Minutes of the ABP-LCE team meeting of 14.05.04 present: EB, AK, WH, FJ, EM, FR, DS, EV, FZ ----------------------------------------------------------------------- (1) Minutes & Follow-up of Pending Actions ------------------------------------------ OLD ACTION: EM will update the LHC collimators impedance according to the latest layout provided by R. Assmann. DONE. See report below. ACTION => Clarify relation between loss factor & synchronous phase shift. PENDING. EM ? (Energy loss depends on t and theta) ACTION => Clarify why B. Zotter's results are an order of magnitude larger than the results from Vos, Burov&Lebedev. BZ will give a presentation in one of the next LCE meetings. AK plans to repeat the derivation with field matching and four em potentials. PENDING. (2) Update on LHC Impedance I - Collimators for Phase I (EM) ------------------------------------------------------------ EM presented an update computation of the collimator impedance using latest parameters provided by R. Assmann, for LHC optics version 6.5. The collimators are here reduced by 30% both in number and total length. The total heat load of the graphite decreases accordingly. The coherent tune shifts for bare graphite are smaller than before, but lie still outside the stability diagram. A 5-micron Cu coating will stabilize the beam with some safety margin. (3) Update on LHC Impedance II - Res.Wall Impedance for TCDQ (EM) ----------------------------------------------------------------- The TCDQ is a 6 m long all-graphite structure, which is placed 8 sigma from the beam, both at injection and at top energy. The resistivity of the graphite is taken to be 6e-5 Ohm meter, which is to be compared with 1.4e-5 for the collimator graphite. ACTION => Clarify the origin of the difference in graphite resistivity. The tune shift induced by this device is large (almost 3e-4) without coating and it is still significant with coating. The beta function at the TCDQ is 10 times larger than the average beta function, which enhances the impedance effect. There are gaps between the graphite blocks. Trapped modes are unlikely to be important, since the LHC bunches are long, but the surface impedance might be affected. After the LCE meeting, FC recommended to check that adequate metallic structures provide continuous conduction paths for the low frequency image currents. (4) Report from Visit to USC and Future E-Cloud Plan (EB) --------------------------------------------------------- A primary objective of the visit was to learn the use of the QUICKPIC code. QUICKPIC is a 3D parallel simulation, originally developed for plasma wake field acceleration. The parallelization is based on the MPI protocol. In 2002 QUICKPIC was modified for electron-cloud applications in collaboration with G. Rumolo. A preliminary benchmark is available where a result obtained by QUICKPIC in a 1-kick approximation is compared with the corresponding HEADTAIL output. The emittance growth agrees within a factor of two. EB described the algorithms used by QUICKPIC to push the electrons and the beam. Among other features, QUICKPIC assumes a frozen-field approximation which reduces the beam-electron interaction to a two-dimensional problem. During EB's visit the smooth approximation of the transverse ring optics was abandoned, and replaced by a FODO cell model. Beam envelopes and betatron tunes were computed without electron cloud. It remains to resolve some discrepancies between this model and the actual SPS optics. As a next step, the electron motion must be calculated for the same magnetic field as that experienced by the beam at each location around the ring. EB plans to update the HEADTAIL code and to combine two recent extensions written by FZ (with localized rf cavity, e.g., for LHC) and by GR (with coasting beam option). The manual on the web will be updated as well. There is no new result on the code comparison at the moment. FR pointed out that the treatment of chromaticity by a delta-dependent rotation matrix renders the total 6D transformation non-symplectic unless a completing term is added to the longitudinal motion as well. He also hinted at possible synergies with the adaptive mesh used in the parallel code BeamX and at the concrete possibility of running QUICKPIC on a new parallel cluster being set-up at CERN with the help of E. McIntosh for beam-beam simulations. (5) Parallel Beam-Beam Simulations (FJ) ---------------------------------------- FJ reported on the status of the BeamX project, pursued in collaboration with WH. BeamX is a 3D self-consistent strong-strong simulation code, which includes many advanced features, such as crossing angle, hour glass effect and longitudinal bunch profile. For the moment, computing time has limited the application to a single bunch pair colliding at one IP. The 2D field solver is based on the FMM technique. The code employs Hirata's synchrobeam mapping. FR asked whether superbunches could be simulated. This is the case. The slice-pair interactions for one time step can be run on different processors which allows for a parallelization. The protocol is MPI. DS suggested that a further factor 2 in speed might be gained by computing already the next slice collisions, when not all processors are used. FJ outlined the FMM quad-tree adaptation subdivision, which ensures that the charge per grid cell is roughly constant independent of the beam density. Typical parameters in use for LHC simulations are 5 longitudinal slices and 1e5 turns. Further parallelization is being thought of. Parallelizing the FMM would be a major effort. An alternative approach might be the force decomposition proposed by Ellison, Vogt and Sen. The data flow and the tasks of masters and slaves were explained. The two beams are treated differently. Various performance tests were done. The dependence on the slice number demonstrated the merit of the parallelization. Several refinements of the model and of the computation are being considered. There are only few strong-strong beam-beam simulations in existence. Possibly BeamX is the only truly self-consistent code. There is a code by Chiang-Furman-Ryne which could be used for benchmarking. A. Kabel, Y. Cai, J. Shi, K. Ohmi, J. Rogers, and S. Krishnagopal have other strong-strong codes, which may not be 3D and not be self-consistent. (6) Report from ESRF (FZ) ---------------------------------------- FZ participated in an impedance measurement by his student T. Perron and L. Farvacque. The local impedance of insertion devices can be measured with a precision of a few percent. The impedance is determined by detecting the non-closure of local bumps at high single-bunch currents, for various gap sizes. Other measurements at various bunch lengths with a bump in an achromat confirmed a broadband impedance model predicted by GdfidL simulations, and suggest that an older model derived from coherent tune shift and growth rates was not a good representation. Global impedance measurements were done as well, using a similar recipe, and single-corrector excitation. T. Perron will give an AB forum on June 1. For longitudinal studies the streak camera will be re-commissioned. Surface measurements of the NEG unveil a a rough surface with spherical or column-like objects of micron size. FR emphasized that the resulting impedance should not be important for the LHC, but he agreed with FZ that single-pass effects deserve further investigation. (7) AOB ------- FR mentioned an upcoming experimental beam-pipes meeting on Thursday 3rd of June. Perhaps nobody from the team needs to attend. CMS had considered enlarging the vacuum chamber 17 m fro the IP for mechanical support reasons. They would be happy to reduce the radius, following DS' recommendation. Feedback from the vacuum group is now needed, considering the pumping. FR emphasized that the official baseline for the LHC crossing scheme is alternating H-V in the two main IPs. This is based on tune footprint considerations, which are more solid than simulations of nonlinear effects. Thus these should never be questioned outside the ABP group, unless new data become available. Some information may come from a dedicated refined beam-beam simulation study to be launched asap by WH and Dobrin Kaltchev (first priority: tune scan, second priority: investigation of alternating vs non-alternating crossing planes), other from this year's BBLR experiment. It is confirmed that GdfidL simulations are henceforth handled by Erk Jensen and colleagues in the rf group. Alexej Grudiev has already started computing the BBLR impedance. Attached: slides by EB, FJ, EM, and FZ