Minutes of the ABP-LCE team meeting on 05.12.03 present: RA (partially), EB, WH, AK, EM, FR, DS, EV, FZ ---------------------------------------------------------------------- (1) Minutes & Pending Actions ----------------------------- WH and FR suggested that in the LCE meetings relevant reports from other meetings be made. In the minutes, 'old' and 'new' Actions should be marked in a different way from now on (use small and capital letters). ACTION -> FR will contact LV concerning factor 4 Pi. Pending. This factor and implications still to be confirmed with LV. ACTION -> Present example of Zbase usage (young ZBase users). Some problems with Zbase encountered (see below). ACTION -> FR will check the Zbase afs access rights Still to be done. ACTION -> CB head-tail growth rates for updated LHC impedance (FR) Pending. Requires response to JPK. During SPS scrubbing large values of xi (1-2) are required to suppress e-cloud instability. Bounds on Q' in LHC look soft. They are determined by dynamic aperture. Q' values of 10-20 units might be acceptable. action: DS will cross-check Roman-pot estimates by numerical simulations and follow-up the impedance of Roman Pots. Calculation in the open position gives R/Q=9 Ohm at 2.4 GHz. Significance to be evaluated. Some results were lost by computer problem. ACTION -> AK will check CB rise time for this impedance. ACTION -> Stability Analysis for TOTEM Beam (EM) Pending. ACTION -> Revise Thresholds for Single-Bunch Instabilities (with TOTEM?) (EM?) Pending. Answer to be provided before the end of the year. ACTION -> Study McGinnis paper and impedance of of Schottky monitor for LHC (EV) The interest is only in the Schottky monitor, not in the stochastic cooling. The question is whether the pick up is compatible with the LHC impedance budget. Results from FNAL may need to be scaled, unless the same device is installed. Then scaling law needs to be found. Discussion with FC recommended. ACTION -> FR will read and approve a paper by EM on "Longitudinal Bunched-Beam Coherent Modes: From Stability to Instability and Inversely". Pending. (2) Collimator Impedance (EM) ----------------------------------------------------- EM discussed the coherent tune shift due to the collimator impedance, combining the effects of IR7 (v26top6n1b1) and IR3 (June version). He scanned resistivity and gap of the secondaries for the cases without coating and for a 1-micron Cu coating, whose resistivity is taken to be 15e-9 Ohm metre. It was remarked that the coherent tune shift is not linear in the resistivity, due to the inductive bypass. Without coating and with a 7 sigma secondary gap, the beam is always unstable. It can however be stabilized if the secondary collimators are opened by 40% to about 10 sigma. The dependence of the tune shift on the gap appears to be inversely quadratic. Even with 1 micron Cu coating the beam is still marginally unstable, but it might become stable for a 10-micron coating (last hypothesis to be confirmed). A factor of two decrease in resistivity does not have a dramatic effect on the coherent tune shift. These slides will be presented by RA at the upcoming LHC MAC meeting. EM will attend the MAC closed session on collimation. It was suggested to prepare a few back up slides explaining the calculation and in particular the inductive bypass. (3) Progress on ZBase and Impedance (AK, DS, FR, FZ) ---------------------------------------------------- A meeting was held by EM, TdA, and FZ. ZBase was successfully launched and a new directory was constructed. Zbase consists of 8000 lines of script. How new formulae or additional data feature are added is still not transparent. There does not seem to be an option to convert impedance into wakes or vice versa. It might be that most results are computed beforehand with standalone programs. TdA suggested to organize a talk or tutorial by OB. First impedance item FZ is looking at in detail are the BPMs. Impedance for three types of BPMs were computed by DB, LV et al. The BPM list from BDI counts more than 20 types of BPMs, the MAD file has about 7 types. The interrelation between these items is yet to be determined. Formulae for stripline cavity might have missing factor two, which is to be checked. AK has interpolated the Yokoya factor as a function of aspect ratio. FR mentioned that ABP feedback is expected on the impedance of the VELO detector, especially the necessity of coating. Team endorses the suggestion to apply 10 micron Cu coating on stainless steel. FR pointed out that in the future other team members should follow up such type of questions. (4) Memo on Q meter & Beam-Beam (WH,EV) ---------------------------------------- A draft memo to JPK is being prepared. It contains simulated tune spectra for various scenarios including head-on and long-range beam-beam. there is a huge difference in the tune spectra for parallel-plane and alternating-plane crossing. A doctoral student will start working on beam-beam in March. EV is organizing a meeting with the RF (WH) and BDI groups to discuss the possibility of using the feedback for beam-beam diagnostics. ACTION -> Organize ABP-RF-BDI meeting on beam-beam diagnostics with feedback (EV). (5) Electron Cloud (EB, DS, FZ) ------------------------------- FZ compared simulations for an elliptical and original geometry for the LHC at injection. Above 4e10 protons per bunch there is a large difference in the results. Which one is more correct is not clear. It was suggested to compare with the result for two parallel plates or to implement the exact boundary condition for the original geometry (flattened circle - space charge presently calculated from the inscribed ellipse). First results are available for WAMPAC1 and WAMPAC3. The former shows no multipacting and negligible electron-cloud heat load up to a delta_max of 2.0. For WAMPAC3 the simulation shows a large heat load (several W/m) already with delta_max=1.5. The implementation of the bias voltage does not look correct. Simulations are being continued. DS continued SPS simulations for the cold strip detector. Assuming that the scrubbing was inhomogeneous, he can qualitatively reproduce the observed two stripes, but their distance still differs from observations by almost a factor of 2. Elliptical chamber geometry gave a good agreement for the dependence on the magnetic field. The rectangular chamber gave quite different results, which might be due to the crude handling of the beam image charges. Results with an improved version are expected by today. Qualitatively, the simulations for an elliptical chamber show that the build up starts earlier in a dipole field than in a field-free region, consistent with the measurements. Until now, for the rectangular case it has been the opposite. The simulated dependence on bunch length is also similar to the experimental observations. DS observes a clear well-defined threshold in the simulation, while FZ expressed some doubts that such a threshold could be defined unambiguously, especially in cases where photo-electrons are important. EB has checked the sensitivity of the HEADTAIL e-cloud simulations to the number of macro-protons, the number of slices and the number of macro-electrons. It appears that the number of macro-particles is well chosen, while the number of slice might be decreased by a factor of ~2. The convergence appears better in the case of 1 IP as compared with 5 IPs. This might seem counter- intuitive, but it could be explained, if the underlying mechanism of emittance growth is not the same. EB has included a damper in the HEADTAIL code. She presented a result containing only a beam kick, damper and nonzero chromaticity, illustrating that the damper acts on successive synchrotron period, even if the chromaticity initially leads to a fast decoherence. Simulations with electron cloud and damper for the SPS are in progress. EB also simulated the SPS TMCI thresholds for different charges, chromaticities and longitudinal emittances. Her results look consistent with EM's analytical predictions. EM showed that a chromaticity xi of 2 units raises the threshold by a factor of 4. According to EM, only 1 resonator is critical. The frequency of the resonator could be inferred from the threshold behavior. EB will implement several resonators in HEADTAIL. A meeting was held by A. Krasnov, FZ and partially FR. A. Krasnov proposes e-cloud scrubbing & reflection experiment with a closed chamber encompassing a hot filament in Novosibirsk. Many aspects could be studied, especially if the device can be pulsed. Xiaolong Zhang (FNAL) will work on e-cloud simulations for LHC as part of the US LARP. FC asked if FR could present the slides on SPS microwave experiments at the BNL workshop. Attached: Slides by EM and FZ.