Choose timezone
Your profile timezone:
117th Impedance Working Group Meeting
→
Europe/Zurich
6/R-012 - conference room (CERN)
117th Impedance Working Group Meeting
Present: C. Antuono, D. Gibellieri, A. Grudiev, G. Banks, C. Pasquino, G. Favia, S. Kostoglu, L. Sito, J. Sonpar, K. Lasocha, M. Neroni, H. Pommerenke, Hannes ?, I. Valencia Ruiz, P. Bampis, V. Daliento, C. Zannini
ACTIONS (IWG): To simulate SPS BGI with bulk metallic cathode
ACTION (SY-BI): To check if old/diamond grid could be reinstalled in one SPS BGI in the last weeks of run (keeping sealing plate)
High intensity test overview (C. Antuono)
C. thanks Sofia and Benoit for the inputs.
-High intensity tests needed to identify main limitations from hardware due to beam induced heating and injectors readiness. Tests were very successful and went even beyond the identification of limitations (Improvements of e-cloud predictions, collimation, input to RF studies, and general possibility to comple beam tests and MD4 ).
- TCLDs showed vacuum spikes since 2022 especially appearing with different beam types with following conditioning. An important beam dump was observed in 2025 during 8b4e MD and for the first time with 25 ns beam during the HIT. Following increased vacuum interlocks and not anymore beam dumps due to the TCLD but only some vacuum steady rise. However, with single beam at 3 TeV the target was reached (2748@HL bunch intensity).
-From previous impedance follow ups, the impedance has been shown to heavily depend on the fingers contact quality, and X-rays installed TCLD and inspection of spare ones, might induce the suspect that the contact is not perfect. IN 2025, an impedance mitigation solution was found to reduce half gap from 22mm to 10 mm (confirmed by bench measurements) and 2026 beam observations seems to confirm it.
- Consolidation plan/follow-up is put in place: Intervention to inspect one TCLD expected from September and in the meanwhile some testing on the spare TCLD with new fingers concept or testing after bakeout thermal cycle can be done.
-Possibility of influence of cryogenics to be further analyzed.
Several vacuum spikes in 4R2,4L2 limited the HIT at 3 TeV with two beam @ 2220b, they presented non-systematic and erratic behaviour, with suspicion of beam induced heating and e-cloud involvement, the latter was difficult to confirm. Anyway, large heating rate in the ID800 chamber with full beam in the machine.
In these two sectors many vacuum requirements. The ID800 was equipped after Run1 with NEG inserts as it suffered of outgassing from e-cloud.
Preliminary simulations show transmission line resonances corresponding to the length of the insert. There should be some point of contact at the bottom of the chamber but not clear until the real layout will be shared. The impedance resonances can change frequency of resonance depending on the real length of the inserts. The gap between the inserts acts as a capacitor and the smaller the gap the larger the electric field peak. The trend indicated that 1MV/m could be reached for 300 um gap.
Some preliminary calculation of beam induced were carried out with expectations of 100 W/m on the insert. However, numbers to be confirmed with a more realistic model.
Some solutions for consolidation were proposed by the vacuum team and they need to be evaluated.
Other locations are worth to be mentioned and follow-up (CT-PPS, LHCb velo, TWOCRYST,MKI)
Discussion
C. Pasquino asks what is the follow up plan for the Schottky. Just inspection can be done during LS3. In the past no clear sign was observed. Sofia mentions that the pressure spikes were observed just at injection moment and with long trains.
Sofia asks the plan for the TCLD. Carlo answers that in September there will be an inspection to check the status of the currently installed collimators. Not to be excluded that any other neighbour element could give the spikes. Also to be checked if cryogenic issue can be excluded. If new fingers are installed in the spare collimator, then the spare will be measured. Not easy to judge the quality of contact also during the impedance measurements, also thermo-mechanical and material considerations might help.
SPS BGI (C. Pasquino)
No signals was detected on both instrument as of the beginning of the run. During the TS intervention, lack of DC continuity confirmed on both instruments, verified between the HV feedthroughs and the cathode. Additional findings:
- Pogo pin still springing correctly, but tip shows visible coloration à Suspected sparking between the pin tip and the HV socket on the cathode.
- Silver paint was added in the cathode pocket where the pogo pin is contacting during the installation in 2023. Now, silver paint bond (cathode → HV feedthrough) shows coloration and clear degradation, signs of sparking and localized heating.
- 3 out of 3 NEG-coated cathodes inspected show the same defect:
o No DC electrical continuity over a localized area on the HV feedthrough side
o Open question on plasma instability during NEG coating, or local thermal deposition induced by the beam.
o Checking with the coating team, all the cathodes’ coatings were verified with the 4 pins measurements in 3 random locations è the area where the HV pin is touching might not have been measured. Uniform coating thickness of 60 nm was measured.
o From preliminary calculations, about 100°C on the cathode might be reached therefore the coating resistivity could have been modified by beam induced heating.
Currently both H and V BGIs are back in operation and taking acquisition thanks to a metallic pad creating DC continuity between HV and cathode (just on top, so the cathode is floating). Still to be understood root cause of the NEG cathode non-conformity (coating vs. beam-induced) and long-term impact of the floating cathode on instrument performance.
Chiara asks for impedance simulations with metallic cathode option which would be the easiest implementation to restore full functionality of the instrument and in line with the current implementation in PS and future HL-LHC BGI.
Discussion
Carlo answers that simulations with thicker coating or metallic cathode can be done.
Leonardo mentions that some recent simulations by Valentina Daliento show that the old grid (diamond shape) would not lead to much larger field leakage in the detector area with respect to the currently implemented one (1 mm diameter holes). Leonardo mentions that the leakage might actually be coming from the region around the grid, which was modified together with the grid itself.
Request by Carlo and Leonardo to consider the reinstallation of the old grid in one of the instruments before end of the run. Chiara will check if it is feasible.
Open and upcoming ECRs
Carlo and Michela present the impedance comments to open ECRs:
- CMS PPS (Roman Pots) à LMC postponed approval. Good solution for ferrite cooling channel to be found. Impedance team can follow up with some mitigation solution in case no good solution for the cooling is found.
- Installation of Combined Fast Beam Current Transformer and Electrostatic Intensity Pick-Up in AD ring à beam induced power expected well below the mW range, so not expected to be significant limitation. Impedance should still be properly evaluated and controlled, even for AD ring.
- Beam screen treatment on 120 half-cells à impact below 0.5% for both longitudinal and transverse. Impedance assessment overview presented. IWG fully supports the implementation of the treatment for e-cloud mitigation.
Upcoming ECR:
- PS kicker KFA4 à Inductive Adder Pulse Generator. To be followed up with ABT group.
There are minutes attached to this event.
Show them.