Minutes of the ABP-LCE team meeting on 02.04.04 ------------------------------------------------------------------------- present: EB, WH, JJ, AK, FR, DS, EV, FZ ------------------------------------------------------------------------- (1) Follow up on action items ----------------------------- => WH, DS, and FZ will assess specific CPU needs/plans for LHC beam-beam tracking and e-cloud simulation campaigns -> partly done? FZ and DS made independent estimates of the ecloud CPU needs. More important than CPU may be the disk space. 20 Gbytes of non-backup-ed afs disk space would be helpful for simulation campaigns. => perform full simulation for the SPS (AK) -> ongoing ACTION => compare with analytic estimates for uniform bunch filling (AK) A coasting beam model for 270 GeV gives 12000 turns growth time, while a simulation with 924 bunches and the same current yields 416 turns. => compare results and mathematica notebooks (FZ, EM, AK) Discrepancy was resolved. Luc Vos model and Burov-Lebedev now roughly consistent. => what can be measured in the SPS? -> pending ACTION (AK,EM,FR,DS,FZ) Presentations to the APC in the afternoon. A preview was given by FZ (see below) => FR will propose to provide DS with a fast desktop machine for e-cloud and possibly HFSS calculations -> pending ACTION DS will discuss with E. Jensen and provide more information to FR. In progress? ACTION => EB will extrapolate HEADTAIL results for 30 min operation and extend the simulations to SPS conditions (with feedback and dipole field). Partly done. Presentation below. (2) e-cloud build-up for several bunch trains in the SPS (DS) ------------------------------------------------------------- DS performed simulations for different electron reflectivity at low energies, and found that the build up in the second batch is sensitive. The onset of electron cloud can shift by 10 bunches, if reflectivity changes from 50% to 0%. Dependence on the pressure is also under study. DS computed the coupled-bunch wake on the next bunch. He confirmed his earlier result that the wake changes sign inside the bunch, possibly driving higher order modes. The wake could be used as input to multi-bunch simulations. DS also studies the effect of bunch-to-bunch intensity variation. There are two likely patterns: (1) intensity change for 12 successive bunches, which will be corrected in operation; (2) intensity difference for every other bunch. The second pattern is more difficult to adjust, and DS is performing simulations for this case, with +/-10% fluctuation. Lastly, DS is simulating the local scrubbing in dipole field, assuming the logarithmic dosing effect. There is a rather uniform scrubbing due to the logarithm. FZ recommended that the vacuum group apply a dipole field at the in-situ SEY detector, in order to get an experimental value for the width of the scrubbed surface. (3 LHC crossing angle topology (WH) ----------------------------------- Layout of the luminosity monitor depends on the plane of crossing. Cost savings can be made if the plane of crossing is fixed to within a few degrees. BDKI posed the question whether the crossing angle should stay flexible over a range +/-45 degree or whether the detector acceptance can be reduced. WH drafted a response to BDI which explains the requirements at each IP. The draft was sent to OB and FR. He remarks, for example, that in IP1 and IP5 the sign of the crossing angle could change, and that, if beta-star is increased, also crossing at 45 degree becomes possible. From the ABP point of view there is no strong reason not to fix the plane of crossing. However, background in the experiment might depend on the crossing geometry. FR suggested that it is not up to ABP but to the experiments to decide this question. JJ noted that CMS requires a small crossing angle for ion running. (4) CMS impedance (DS) ---------------------- DS mentioned that in impedance estimates for a modified CMS chamber, the average beta function was used. The real beta function at the location of the chamber, increases the expected tune shift due to trapped modes by a factor 10 to 1e-5. This chamber is 17 m from the IP, contact P. LePeuple. FR recommended that the experimental pipe impedance is considered as a package and not piece by piece. (5) HEADTAIL simulations for LHC (EB) ------------------------------------- EB described recent simulations, concerning the maximum acceptable e-cloud density in the LHC at injection. She showed the rise time as a function of density. Chromaticity may not helo much at low densities. EB also presented the rise as a function of chromaticity for a constant density of 6e11 m^-3. There is a change of regime near Q'~30 above which the rise time is nearly constant. If this rise time is real or the result of the simulation procedure is an open question. The present results suggest that increasing the chromaticity may not raise the e-cloud density required for a 30 minute emittance lifetime. (6) LHC collimator test in the SPS (FZ) --------------------------------------- FZ presented a review of his APC presentation, listing the main results. He found that the collimator impedance introduces a small effect. Best sensitivity is achieved with a single bunch at low emittance and a small gap. The signals that could be measurable with averaging are the change in the vertical closed orbit when one jaw is closed, with a predicted orbit change of the order of 15 microns, and possibly the coherent tune shift with a predicted shift of 2e-4. Horizontal orbit changes, current-dependent phase advance, head-tail growth rates, and energy loss should be less sensitive. (7) Experimental beampipes meeting (FZ) --------------------------------------- An experimental beampipes meeting was held on April 1st, organized by Ray Veness. Topic was the region between D1 and D2 in IR2. Questions concerning ABP referred to the aperture, the necessity of NEG coating, and the location of ion collimators and related loss map calculations. FZ informed JBJ and HB about pertinent points. The next meeting will be on June 3 and devoted to IR5. JJ recommended that a representative of the ion project be invited to these meetings. (8) Highlights from