----------------------------------------------------------- Minutes of the ABP-RLC team meeting of 04.11.2005 present: UD, WH, EM, TP, FR, GR, RT web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/ ------------------------------------------------------------ (1) Progress report on coherent beam-beam simulations (TP) ---------------------------------------------------------- TP presented an overview of her present activites on coherent beam-beam effects (see slides attached). She explained that the main objective is to extend the present understanding and models to include a large number of bunches and arbitrary collision schemes. It is expected that this will change the behaviour of coherent beam-beam modes very strongly. Applications to beam diagnostics such as tune measurement procedures and feedback systems are foreseen. To address these questions she presented three different models she has developped: a semi-analytical model based on the analysis of the full one turn map, a simulation program using rigid bunches and a fully self-consistent multi-particle simulation. The latter is presently using a soft Gaussian approach for the field calculation. TP presented the advantages and disadvantages of the three models and their relative merits in her present studies. She proceeded by showing several collision scenarios and the consistency of the models and how she uses them to explain the observations. In particular, parameter variations from bunch to bunch and the consequences are investigated since all models are optimized to allow such variations. An unresolved question is a Yokoya factor she has obtained with the multi particle program which is larger than expected from previous simulations using the same approximation and which is closer to the theoretical value. A future extension could resolve this puzzle when the present approximation for the field calculation is replaced by the exact Hybrid Fast Multipole Method (HFMM). The disadvantage of this method is the largely increased computing time needed and it is foreseen to improve this by a parallelization of the code. Since most time is spent in the field calculations of simultaneous beam-beam interactions which can all be performed independently, this parallel mode can make it possible to study multi-bunch effects presently out of range. A proposal is submitted to the EPFL (Lausanne) to use the BlueGene machine where more than 8000 processors are available. TP presented first ideas on a possible implementation. (2) Operational scenarios for LHCb spectrometer magnet (WH) ----------------------------------------------------------- WH addressed two questions which were recently brought up concerning the operation of the LHCb spectrometer magnet and smaller beta* in IP8 for lower bunch intensities (see slides attached). He first explained the constraints imposed by the crossing schemes and in particular on the sign of the crossing angle. The latter is imposed by the need to avoid additional crossings and is fixed (negative for beam 1) due to the change from the outside to the inside aperture in IP8. The LHCb spectrometer magnet together with its compensator magnets introduces a horizontal crossing angle of +- 135 murad at top energy. The crossing angle needed for the separation at the beam-beam encounters is added in the same plane and must always be negative for beam 1. Changing the polarity of the LHCb spectrometer implies a different sign of the crossing angle produced by the spectrometer and therefore requires different external crossing angles to ensure the correct sign of the net crossing angle. This is foreseen and implemented in the nominal optics Version 6.5 for beta* = 10 m. To keep the luminosity above the required 10^32 cm^-2 s^-1 when the bunch intensity is smaller than nominal, it is requested to operated with reduced beta* in IP8. Running at smaller beta* imply different contributions of long range beam-beam effects and stricter requirements on the available aperture. WH showed the necessary crossing angles to ensure the nominal beam separation. For the case of beta* = 2 m this can be achieved for both spectrometer polarities with a maximum crossing angle of 210 murad. In the case of beta* = 1 m the necessary crossing angle of 255 murad for one of the polarities is not accepted for reasons of limited aperture. A reduced crossing angle of about 200 murad or below provides a significantly smaller separation at the long range beam-beam encounters (about 5 sigma). Although it is only foreseen for low bunch intensities, we do not consider this a robust running scenario. A further request concerned the ramping strategy of the spectrometer magnet and its operation at full field already at injection energy was discussed. The crossing angle in such a case would be as large as +-2.1 mrad. While for one of the two polarities this can be accepted, it is excluded for the other polarity since it would require an external crossing angle larger that -+2.2 mrad. We should therefore like to draw the following conclusions: 1. The LHCb spectrometer magnet cannot be operated at full field for one of its polarities. 2. We strongly suggest to drop the case beta* = 1m as an option to run with lower bunch intensities. (3) Update on the impedance of the "sausage" (EM) ------------------------------------------------- Last RLC Section meeting, EM presented first results on the calculation via HFSS simulation of the impedance of the "sausage" (round->round->elliptical connector). EM had shown a table with the resonance frequencies of different modes and the respective quality factors (see minutes of the last meeting). This time EM has given an estimation of the R_s factors (~10 Ohm) and power loss per mode for the first 10 modes. The power loss can be calculated pessimistically, if the frequency of the mode is a multiple of the bunch frequency, and values show peaks of about 14 W in the range 4-7 of the mode numbers. The power loss evaluated with HFSS considering the real frequencies of modes in the structure only show peaks up to 3.5 mW. FR says that there is a proposal for an experiment close to ATLAS, or to CMS or to both, for which a 8m long movable wall should go very close to the beam. A statement is awaited from our group saying whether that could potentially be a killer. A post-doc from Daresbury will soon start to work on this issue. FR will not be able to attend the TOTEM review committee and therefore has given the name of EM to replace him and give recommendations from the impedance point of view. Concerning the progress on HFSS simulations for the impedance of the LHC collimators, EM has shown plots in which there is a very good agreement between the analytical formula for the longitudinal impedance (Z_||/L) and the value computed with HFSS (FR argues that, as both results of analytical estimation and of simulation lie in the range 01-10 Ohm/m, it would be better to display the results on a scale which in not logarithmic and spans over more than 6 orders of magnitude, EM replies he has only used the same scale used by Tsutsui in his paper). There is already indication that some minor discrepancies should be corrected by re-launching the simulations with a finer mesh and higher precision. On the discussion of the previous meeting about what impedance one effectively measures when using one or two wires, EM shows results from Tsutsui (CERN-SL-Note-2002-034-AP). Tsutsui's formula clearly shows that the measured quantity reduces to the longitudinal impedance if the measurement is carried out with one single centered wire (even correctly scaled by the Yokoya factor accounting for the non-roundness of the chamber). With one displaced wire, one sees zero in the horizontal plane and the vertical impedance (some of its dipolar and quadrupolar components). With two wires, one sees the sum of horizontal and vertical dipolar impedances. Measuring with wires displaced at different distances, one should observe a parabolic increase of the "transverse" contribution to the measurement. This explains why there are different results when measuring with one or two wires, but it still does not explain why from measurements there appears to be a negative real part of the horizontal impedance. This might be due to the different materials in the two planes (good conductor in the horizontal plane and ferrite in the vertical plane), or to the finite length, or... GR says that preliminary results from HEADTAIL simulations on the TMCI thresholds in the SPS show a clear increase of the threshold with the injection energy. Going from 26 to 40 GeV, the TMCI threshold jumps from ~0.6 10^11 p/bunch to 1.5. At 60 GeV injection energy the threshold reaches 1.9. These thresholds have been evaluated without taking space charge into account. Space charge can raise the threshold at 26 GeV up to about 0.8, and surprisingly enough it still seems to affect the threshold even at 40 and 60 GeV. Simulations were done keeping the same longitudinal emittance in all cases and re-matching the bunch to bucket each time. Also, the transverse normalized emittance was kept constant, which translates in smaller beam sizes for higher injection energies. EM observed after the meeting that the increase of the TMCI threshold is perfectly consistent with his formula, which has a proportional to eta dependence. Posted on the web: slides by TP-WH, EM-AG (sausage), and EM (Tsutsui). Web site: http://ab-abp-rlc.web.cern.ch/ab-abp-rlc/