Participants (remote and in person): Ludovica Bertolozzi, Enrico Bravin, Roderik Bruce, Nuno Duarte, Jorge Guardia, Birk Karlsen-Baeck, Anton Lechner, Daniele Mirarchi, Sara Morales, Belen Salvachua, Dora Veres

Power deposition from TCLD pressure spikes (TCLD.11R2 beam-gas simulations) - Nuno Duarte

• Pressure spikes in TCLD.11R2 (installed in LS2) observed since 2022, occasionally triggering beam dumps
  • FLUKA simulations performed to evaluate risk to downstream magnets from beam-gas interactions
• Analysis of fill #11164 --> Dumped on pressure spike by vacuum, not BLMs
  • Vacuum pressure reaching 4e-6 mbar --> ~5s with at least one gauge over threshold
  • High pressure for >140cm
  • BLMs did not dump but can see losses due to the pressure spike --> 6 BLMs on Q11R2, TCLD.11R2 and MB12R2
  • Max RS09 BLM signal used to benchmark simulations
• Beam-gas FLUKA simulations --> Because of the low probability, inelastic interactions are forced for every primary and then normalize to pressure.
  • Location of inelastic interaction randomly sampled along particle trajectory
  • Energy deposition scored on BLMs (benchmark), MQ and MBs downstream of TCLD.11R2 (power deposition on superconducting coils)
  • Accurate model geometry around TCLD
  • Simplified gas profile in interaction region --> H2 with constant pressure
  • Scale the fluka results and then investigate the factors wrt BLM signals --> Good agreement between simulated and measured BLM signals, but fitted normalization factor is ~3.5x higher than the analytical one
  • With a gas mixture (used for FCC studies), the normalization factor is just 1.4x higher than the analytical one
• Energy scoring from the N2 simulation (more conservative)
  • Normalized for pressure and intensity for the particular moment that is taken 
  • Energy peak happens in first dipole, but very modest (0.33 mW/cm3)
  • For simulated losses with 4e-6 mbar, the estimated maximum power density in SC coils remains well below quench limits
    • Maximum vacuum pressure before quench expected at 4e-6 mbar x 30  = 0.00012
    • Even with 1e-5mbar we will not quench

• Daniele comments that from empirical scaling there should not be a need to increase BLM thresholds
• Belen comments that this factor of 30 is applicable to all BLMs in the area
• Daniele says that even increasing the maximum vacuum threshold we are well below the BLM thresholds
• Belen asks if similar simulations for the beam pipes are foreseen, for the case of vacuum spikes in bellows for example
  • Anton says that it depends strongly on the location, and 6L2 was exchanged already
• Daniele asks about the gas mixture composition 
  
  • Standard mixture used for FCC studies, for LHC it should be different compositions
    • Would it change much the simulation results?

Validation of updated IR3 BLM thresholds - Sara Morales Vigo

• Off-momentum losses at the start of the energy ramp reaching BLM thresholds in IR3 during some fills in Run 3
• Loss plane decomposition algorithm developed to quantify sharing of losses between IR3 and IR7
• Main limitation in 2024 proton run was in the Q6L3 BLM RS10 (~5s), limiting at 20kW --> Applied threshold
  • Threshold increased by a factor of ~2 as a temporary measure before further studies were completed --> Changes on 25/06/2024, limiting at 60kW 
• IR3 BLM thresholds model not reviewed since LHC LS1, at the moment no way to disentangle IR3 and IR7 off-momentum losses
  • BLMs at TCPs and TCSGs master threshold set to 500kW (IR3+IR7 losses), with MF = 0.4 (applied threshold at 200kW)
  • Proposal to update the IR3 BLM thresholds collimation model similarly to the IR7 collimation model that was implemented during LS2
• Collimation model requires:
  • BLM response --> Calculated from dp/p loss maps
  • Allowed number of charge impacts on the TCPs --> Up to 500kW losses at injection probed in 2025 proton MD, ~100kW losses limit expected at top energy (TCLAs)
• BLM thresholds in IR3 updated during the YETS 25/26
  • New model for IR3 collimators
    • 5 new families
    • Master thresholds --> 500kW at injection energy, 100kW at top energy
    • MF  = 0.4 --> Applied thresholds at 200kW injection energy, 40kW at top energy
  • Q6 and warm magnets with corrections on top of the existing models to allow 500kW losses with the master thresholds, top energy untouched
    • MF = 0.4 -->  Applied thresholds at 200kW injection energy
• Validation of new IR3 thresholds from scaling BLM signal-to-dump ratios from:
  • Off-momentum loss maps at injection and top energy
    • Only interesting when enough losses in IR3 --> B1 losses often very low or in IR7
    • Power loss in IR3 calculated from BLM decomposition algorithm
    • B1/B2 BLM ratios scaled to 100%, equivalent B1/B2 power loss at dump calculated
      • Crosstalk between beams affects the analysis, specially when one beam loses more than the other
    • Overall OK, but hard to validate
  • Start-of-ramp losses
    • For fills up to #11474, for B1 the main power loss is in IR3, later in IR7
      • In some cases, both beams with activity in IP7
      • Daniele comments that the switch to IR7 losses for B1 happens in the first fill of 75 bunches
    • Collimator hierarchy still OK in IR7 for B1, but secondary peak of losses appearing clearly
    • Not enough signal to scale for fills with power loss under ~1kW --> Only B2 with losses in IR3
      • B2 seems well aligned to 500kW for master threshold
• Will continue to monitor with start-of-ramp losses and adapt if needed

AoB: Proposal for increase of BLM threshold at TCTs during the ramp - Daniele Mirarchi

• TCTs going to much tighter settings during the ramp this year due to the combined squeeze
  • Combination of loss increase + threshold going down during the ramp (before flattop corrections) --> Some TCTs in IP1 and IP5 with signal to dump threshold above 30% in RS11 and RS12
• Factors needed to avoid being in warning with full machine
  • TCTPV.4R1.B2 RS11 higher than 1.2, RS12  higher than 2.1
  • TCTPH.4L5.B1 RS11 higher than 1.4, RS12 higher than 2.6
• Check of power loss allowed on TCTs for given thresholds 
  • TCT BLM response measured in 2025 at injection and top energy --> Good agreement with FLUKA simulations
  • TCT jaw is designed for a maximum power load of 2kW --> Calculate equivalent power for different thresholds --> Conservative as not all the load remains at the TCT
    
    • If Flat Top corrections propagated down to lower energies --> 3.8kW
    • If RS11 and RS12 propagated to RS10 --> 0.2kW --> Acceptable, safe increase --> From EL21 (5 TeV)
      • Sufficient margin to operate the machine until the TCT thresholds model is reviewed --> Plan for LS3