07.03.2024

Expert discussion on scenarios for operating with sparking RF Fingers

Indico page: https://indico.cern.ch/event/1390492/

Participants:

Giuseppe Bregliozzi, Andy Butterworth, A. Lechner, Belen Salvachua, Josef Sestak, Matteo Solfaroli, Helga Timko, Jan Uythoven, Christine Voellinger, C. Wiesner, D. Wollmann, Carlo Zanini,

Introduction. Follow-up topic of discussions at the Chamonix workshop. 

How to be prepared in case we get issues with the RF fingers and prepare a procedure to be applied in this case.

Main points to cover: 

• Can we run with sparking RF fingers and under which conditions? 
• What interlocks need to be adapted? Under which conditions could we allow to operate and delay a repair to the YETS?

Warm-Module RF Fingers - recap of issues observed in 2023 and repairs in EYETS 2023/24 (Giuseppe)

• Overview of layout in section A4L1 (symmetrically R1, L5, R5):
• 9 modules after TAN and 6 elliptical modules before the triplet; there is only one vacuum gauge (not interlocked) and no sector valve; next sector valve is at the triplet (also with interlocked gauge).
• Consolidation during YETS 2023/24:
• All modules in the affected sector were changes, with DRF modules interleaved with anchored modules ; 
• Elliptical modules have not been consolidated, just springs repaired, where required. … two springs oo 24 were found damaged. 
• One DRF at each side has been installed around the BSRT (pt4)
• Point 8: after TDIS one module was changed, which showed issues, 5 other modules where not changed
• Point 2/8: was not touched around TDIS and TCDD can be done very quickly, as it has very short sectorisation.
• Point 2: large vacuum chamber after the ZDC is critical, due to a large volume and long takeout in case of an intervention. The proposal is to install bellows without RF fingers, as there is no design for this diameter; 
• Point 4 as well untouched;
• No elliptical modules were changed;

Discussion:

• Is there any concern to run with these type of conditions.  In pt2 we have been running for a couple of years in this condition; worst case would be to damage the bellow if the RF fingers touch the bellow. 
• For the moment no detectable precursors have been identified, which indicate that the spring gets damaged.
• Pressure interlocks close to cold machine parts should not be increase beyond 10-6mbar, to avoid pollution of the beam screen surface, which could lead to increased heat load.
• RF fingers could move in the aperture, this would be detectable due to the beam losses.
• No significant increase of activation expected due to increased vacuum pressure at a damage module

Follow-up: 

• Review if there could be a collateral damage on accelerator equipment which makes the situation worse and request for an immediate intervention - Giuseppe

Recap of BLM threshold changes in 2023 due to failure in VMBG in LSS1 & related studies - Anton

• Beams dumped always by one BLM (BLMQI.03L1,B2E30_MQXA)
• BLM thresholds were increased at Q3 after the intervention (MF0.16 —> 0.25)
• Impact of beam gas collisions depends closely on the position, makes strong difference 
• Based on experience from 2017 (septum event), the pressure will likely reach a (lower) steady state following the highest levels at moment of failure and shortly there after.

Follow-up:

• Estimate energy deposition in the triplet before the repair the warm module with full beam & estimate the expected energy deposition in the triplet, if the closest module before the triplet would fail - could quenching of the triplet be avoided? - Anton
• Look back at 2017 LHC septum issue and summarize and derive observed time scales before situated settled in a stable regime. Compare vacuum levels during peak of event and during stable period - Josef

First level action steps:

 - If issue with RF fingers is detected, stop operation and take X-rays to identify the source module

• Perform FLUKA studies to estimate the expected energy deposition in the downstream coils to identify possible safe intensity levels for operation, without risk of quenching.
• Some low intensity operation to cover the time for FLUKA studies

Follow-up:

• Prepare first version of procedure in shared document - Daniel

Next meeting: Thur 14.03. 10:00-11:00