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237th HiLumi WP2 Meeting
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Europe/Zurich
Description
Chair: Rogelio Tomas
No Minutes
HiLumi WP2 Meeting Room
In the absence of detailed minutes for this meeting a short summary of the main points follows:
In view of the high intensity tests foreseen for 2026 it is requested to test with high priority the following two configurations:
- Standard scheme 4x72, 2.e311 ppb, with the machine as full as possible ~2700 bunches at 3 TeV. The bunch length should be aimed at ~1ns (20% below the HL 1.2ns) as proposed by Xavier as a way probe HL bunch lengths for few hours. From simulations performed on-line by Sofia this should allow to probe HL-LHC heat loads for about 2h as bunch lengthening from IBS develops (see plots at the bottom). Various fills with this conditions should be envisaged. Larger transverse emittance from the injections or generated after injection via blow-up should be explored to maximize the time at flattop with nominal bunch length. This configuration suffers from longitudinal loss of Landau damping, as Helga presented, but this might be OK as it would only add a slow blow-up. This configuration is at the edge of cryo-capacity as presented by Lotta. Ideally, the critical points of this configuration (IBS, e-cloud, longitudinal Landau damping) are validated in dedicated MDs in 2025. This configuration is ok for TCDS and TDE (even higher energy would be OK, however e-cloud would be the limit) as presented by Antonio.
- 8b4e, 2.3.e11 ppb, close to 2000 bunches at 6.8TeV. This configuration has no limitations from RF, protection devices or IBS. 8b4e is of high importance as it features spectral lines that do not appear in the Standard configuration, as Chiara and Carlo requested, and for benchmarking e-cloud models. The degradation of the vacuum chamber in LS3 is unpredictable and HL-LHC might need hybrid beams, therefore it is of high priority to test this configuration.
Other configurations were discussed during the meeting that could serve as back-up in case the configuration 1 given above cannot be realized:
- 24b, 2.1e11ppb, ~2200 bunches at 6.8 TeV.
- Standard scheme, full machine, at 5 TeV with single beam as proposed by Guido. This is ok for cryo capacity and probably also for TCDS and TDE. This would require more fills to probe both beams and has the drawback that elements in the regions where the two beams share the same aperture will not experience the full heating.
As an AoB Chiara presented the impact on octupole threshold from the impedance from PPS2. At flat top, the impact of PPS2 is quite small, but a significant increase in octupole strength would be needed at the end of leveling to keep non-colliding bunches stable. On the other hand, the crab cavities dominate the stability situation in collisions, as only standard feedback can be used. Note that the factor 4 lower brithness in the witness bunches has been taken into account by Chiara already, hence the octupole current needed (-160 A) is not DA-compliant with round optics (with Q'=15). Flat optics should be evaluated as they have significantly larger margin on DA and could tolerate the required MO threshold. There is an MD request to test the lower brightness of the non-collider bunches by Xavier. This would happen as a normal physics fill and attention will be put to tune and BLM signals and detector background.
The next meeting will likely take place next Tuesday, dedicated to magnetic ripple measurements.
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