------------------------------------------------------- Minutes of the ABP-RLC team meeting of 29.04.2005 present: EB, UD, AG, WH, EM, TP, FR, FZ web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------- (1) Minutes of the last meeting and pending actions ---------------------------------------------------- => ACTION => compare the size of the longitudinal geometric wake with RW longitudinal wake from A. Koschik (FR) STATUS: FR will talk to A. Koschik => ACTION => attempt to derive a general nonlinear theory including the inductive bypass (EM) STATUS: PLANNED (?) (2) Reports from other meetings & discussions --------------------------------------------- EM reported from the Collimation Working Group meeting. The collimation team has asked for the sensitivity of the tune-shift predictions to the centering of the beam between the collimator jaws. => ACTION => check sensitivity to centering with Piwinski theory (FZ) Further data analysis by Stefano Redaelli is needed for the actual centering in the experiment. But, roughly, a positioning accuracy of 50 micron is expected for each jaw, allowing for a shift in the beam centering by up to 100 microns. The DC resistivity of the collimator graphite was measured to be 10 microOhm-metre in all directions. Bench measurements on a collimator prototype will start in 2 weeks. EM mentioned a discussion with Fritz Caspers on the low-frequency impedance, and a confusion between thick-wall and thin-wall formulae. He points out that there are two different effects: inductive bypass and current redistribution. EM plans to still examine the possibility of an inductive bypass effect for the longitudinal impedance starting from B. Zotter's general approach. Luc Vos' paper had suggested that there is no inductive bypass longitudinally. FR related a new explanation of the SPS microwave experiments in terms of a 'magnetron effect' by Fritz Caspers. FZ added that a lab experiment of the magnetron effect is foreseen for next week. (3) Pressure rise induced by the electron cloud in the LHC (FZ) --------------------------------------------------------------- The question arose whether e-cloud scrubbing would be needed in the LHC. Effects possibly requiring scrubbing are the vacuum pressure increase, the heat load, fast single-bunch instabilities, and long-term emittance growth. FZ investigated the vacuum pressure evolution by discussing with Vincent Baglin. The vacuum model goes back to W. Turner. In the presence of electron cloud the vacuum pressure is directly proportional to the electron flux and to the desorption yield. Both factors decrease with time. The surface coverage of the beam screen is predicted by Noel Hilleret to be 10% of a monolayer in the worst case. The gas density can exceed the 100-hr lifetime limit for a short time, for nominal electron flux, but will drop below the 100-hr limit after 17 hours of nominal operation. Synchrotron radiation alone would lead to a rapid improvement of the vacuum to a level corresponding to 10000 hr beam lifetime. A combined calculation of SR and e-cloud induced pressure rise is not available. A delta_max of 1.5 required to store the nominal intensity, could be reached after 4-12 hours. A few weeks of TOTEM beam operation could contribute to the scrubbing of the beam screen by synchrotron radiation and photo-electrons. FR remarked that the initial gas densities employed in the model calculation could be far too optimistic. He also suggested that the emittance growth may set a tighter limit than the beam lifetime. FZ pointed to a recent report with B. Jeanneret for HHH-2004 where the emittance growth was estimated. (4) Electron cloud in the quadrupoles (FZ) ------------------------------------------ FZ presented a comparison of simulated e- trajectories in a quadrupole field using 5 different integration routines. All methods exhibits a slow drift of e- towards the two diagonals. The simulated heat load indeed is concentrated in a narrow region at the diagonal. Two reasons for the drift could be identified: one is the combined effect of elastic reflection plus the geometry of field lines and chamber boundary, the other is an artificial effect arising from the algorithm used to determine the secondary emission point on the surface in the case of particle loss. The artificial effect might be removed by some modification of the e-cloud code. FZ mentioned that the space charge forces are not calculated correctly for a narrow e- distribution on the diagonal. AG and FR suggested that an analytic estimate should be possible, assuming a uniform electron distribution along the diagonals. (5) Electron-cloud induced emittance growth below the threshold of the TMCI (EB) --------------------------------------------------------------- In collaboration with G. Franchetti (GSI), EB explores a model of emittance growth based on a trapping/detrapping mechanism similar to that successfully used to reproduce the results of space-charge experiments (see G. Franchetti's talk at HHH-2004 workshop). Giuliano considers a simplified model of the e-cloud distribution, where the cloud density increases linear in z and has a transverse Gaussian distribution with an rms size equal the beam size divided by sqrt(2). He finds an emittance growth which occurs mainly in the tails (halo formation). HEADTAIL uses a more realistic e- distribution which is strongly pinched during a bunch passage, giving rise to a narrow spike at the center and rings moving radially outwards. The HEADTAIL simulation primarily shows a beam-core blows up, which in addition is faster than the growth observed by Giuliano's code. K. Ohmi at KEK followed an approach whose basic model is similar to Giuliano's, and where the emittance growth is attributed to Arnold diffusion. Several team members remarked that differences in the electron distribution could explain the difference in the simulated emittance growth between HEADTAIL and Giuliano's code. FZ mentioned that another difference is the use of an analytical formula for the e- force by Giuliano and the interpolation on a grid in case of HEADTAIL. => ACTION => Implement analytical formula of Giuliano in HEADTAIL to see whether his result can be reproduced (EB). => ACTION => Repeat simplified simulation for a much narrower e-cloud distribution (GF) FR suggested to look for the position of resonance islands in phase space. He also remarked that the model of the e-cloud field is not symplectic, since the transverse kick is taken to change with longitudinal position, but there is no corresponding longitudinal kick. (6) AOB ------- Following a request by Volker Mertens, there will be a meeting with the LHC Injection WG to discuss the overhall vacuum chamber layout around the injection zones. Posted on the web: Slides by EB and FZ (2x) and movies by EB (2x) Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/