----------------------------------------------------------------------- Minutes of the ABP-LCE team meeting of 22.10.04 present: EB, AG, WH, TP, FR, DS, EV, FZ excused: EM ----------------------------------------------------------------------- (1) Minutes of last meeting, pending actions & news --------------------------------------------------- ACTION -> EM will check with BT experts and/or with FC whether 4A4 ferrite data is available at higher frequencies. Otherwise some analytic model will be used to extrapolate the available data to higher frequencies. STATUS: Pending. ACTION -> Estimate emittance growth time and beam lifetime from gas scattering in the SPS (FZ) STATUS: GA provided some vacuum data for 270 GeV/c. Data at injection will be recorded during two 26 GeV/c coasts in November. ACTION (EB) -> Plot results as a function of effective phase advance. Perform tune scan with a constant electron density Simulate the situation of the recent SPS experiments STATUS: Tune scan ongoing. ACTION -> Estimate whether/which measurements can distinguish between resistive-wall theories (EM) STATUS: FZ presented some results on this topic (see below) ACTION (FZ) -> Organize meeting of VB, JMJ, Ds, FR, FZ to discuss outstanding simulations. STATUS: No common free time slot could be found in week 43. Meeting is now planned for week 44 without FR. ACTION -> EM and/or FZ will review Landau-damping mechanisms. STATUS: pending. ACTION -> Finer tune scan near resonance for constant density (EB) STATUS: pending. ACTION -> EB will redo this comparison either without synchrotron motion, or with QS, Vrf and alphaC all scaled by a factor 2 as well. STATUS: pending. ACTION -> Study sensitivity of TMCI threshold for LHC beam to various impedance parameters (EM). STATUS: pending. 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. STATUS: Bruno Zotter will give a short talk mid or end November. He also recommended to move the tune closer to an integer to detect the low-frequency part of the collimator impedance in the SPS experiment. A similar suggestion was made by FZ during last week's MD. However, this could not be realized, since, approaching the integer from below - for an unstable situation - implies crossing the half integer, which according to GA is impossible at 270 GeV/c. FR announced that he will be absent for the next two weeks. He suggested to hold a meeting without him on October 29. (2) Preliminary results of the SPS collimation test: trapped modes (FR) ----------------------------------------------------- FR reported that Fritz Caspers and Tom Kroyer have observed trapped modes, when the collimator jaws were moved inwards. The mode power was more than 20 dB above the noise level. This means FC has won a bet. (3) Preliminary results of the SPS collimation test: measured vs. estimated impedance effects (FZ) ----------------------------------------------------- FZ compared predictions for tune shift, orbit deflection, head-tail growth rates, and multi-bunch instabilities, with preliminary measurement results. The predictions were calculated for the Burov-Lebedev impedance and for an approximation to Bruno Zotter's calculation result. Both predicted and measured tune shifts are of the order of 1-2e-4 in good agreement. However, the predicted dependence on the gap size is an inverse cubic one, while the measured dependence appears about inversely proportional to the gap, characteristic of a geometric impedance. FZ studied whether the nonlinear part of the wake field together with the finite size of the beam could explain the difference. He computed a Piwinski form factor as a double integral over source and test particle distributions cut off at the collimator gap. The result was that the nonlinear part of the wake should enhance and not lower the tune shift. Orbit deflections of a few 10ths of a microrad were expected. No deflection was observed by Ralph Steinhagen with a measurement resolution of about 1 microrad. To get a deflection above the noise level, the beam center should be closer than 200-300 micron to the jaw, at which point a large beam loss would occur. The expected change in the head-tail growth rates from closing the collimator is small, and comparatively largest at low chromaticities. Measurements were taken by G. Arduini and FZ. Within the error of the measurement the effect of closing the collimator was either invisible or with a sign opposite to expectation. All these single-bunch results were calculated to be the same for Burov-Lebedev's and Zotter's impedance. The two impedances make very different predictions for multi-bunch growth rates. For a uniformly fille dring, a few mutlibunch modes should have up to 100 times larger growth or damping times. In the SPS experiment, 4 trains of 48 bunches were stored. There was no obvious change when the collimator was closed. (4) Weak-strong simulations of LHC crossing schemes (FZ) -------------------------------------------------------- FZ presented the results of weak-strong beam-beam simulations for the effect of long-range collisions alone and the combined effect of long-range and head-on collisions. He compared results for xx, xy and yy crossing at the two main IPs, scanning the vertical tune over a total range of 0.05. The 0 amplitude tune was kept identical for each of the three schemes. Simulations were repated for both the nominal crossing angle and integer tune, as well as for a reduced crossing angle of 200 microrad and also for a horizontal integer tune of 63 instead of 64. In addition, start amplitudes were chosen along the lines x=y and y=0. In all cases, except on the coupling resonance, where the invariant calculation may encounter problems, the xy crossing exhibits a smaller diffusive aperture. The difference in diffusive aperture varies between 1 and more than 6 sigma depending on the tune and the scheme. In several cases, the diffusive aperture for equal-plane crossing is above 9 or 10 sigma. Frequency maps illustrated the qualitative difference between the crossing scheme. With xy crossing long wings expand in two directions, with equal plane crossing only in one. Besides, more resonances are excited with xy crossing. The frequency map results appear robust against small tune changes. A tentative explanation is that xy crossing cancels the dodecapole and 20-pole terms between IPs, leading to a different 'folding' of the frequency map. It also excites twice the number of resonances. (5) Preliminary results of beam-beam simulations (TP) ----------------------------------------------------- TP presented results of a strong-strong beam simulation with 4 bunches colliding against 4 bunches at 8 IPs. Varying the intensity of one of the 8 bunches can produce an additional line in the frequency spectrum of multi-bunch oscillations, for a certain range of relative intensity. It remains to be investigated whether this new line reflects a growing or damping phenomenon. This study attempts to understand in a systematic way, earlier observations, made when all bunch intensities were varied randomly. (6) LCE contributions to CARE HHH-2004 workshop ----------------------------------------------- FZ is coauthor on 3 posters, addressing vacuum effects in LHC, nonlinear collimation, and electron-cloud instabilities. FR recommended to DS and FZ to consider a poster on certified heat load predictions, which will no longer be subject to change. This can be a collection of results published earlier, with possibly a few additional ones covering the 75-ns upgrade option. FR suggested that the 12.5-ns option would also be looked at. DS was invited to be a secretary and participant in the panel discussion on electron-cloud effects. WH chairs the beam-beam panel discussion. TP may present a poster on beam-beam simulations. DS pointed out that present limits from heat load for LHC are delta_max=1.1 for 100% reflectivity of low-energy electrons, delta_max=1.2 for 50% reflectivity and delta_max=1.3 for 0% reflectivity. These numbers are not very different from what was published in the past. EV suggested to look at limits from longitudinal coupled-bunch instabilities. He may present a poster on this topic. (7) AOB ------- EV performed an error calculation for the measurements at the beginning and end of the batch for the CGNS extraction tests. The rms value is not a good measure. Considering the bandwidth and digitization accuracy he estimates the error to be about 20%. This means the measured performance is at the edge of the specified limit of acceptability. Therefore, he is now waiting for a response from CNGS. Attached: Slides by TP, FZ