Minutes of the ABP-LCE team meeting on 28.11.03 present: GA, EB, WH, JJ, AK, EM, FR, DS, EV, FZ ---------------------------------------------------------------------- (1) Minutes of Last LCE Meeting and Pending Actions ------------------------------------------------------------------ ACTION -> EM will check possible longitudinal 'Yokoya factor' relating round and rectangular cases. EM verified that the longitudinal Yokoya factor is 1 in the two limits of a square and of two infinite plates. It is slightly smaller for intermediate values. So all previous calculations are exact. FZ confirmed after the meeting that the factor pi^2/8 mentioned by Karl Bane referred to a collimator impedance in the transverse plane. After the meeting FR pointed out the calculation in his paper "Resistive wall impedance as derivative of the electric capacitance for a beam pipe of arbitrary cross section" published as CERN-SL-95-95-AP, which is also found at http://prola.aps.org/abstract/PRE/v53/i3/p2802_1 The longitudinal loss factor here is the same for a beam travelling in the centre of a circular pipe of radius b or parallel to an infinite metallic plane of equal resistivity at a distance d=b. Moreover, a square pipe has the same longitudinal impedance as the inscribed circular pipe. Finally, for a beam travelling in the centre of a rectangular pipe, the resistive loss reaches a minimum value about 6% lower than that corresponding to a square pipe when the ratio between the sides of the rectangle is around 1.33. EM asked about the factor of 4 Pi found by E.S. This was related to LV's paper on the SPS impedance. ACTION -> Find ES' email (FR) Done. Email to LV is enclosed. ACTION -> Present example of Zbase usage (young ZBase users). EM has started discussions with TdA on ZBase. The ZBase directory is on afs. ACTION -> FR will check the access rights The ZBase program can be executed on linux and on windows. Young users are supposed to put numbers and formulae into ZBase rather than do offline calculations, e.g., length, resistivity, gaps, cavity trapped modes etc. The ZBase documentation is NOW linked to the collective effects page and can be accessed directly at the web address http://bruening.home.cern.ch/bruening/ZBASE/zbase.html AK has tried the ZBase program and found that most of the values are old. ACTION -> CB head-tail growth rates for updated LHC impedance (FR) Pending. ACTION -> DS will cross-check Roman-pot estimates by numerical simulations and follow-up the impedance of Roman Pots with Roger Perret. DS again discussed Roman Pots with Roger Perret. Drawings have changed since the last time. DS obtained drawings and explanations. Thickness of coating not yet well defined (about 10 micron Cu?). Two cases need to be calculated, with pots in and out. GA mentioned that a prototype will be ready for installation into the SPS by the end of the year. ACTION -> AK will repeat simulations with a constant injection offset. Done. ACTION -> FR will read and approve the draft note by AK. Done. ACTION -> EB, FR, DS and FZ will further discuss electron cloud instability studies and simulation strategies Done. List of actions was established. Presentation at LTC in 2 weeks or (preferred) early January before Chamonix. Computer problem with batch cluster. Linux is being installed on a computer in EB's office. (2) Collimator Impedance and Stability Limits (EM) -------------------------------------------------- Final data are available for IR7 (no information on IR3 yet). The data contain collimator positions and settings for 6 and 7 sigma aperture and the two beams. With 7 sigma aperture the two beams may be marginally stable both at injection and top energy. These results are without any coating. IR3 might slightly change the picture. FZ asked whether it is coincidental that the aperture in units of sigma is the same at injection and at top energy. FR mentioned a request to study the stability of the TOTEM beam, with 1/3 bunch intensity, 1/4 emittance and 2 times tighter collimators, 156 bunches. Rough estimate indicates some margin. The experiments also would like to decrease the collimator gap by another factor of 2. ACTION -> Stability Analysis for TOTEM Beam (EM) ACTION -> Revise Thresholds for Single-Bunch Instabilities (EM?) (3) Progress Report on GdfidL, ZBASE, and Multi-Bunch Instabilities (DS+FZ+EM+EV+AK) ----------------------------------------------------- EM discussed the Yokoya factor as mentioned above, using CAS CERN 95-06, and papers by Zotter, Gluckstern, Palumbo et al. FR mentioned two related papers, by Kurennoy and himself. DS reported that the new version of Gdfidl including the Napoly option is slow. It is being worked on. I.Syrachev also started running Gdfidl and they are now sharing the computers. Comparison with B. Spataro is on the way. There can be pseudo-trapped modes at the collimators. Results depend on the boundary conditions chosen. AK performed resistive-wall instability simulations with random and constant initial offset. No Yokoya factor was taken into account. Then the expected ratio of growth rates in the two planes is 52, while with Yokoya factor it would be about 2. The simulated vertical growth rate is about 40-45 turns for both types of initial offsets. The horizontal growth rate for a constant offset is 2000 turns. AK notes that there always is a 'latency' time until the exponential growth sets in. FR remarks that this depends on the initial bunch offsets: if they happen to correspond to the most unstable mode, no latency time is expected. (4) Progress report on Electron Cloud Studies (EB+DS+FZ) -------------------------------------------------------- EB summarized meeting with FR, Ds, and FZ. The HEADTAIL code will be speeded up with the help of DS. The results poorly converge with the number of IPs, which is ascribed to the integer part of the tune. Number of macroparticles and slices will be varied to check the sensitivity of the results. Experience at the SPS suggests a large chromaticity suppresses the instability. EB is now performing simulations for SPS conditions including damper and a large chromaticity. FR raised the question whether an analytical model can be constructed. This is already being worked on, but explicit solutions are difficult when nonlinear motion is taken into account. DS suggested to seed the instability with a well-defined initial condition and to look at a uniform profile. The analog or digital noise of the damper will be quantified and included. EV pointed to a paper by WH. EB also mentioned ongoing collaboration and comparison of HEADTAIL with QUICKPIC code. Contact is Ali at USC. FZ showed simulation results for e-cloud build up at LHC injection. There is a resonant like dependence with maximum heat load occurring for about half the nominal charge, and becoming negligible above 7e10 per bunch. The simulation considers gas ionization. The resonant dependence (which needs to be confirmed for elliptic chamber geometry) was seen for three types of field. The dependence on the secondary emission yield was also explored, for a dipole field. For preparing a baseline parameter set for the SPS MDs, accurate input numbers and several data files were provided by GA. FZ showed the apertures and computed beam sizes at 14 detector locations. Complete sets of numbers are now available for the two bunch spacings at injection and top energy. The transverse beam emittances are a factor 2 different for the 25-ns and 75-ns spacings, which might have some impact on the simulations. The bunch lengths from TB are smaller than previously assumed, but could have a systematic error. FR and EM remarked that the LHC beam parameters should be the same for the two bunch spacings. (5) Q-meter Specs and Spectra with Beam-Beam Collisions (WH) -------------------------------------------- WH has restarted coherent beam-beam simulations, extending them to multiple bunches. An increased forest of pi modes was expected and is seen. Aim of the study is to respond to a question from BDI (JPK) and to compare single- and multi-bunch tune measurements. The preliminary results confirm that excitation and measurement of a single bunch is clearly preferred. If several bunches are excited due to limited bandwidth, it would still be best to excite regular, rather than PACMAN, bunches to measure the response of one of the excited bunches only. (6) AOB ------- FR mentioned that Fritz Caspers is studying the possibility of a Schottky monitor in the LHC. Impedance implications should be estimated. EV will look at a related paper by McGinnis from FNAL. ACTION -> Study McGinnis paper and impedance of of a Schottky monitor for the LHC (EV) Enclosed: Slides by EM.