Minutes of the ABP-LCE team meeting on 24.10.03 present: EB, WH, JJ, AK, EM, FR, DS, LV, FZ (1) Outstanding Actions ----------------------------------- ACTION -> Check sign of real part of impedance (DS). Perhaps done? ACTION -> EXCEL spreadsheet for collimator impedance (EM). Done (see below) (2) Automatic collimator impedance calculation (EM) --------------------------------------------------- EM prepared a program which computes various impedance properties of the LHC collimators, using as input the excel spreadsheets sent by Ralph Assmann in regular intervals. There are 4 primary and 16 secondary collimators, all made of carbon with an assumed resistivity of 14e-6 Ohm m. Length of the primaries is 0.2 m, length of the secondaries 1 m. Rotation angles and plane (H,V,S) are defined for each collimator. ACTION -> Ambiguity in the angle definition needs to be clarified. FZ asked why some angles differ by odd amounts from 0 or 45 degree, etc., reminiscent of sextupole rotation angles in the SLC RTL. For the longitudinal impedance, EM used the formula for a round pipe. This should be a pessimistic estimate. ACTION -> check possible longitudinal 'Yokoya factor' relating round and rectangular cases For the transverse impedance, EM includes Yokoya factors, Luc's expression for the inductive bypass, the tensor transformation by Francesco, and the beta functions at the collimators. He sums over all collimators to compute and plot the total real and imaginary impedance, as well as the ratios between the two planes. The standard collimator version 6.4 has a 20% larger impedance in the vertical direction, while the so-called 'option 1' promises an equal and 2-3 times smaller impedance in both planes. EM's program also gives the coherent tune shift on the stability diagram for the most unstable coupled bunch mode. The nominal LHC beam is unstable for both collimator versions, but much less so for option 1. Some discussion ensued as to whether the tune shift was consistent with Luc's earlier calculation. EM's program at the moment does not yet handle the case of coating. A further output of the program is the resistive heating of the collimators, which is of the order of 140-200 W/m for a half gap of about 2 mm. The power scales as the inverse of the gap size. ACTION: put numbers into ZBASE (EM) JJ pointed out that a conversion program is available from EXCEL to mathematica format. (3) TMCI threshold in the SPS (EM) ---------------------------------- A few weeks ago an instability was observed when a low-emittance beam of 1.2e11 protons was injected into the SPS. The beam loss occurred after a few ms, and was suppressed by a high chromaticity of 1 or 2 units and also by increasing the longitudinal emittance. This instability could be either a head-tail or a TMC instability. EM presented his general framework for TMCI instabilities, and compared it with earlier MOSES calculation by Elena Shaposhnikova, for leptons at 3.5 GeV. His simplified formula gives nearly the same minimum threshold as Elena's calculation. The dependence on the bunch length is qualitatively the same (quadratic), but there is a factor of 3 difference, which is tentatively attributed to the difference between Gaussian and parabolic bunches. It is remarkable that the simplified formula extends over a huge range of the MODES calculation, over which various different mode mergers define the threshold. The formalism can be applied to the e-cloud instability, where EM found that shorter bunches are more stable. Also the longitudinal TMC instability can be treated in the same manner, after including the potential well distortion from space charge and wake fields. Linear coupling can increase the threshold, but so far no experiment was done to demonstrate this for the e-cloud instability in the SPS. (4) ZBASE GdfidL work (DS) -------------------------- DS computed the res. wall impedance for a 10 cm long carbon collimator. He plotted the ratio of the computed real and imaginary impedances divided by the Piwinski prediction and found a difference by at least an order of magnitude (Gdfidl giving the smaller number), which increased for larger frequencies. After contacting Warner Bruns the conclusion is that GdfidL cannot be used to compute the impedance for carbon, since it is still too good a conductor (the ratio of epsilon/kappa determines the time step required). A factor 1000 times shorter time step would be required, which is not practical. Presumably HFSS has to be used instead. As a further