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17th HiLumi WP2 Task Leader Meeting
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Europe/Zurich
6/R-018 (CERN)
Minutes of the 17th WP2 Task Leader Meeting held on 20/09/2013
Present: G. Arduini, D. Banfi, R. De Maria, S. Fartoukh, M. Giovannozzi, E. Métral, T. Pieloni, R. Tomàs, A. Valishev, A. Wolski
Minutes of the last meeting and follow-up of actions
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The minutes have been approved without changes
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Actions:
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The actions for the impedance team concerning heating are on-going. Gianluigi reminded that it would be important to analyse the effect of bunch lengthening with 800 MHz system on transverse stability. Elias noted that it might be difficult to address this issue this year. Gianluigi suggested asking the Frascati collaborators to look at this aspect.
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Gianluigi asked for the new definition of n1 and the underlying assumptions (to become new design targets) that make the specifications less strict than the design report parameters. Massimo replied that for n1 the beta-beating and orbit margins are defined, it was also decided to remove the margin for the momentum offset due to chromaticity measurements, however it is still under study how to specify the momentum offsets to take into account dispersion and beta-beating effects.
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Tatiana asked for optics files that could be representative of the PIC and US1 optics, Riccardo is going to provide the 40/20 cm and 40/10 cm within next week on the SLHCV3.1b layout. This has not exactly the same features of the PIC/US1 scenarios but it can be easily compared with the previous simulations.
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Gianluigi reminded the task leaders to provide feedback on the proposed upgrade scenarios.
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Sixtrack - LifeTrack benchmark – A. Valishev
Alexander highlighted the additional features added to LifeTrack and Sixtrack. One notable difference is that in Lifetrack each point is an average over several linear action phases. Massimo and Stephane noted that this should not make any significant difference as in a limited number of turns each particle should scan the different phases provided that the starting betatron tune is not on a resonance.
Some points concerning the interpretation of negative values for the dynamic aperture provided by SixTrack were noted and will be addressed by Massimo and Alexander (done after the meeting). Some differences were still observed between the two codes without beam-beam and should be clarified. Action: Sasha.
Few cases have been analysed with beam-beam effects in IR1 and IR5 without multipolar errors. The simulations with Lifetrack seem to indicate a dependence of the Frequency Map Analysis plots on the linear action phase which is not understood. It was suggested to run a similar FMA analysis with Sixtrack as this is now possible.
The comparison between Lifetrack and Sixtrack shows discrepancies for the cases where beam-beam is considered. Stephane asked whether the 6D and 4D results are equivalent when deltap=0, Tatiana replied that this has been checked for the Sixtrack data. Tatiana asked whether it would be possible to run Lifetrack with a 4D lens. Alexander replied in the affirmative or at least it should be possible to reproduce the 4 D lens behaviour. Action: Alexander to compare 4D and 6 D lens behaviour in Lifetrack. Tatiana mentioned that 4D and 6D lens Sixtrack simulation results disagree only when long range is included. Similar tests should be done with Lifetrack. Action: Alexander.
Stephane suggested verifying the dependence of the results on the horizontal-vertical fractional tune split as the dependence on phase is not there in the absence of beam-beam and could be due to coupling introduced by beam-beam. Action: Sasha to run the proposed tests.
Gianluigi proposed to schedule a presentation on the SixTrack comparison between 4d and 6d models. Andy asked whether SixTrack can do FMA. Tatiana and Massimo replied in the affirmative.
Crossing angle scaling with beta* ratios and intensities – D. Banfi
Several scans using SLHCV3.1b optics and tools were performed both for flat and round beams. Few improvements were done on the tracking environment to speed up the post-processing.
In some cases there are important differences in the DA between 4D and 6D beam-beam models that are not fully understood yet. The scans performed for different beta* ratios and intensities cannot still be used until the physical effect underlying the differences between the models are clarified.
Stephane noted that the 3.1b optics has a 2pi phase advance in the vertical plane between IP1 and IP5 that may artificially enhance some effects.
Riccardo suggested to consider the nominal LHC bunch intensity and beta* and to benchmark with LHC nominal simulations.
The following additional checks to be performed emerged during the discussion:
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Beam-beam kicks definition and effect on orbit;
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Qs dependence;
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Tune scan perform additional the tune scan
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The error table used in the simulations should be verified with Massimo
Action: Sasha, Tatiana.
IBS blow-up estimates and alternative scenarios for HL-LHC (R. Tomàs)
Rogelio review the proposed beam parameters for the PIC-US1-US2 scenarios.
Besides marginal differences, the only disagreement comes from a different value of cross section used for the estimation of the intensity decay due to burn-off (total cross section of 100 mbarn). The IBS blow-up for different scenarios has been checked with a detailed model using an RF program prepared by Elena Shaposhnikova and Philippe Baudrenghien keeping a 10 cm r.m.s. bunch length.
It shows that for very small emittance cases the 20% margin in emittance blow-up can be reached by IBS only. An additional scenario (80/20 SF) has been analysed for PIC using larger beta* in the crossing plane and potentially compatible with present triplets (triplets at 10.5 sigma after meeting check RDM). This scenario has marginally lower if not equal performance with the advantage of using small aperture triplets and better pile-up density.
Alternative scenarios are proposed. 12 bunches gap can solve e-cloud issues at the cost of 20% of availability. 200 MHz in the LHC will remove some intensity limitation since at higher intensity injectors are limited by intensity and not brightness.
A different splitting leading to 4 batches of 48 bunches instead of 72 in the SPS at very high intensity 6x(8b+4e) could serve as an effective backup for e-cloud.
Reported by Gianluigi and Riccardo.
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