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Special Joint HiLumi WP2/WP4/WP5 Meeting

→ Europe/Zurich
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Chair: Rogelio Tomas, Rama Calaga, Stefano Redaelli

Minutes: no minutes

HiLumi WP2 Meeting Room
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67218398114
Host
Hannes Bartosik
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RF, Birk

-Needed to launch full detuning and wait 10min before ramp to avoid dumps. Need to explore ways to avoid this in HL but likely some minutes could be needed. To be reviewed after the HI tests.

-Full detuning looks very promissing to reduce RF power at injection to allow HL intensities. SPS RF phase modulation is very important for this scheme.

-The 8MV goal of HL-LHC is at reach now. To use this at injection is easy. First software will be available to operate this.

-Try to reduce bunch length at injection towards the end of the HI tests.

-Try to find balance for HL to have at a long-safe bunch length at start of Ramp (~1.4 ns) and short enough for injection (~1.25 ns)

 

Longitudinal instaibilities, Ivan

-MD done with similar instability threshold than expected in HL (by reducing main RF gain).  Blow-up seen at the end of train with clear coupled-bunch instability signature on the phase.

-Further MDs to probe the other beam and test mitigations will be pursued and probe different bunch lengths (shorter bunches, faster instability).

-Shorter trains could be a mitigation in this configuration, but unclear if for CC would also works, this needs to be studied.

-Larger emittance in the SPS would help mitigating the instability. Now possible thanks to full detuning. To be tested.

 

HI tests, Sofia

-Excellent results, with excellent brightness, tails, bunch length, injection losses, etc.,  in BCMS beams 1500-1900 bunches. Injections losses were worse for 4x72.

-Issue: Collimator hierarchy breakage causing a dump.

-Vacuum activity as expected, TCLD and 11L2, and other unexpected regions.

-Allow reducing bunch length to avoid long bunch at 3TeV? Jorg suggests to stay with target and increase voltage. Maybe only at the end test shorter bunch length. -> Stick to 1.3ns.

-Hierarchy breakage seen from the beginning could be mitigated with reduced octupoles.  Worse for 2 beams fills. Bucket area was lower for for the fills with large losses. There are two times with losses at the start of the ramp.

-There is the option to move back to the standard ramp, instead of the PPLP for the high intensity tests (prepare a back-up PELP?), and full detuning could also be a mitigation for the hierarchy breakage. Tentative mitigation list: Lower MO, Full detuning,  PPLP, operate with hierarchy breakage increasing thresholds, Positive MO.

-Roderik suggests that what is seen is a degraded hierarchy rather than a full breakage.

-CMS off OK

-TCLD large spike and strange behaviour with very frequent spikes. Also in B2.

-Various other spikes at locations presented at 4/5/6 L/R 1/5/8

-Sofia suggests that if spike behaviour are reproduced at injection with shorter bunch we might skip the 3 TeV ramps in June. Actual settings of the collimators should also be regarded (TCLD remains at constant gap).

-Hints of conditioning were observed during the HI tests.

 

TCLD observations, Chiara

-Since 2022 preassure spikes and "Runaway behaviour" were observed in the TCLD during regular intensity ramp-up.

-Conditioning signatures were observed in the TCLD.

-Vacuum levels are very low during the strange behaviour. The concern was potential sparking and annheling or burning arch. Other concerns were the spikes in the stand-alone quadrupoles, and helped in the decision to stop the high intensity run.

-These concerns should be less relevant in the June HIT.

 

Beam-beam MD with 72b, Anna

-Strong emittance evolution were observed along with losses. 

-Increasing the tune split change the emittance evolution.

-With single plane kick oscillations are induced in the other plane and in the other beam (coupling + beam-beam coupling). 

-RPTG and RPTG Active filters did not have an impact.

 

Collimator impedance, Nicolas

-Larger tune shifts than expected measured since 2025, but similar between 2025 and 2026. Octupole bias discarded. 

-Some analysis still needed, couple of hypothesis for discrepancy are chromaticity and collimators tilt.

-Measurements of growth will be reassuring concerning impedance of the machine. 

 

e-cloud, Lotta

-Comparison between 2018 and 2022 and 2026 heat load clearly shows degradation in most arcs.

-From last measurements full BCMS beams should fit at 3 TeV, however 4x72 would not, it should be below 2

 

 

 

 

 

 

 

 

 

 

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