check, DS is running a GdfidL job with much increased resistivity. DS also simulated the impedance and wake from a smooth tapered step, and got a reasonable result, except for long-term growth which appears to be a numerical artifact, since it decreases for a finer mesh. He checked the impedance for an unshielded LHC bellows, comparing the ZBASE data from ABCI with new GdfidL results. The wakes look quite different. Nevertheless the impedances computed by the two codes seem similar. FR encourages DS and other young ZBASE users to check the element description with real world. There was no urgent desire to re-distribute the workload for the ZBASE review and update. (5) LHC Ions (JJ) ----------------- A substantial revision of parameters has taken place. No longer are there 200-MHz capture cavities. The longitudinal emittance was reduced accordingly. The emittance is later blown up to reduce transverse IBS growth rates. At top energy the IBS growth rates are comparable to the radiation damping. Therefore, no emittance blow up is expected. The radiation damping time for ions is half that of the protons. JJ is studying the possibility of changing the partition number to shrink the horizontal emittance. There is no crossing angle, and the geometric luminosity loss factor is 1. JJ showed beautiful 3-D animation of beam sizes and apertures around the IP, which can be created with a mathematica package. (6) Update on e-cloud simulations (EB) -------------------------------------- EB compared simulations of emittance growth due to electron cloud with and without conducting boundary condition. The latter reduces the growth by up to 50%. It also affects the centroid motion and the head-tail oscillations along the bunch. If the emittance growth for low electron densities is real or not is still an open question, but a benchmarking study is underway in collaboration with Ali Ghalam at USC. Restricting the QUICKPIC code to 1, 2 and 4 interaction points (IPs), instead of the usual quasi- continuous description, the emittance growth in QUICKPIC is seen to depend strongly on the number of IPs. This looks similar to the HEADTAIL behavior. Detailed comparisons will be performed between the two codes. In particular the low-density region is of interest. A study of the geometric collimator wake field using HEADTAIL is also in progress. (7) Report from PEP-II MAC and Factories'03 (FZ) ------------------------------------------------ PEP-II and KEKB pursue upgrade programs which in the short-term should increase the luminosity by a factor of order 4 and in the longer term should push the luminosity to 1e36. The recipe is more bunches, higher bunch current, higher beam-beam tune shift, and lower beta-star (plus shorter bunches). DAFNE considers strong rf focusing as approach for the future. Electron cloud observations at KEKB suggest that the solenoids have no effect for 4-ns bunch spacing, unlike for 6-ns and especially 8-ns spacing. There is a concerted effort to benchmark strong-strong beam-beam codes, involving 5 codes SLAC, LBNL, KEKB and Cornell. Many of the codes were also benchmarked against one or several colliders. FZ prepared a summary table in response to a suggestion by D. Rice. K. Ohmi showed that the beam-beam limit in the strong-strong simulation arises from an incoherent effect, and that it can be reproduced in a much faster 'quasi-strong' simulation. Upgrades for the PEP-II multi-bunch feedback system work with a time resolution of 0.5 ns, and can damp the m=1 head-tail mode in addition to the rigid bunch motion. (8) Report on other ongoing e-cloud activities and ZBASE (FZ) ----------------------------------------------- G. Bellodi performed a very detailed comparison between ECLOUD and POSINST parametrizations of secondary emission. The simulated line densities is almost a factor 10 different between the two models. POSINST includes re-diffused electrons, and the energy distribution of the true secondaries is markedly different. The ELCOUD'04 workshop is scheduled for April 2004, and CERN is one of its sponsors, contributing to the web page and to the poster design. The progress on the ZBASE assessment is slow. Two persons promised to send MAFIA files and data for the cavities (D. Angal at ASTEC), for the Y chamber (D. Li at LBNL). W. Hofle sent information on the transverse damper, and asked for ABP/LCE resources to update the MAFIA computations. B. Spataro lost all his LHC MAFIA files due to a computer crash in Frascati. Attached: slides by EM, EB, FZ.