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19th HiLumi WP2 Task Leader Meeting
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Europe/Zurich
6/R-018 (CERN)
Minutes of the 19th WP2 Task Leader Meeting held on 18/10/2013
Present: G. Arduini, R. De Maria, G. Iadarola, T. Pieloni, B. Salvant, A. Valishev, A. Wolski, P. Fessia
Minutes of the last meeting and follow-up of actions
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The minutes of the last (joint WP2-WP5) meeting have not been issued yet. Action: Stefano.
Electron cloud in the HL-LHC triplets – G. Iadarola
Giovanni illustrated the simulation setup for LHC triplets using Py-ECLOUD. 10 slices per triplet have been used to see the effect of the varying spacing. The shape of the beam screen is critical and it has been modelled accurately both for the present LHC triplet and the HL-LHC triplet.
For the present triplet geometry:
The e-cloud heat load is smaller at the location of the parasitic encounters. The SEY threshold goes from 1.6 for single beam to 1.15 for two counter-rotating beams with 50ns. For 25 ns the threshold with one beam is already low and it is very close to 1 for both beams. Above threshold the difference in heat load between 25 ns and 50 ns is smaller. There is little effect of ramp, squeeze and adjust on the heat load. After operation for 2 years with 50 ns beam and the 25 ns runs the estimated (from several independent observations) SEY has reduced to 1.2 to 1.3. Gianluigi suggested to analyse the heat load data at injection during the BSRT high intensity tests and during the intermediate energy run at the beginning of 2013 in order to have additional information on the effect of the 25 ns scrubbing run on the SEY.
For the HL-LHC triplet geometry:
For HL-LHC beams the scenario the nominal HL-LHC beam parameters have been used. The simulations show a qualitative difference in the electron distribution during the beam passage with a wider distribution of the electrons along the poles and with electron trajectories connecting adjacent poles. The heat load has been estimated for different values of the SEY. SEY close to 1 are required to suppress multipacting. The heat load in D1 and in the correctors has not been estimated yet. A pessimistic extrapolation can be made by assuming a constant longitudinal heat load density distribution. Measures to suppress the electron cloud are foreseen for HL-LHC:
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Coatings (note after the meeting: Lucio communicated that carbon coatings with SEY as low as 0.95 are going to be tested in COLDEX at the SPS for possible application in the HL-LHC triplets);
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Clearing electrodes.
In the discussion the following points emerged:
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SEY as low as 1 might be difficult to achieve (in particular in a cryogenic environment) and for that reason it is proposed to include the heat load corresponding to an SEY of 1.1 (~300 W in the approximation above indicated for the correctors and D1) for the estimate of the heat load on the beam screen. Note after the meeting: Lucio communicated that carbon coatings with SEY as low as 0.95 have been produced in the laboratory by P. Chiggiato and team and they are going to be tested in COLDEX at the SPS for possible application in the HL-LHC triplets).
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The design parameters of the clearing electrodes should be studied (e.g. required voltage and polarity to suppress multipacting) and discussed with TE/VSC;
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The heat load in the present triplets should be evaluated for the IP2 and IP8 configurations and the nominal HL-LHC parameters;
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It was suggested that similar suppression/mitigation measures should be considered for the new components of the matching sections in IP1/5.
Action: Giovanni to follow-up on the above aspects.
The above results should be transmitted to Laurent Tavian: Action Riccardo.
Update on beam-beam and crossing angle scaling for RLIUP scenarios – T. Pieloni, A. Valishev
In the filling schemes for post-LS1 and HL-LHC non colliding bunches in IP1 and IP5 requested by the experiments for beam gas calibration might become unstable if they are not colliding head-on in IP2/8. All the simulations shown in the presentation assume:
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only collisions in IP1 and 5 without crab cavities. No collision in IP8 and 2. In the past simulations had shown that collisions in IP8 are not affecting significantly the dynamic aperture.
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The dynamic aperture is provided in nominal HL-LHC sigmas (2.5 mm).
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No imperfections have been assumed in the model.
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The energy of 7 TeV has been considered.
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The simulations refer to the bunches with the maximum number of long range encounters.
Gianluigi asked Sasha and Tatiana to indicate clearly the parameters used for the simulations in the plots. Action: Sasha, Tatiana.
Tatiana mentioned that the LHC was designed to have 8 nominal (i.e. corresponding to an emittance of 3.75 m) sigma dynamic aperture with beam-beam and magnetic imperfections included and that losses have been measured in machine studies when the dynamic aperture was expected to reduce to 3-4 sigma (corresponding to ~5 sigma for nominal HL-LHC sigmas).
Gianluigi noted that the dynamic aperture should be larger than the collimator aperture (i.e. larger than ~7 sigma in nominal HL-LHC sigmas – i.e. for an emittance of 2.5 m). Sasha noted that we could accept even smaller values and pointed out that the diffusion at low amplitude should be considered for luminosity lifetime considerations and we should not consider dynamic aperture only. Frequency Map Analysis can provide useful information in that respect.
The guidelines for the minimum acceptable dynamic aperture should be reviewed based on the experimental evidence. Action: Tatiana in collaboration with Sasha and Massimo to review the experimental evidence and the results of the benchmark with simulations for a possible recommendation.
The dependence of the dynamic aperture on bunch population, crossing angle and for different beta* (both for flat and round optics have been studied) but the Frequency Map Analysis for all the cases is being considered to identify possible effects of tune shifts resulting in pathological working points.
Riccardo noted that for the beta* levelling scenarios the smaller beta* and therefore the smaller beam-beam separation is reached at low intensity.
On a question by Gianluigi Sasha commented that levelling at constant beta* ratio should be pursued.
A wire model is present in SixTrack and it is being tested.
Sasha clarified that the differences in DA between Sixtrack and Lifetrack presented in a previous meeting came from the definition of initial amplitude.
Heating issues from impedance – B. Salvant
Benoit presented some considerations on the effect of a double harmonic RF system (800 MHz) used in bunch flattening mode on heating. Overall effect of flat bunches on heating is not evident due to for broadband impedances. A priori the 800 MHz system can offer more freedom to try to avoid specific resonances allowing tailoring the beam spectrum.
Benoit continued with a review of expected heat load in the existing HW for different beam configuration corresponding to the PIC, US1 and US2 scenarios:
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The heat load on the beam screens due to image currents have been evaluated. The heat load in the triplets taking into account of the off-centred beam position and of the presence of both beams is being evaluated. A pessimistic approximation is provided for the moment. Action: Benoit to communicate this information to Laurent Tavian.
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It is expected that the present TDI will be upgraded for the PIC scenario.
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The upgrade of the MKI kickers during the present shut-down will be compatible with the PIC and US1 scenarios and might be marginal for the US2 scenario.
An upgrade of the analog front-end of part of the instrumentation might be required (e.g. on the stripline beam position monitors). Signals can be perturbed with large attenuation.
Reported by Gianluigi and Riccardo.
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