FNAL, SNS, KEK, and APAC (FZ) and from BNL (JJ) ------------------------------------------------- FZ summarized impressions and highlights from recent trips. In particular he discussed a simulation+analytical study of wire beam-beam compensation for Tevatron Run II and the results of a machine experiment with the Tevatron Electron Lens. Electron cloud studies at BEPCII show effect of octupoles, solenoids, and BPM bias voltage. Beam lifetime reduction due to the TEL could be similar to effect of electron cloud. See attached slides for details and the other items. JJ reported on e-cloud problems at RHIC. At BNL P. Thieberger gave a talk on secondary emission from ion impact. An optimum surface shape for minimum emission was determined. There may have been measurements of e-cloud in the cold arcs. A measurement of transverse beam echoes was performed by R. Tomas and coworkers. Attached: slides by FZ Comment by Werner Herr: Dear colleagues, a few comments and clarifications to the minutes of the last LCE meeting (2.4.04) Point number 3: a) The layout of the luminosity monitors does not depend on the plane of crossing, at least this was not the point of the discussion. As I have reiterated several times at the meeting: given a crossing plane the question was whether the monitors should have a 180 degree acceptance as written in the original functional specification. This would allow moving the crossing plane by +-45 degrees from its original plane. It does NOT allow to change to the orthogonal plane since a 360 degree acceptance was never considered. b) The sign of the crossing angle can change in IP1, but NOT in IP5 as clearly said in the draft. c) Crossing planes of 45 and 135 degrees in IP1 and IP5 as I had proposed in 1993 to minimize long range beam-beam effects would indeed be possible for a significantly larger beta*. However, such a scheme would introduce other problems such as coupling. Furthermore, if the additional loss of luminosity due to an increased beta* is acceptable, it could as well be used to increase the separation in the standard crossing schemes, avoiding additional complications. d) It was mentioned by WH in the meeting that the question of HV, HH or VV will be addressed again in studies this year. The final decisions should be taken after the studies are concluded, in collaboration with equipment groups and the experiments. e) To answer the original question, an action was assigned to FR and WH to reformulate the draft to avoid giving up the flexibility or the original specification. Werner Communication by Werner Herr From: Werner Herr Sent: Wednesday, March 31, 2004 9:23 AM To: Francesco Ruggiero; Oliver Bruning Subject: communication to JP Koutchouck Dear colleagues, JPK has contacted me again on the issue of the crossing angle topology. I had discussed with him some time ago and gave him my opinion but he also wants a formal statement by the relevant sections. Below I try to summarize my present understanding and suggest to send it to him as soon as possible together with your input as our common position (and that of the group). Possible requirements from future upgrades are not included. JPK wants this rather urgently so please come back to me quickly. If you want to discuss these issues more thoroughly, I can send him my personal comments first with the necessary reservations. IP2: The crossing plane is fixed by the orientation of the experimental spectrometer in the vertical plane. The sign of the angle can (and will) change when the polarity of the spectrometer is changed. Should be possible on the run to run basis. The precise values of the angles are presently discussed in the LOC section. From beam-beam considerations there is not need for significant adjustment of the crossing plane. For proton operation the beams are separated which probably has no consequences for the luminosity measurements (I believe). IP8: The crossing plane is fixed by the orientation of the experimental spectrometer in the horizontal plane. The sign of the angle cannot change when the polarity of the spectrometer is changed to avoid double crossing. The crossing geometry is therefore completely locked. The precise value of the angle is presently discussed in the LOC section. From beam-beam considerations there is not need for significant adjustment of the crossing plane. Please remember that the crossing plane is not exactly horizontal but also slightly vertical due to the orientation of the spectrometer and its compensators (Project Report 367). An implication for the luminosity measurement I do not expect. IP1: The present base line for the LHC foresees vertical crossing in IP1 together with a low beta configuration. The sign of the crossing angle can change and this flexibility is desirable for possible optimization (background etc.). This should be possible on the run to run basis. From beam-beam considerations there is not need for significant adjustments of the crossing plane. In particular an adjustment of +-45 degrees is not foreseen and excluded due to aperture considerations. IP5: The present base line for the LHC foresees horizontal crossing in IP5 together with a low beta configuration. The sign of the angle is locked to avoid double crossing. From beam-beam considerations there is not need for significant adjustments of the crossing plane. In particular an adjustment of +-45 degrees is not foreseen and excluded due to aperture considerations. IP1 and IP5: In the very undesirable case of a necessary change of crossing plane (after start of operation), the present configuration (beta and angle) is incompatible with the aperture requirements. (All for nominal parameters, parameters for commissioning not considered). The necessary reduction of the crossing angle must be compensated by an equivalent increase of beta* to maintain the necessary beam separation. Its minimum value is determined by the available mechanical aperture. This issue I cannot judge. For sufficiently reduced beta* a range of +-45 degrees becomes possible. -- --- W. Herr, AB-Division, CERN, CH-1211 Geneva 23, Switzerland Tel. +41 22 767 4781, Fax. +41 22 767 8480, E-mail: Werner.Herr@cern.ch Internet: http://cern.ch/Werner.Herr/