperLHC requirement. (6) Highlights from the FNAL IR Upgrade Workshop (FR) ------------------------------------------------------- Tevatron reduced beta* from 35 cm to 28 cm, increasing the peak luminosity by 10% and end-of-store luminosity by 5% (hourglass effect). Further decreases in beta* are excluded by a dramatic increase in Q'' and in the corresponding chromatic tune footprint. This should be one of the criteria for choosing the LHC IR layout. Crystal extraction with 78+/-12% efficiency has been demonstrated at the Tevatron, giving a boost to the SPS programme. Quench limits, radiation damage, and dynamic heat load of the IR magnets were discussed. A stainless steel absorber of 5 mm thickness, and correspondingly larger magnet coil aperture reduces the peak power deposition by a factor of 2. A magnetic TAS would need an integrated field of 15-20 Tm to be effective. A symmetric optics for flat beams was presented by John Jonstone. The tune footprint is much larger than for the nominal LHC. (SF is working on an asymmetric flat-beam optics, using the present magnet configuration). Helical quadrupoles were proposed to spread the debris uniformly in azimuth. => ACTION: the RLC team should provide a proposal for aperture and field requirements. Chromatic performance of all IR solutions should be evaluated. Miguel Furman now finds results in agreement with ECLOUD when re-diffused electrons are not included and a factor ~2 more LHC heat load with re-diffused electrons. Xialong Zhang has sent a draft report about recent ecloud measurements at the Tevatron indicating an emittance growth rate of 28pi/hr. This is an important input/benchmark for our LHC predictions. FR described the roadmap for the LHC upgrade studies, modeled after the ILC GDE. In mid-2006 a reference design report will be finalized. Alternative designs and their risks should be included, with proper definitions. (7) AOB (FR) ------------ Following up on the last APC meeting, FR suggested to put some emphasis on beam dynamics studies at CTF-3. Demonstration of understanding the drive-beam stability limits is important. Posted on the web: Slides by EM, FZ, and FR; email by EM. Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/