Ad-hoc meeting on interlocked BPMs

Tuesday 12 May 2026, 08:30 → 09:30 Europe/Zurich

Present: A.Boccardi, D.Calzolari, K.Lasocha, S.Morales, B.Salvachua, J.Uythoven  

• Kacper asks about the definition of safe beam, he has been in discussions with ABP as they would need to use BPMs for optics measurements during commissioning
  • We do not have an operational scheme yet
  • Action —> Discuss in MP task force the operational scheme, like injection of pilots before higher intensity beams (including definition of commissioning scenarios and the use of Set-Up Beam Flags) and decide from there the BPM resolution needed for different beam intensities
  • Action —> Discuss with ABP (Xavier and Roderik in task force) their needs for optics measurements during commissioning
• Kacper asks what would be the bunch length and if there would be an interlock based on bunch length measurements 
  • Andrea comments that there should be clear specifications in case a device like this is needed, including whether it has to be a safety device, as it would affect most likely the design and the way it is operated/ 
  • Action —> Discuss whether a bunch length interlock would be needed and specifications
• Kacper asks what would be the maximum allowed orbit deviation in the case that the BPMs are interlocked
  • This has not been agreed yet, to be discussed
  • Horizontal beam size much larger than vertical beam size
  • Daniele comments that horizontal collimators are at 9 sigma and vertical collimators at 60 sigma
  • Interlock desired in both vertical and horizontal planes
  • Kacper says that for orbit measurements they have been asked to provide BPMs with a resolution of around 1um at 50Hz. For HL-LHC, they are aiming at having 10-15 um resolution bunch by bunch. Jan says that a resolution of around 50um in two turns might be enough. 
  • Action —> Provide clear specifications to Kacper and Andrea about intensity, orbit excursion and resolution
  • Action —> Decide number and location of interlock BPMs
    • Kacper and Andrea comment that it would be possible to have some BPMs just to interlock and some others to measure, having different electronics or just different settings. For now, there are around 9000 BPMs planned for the FCC, one at each quadrupole
• Jan comments that it would be important to have another interlock in case the synchrotron light misses the absorbers and impacts on other machine elements. In synchrotron light sources a thermal monitor that measures the energy deposition in different slits is used to interlock when there is a deviation.
  • If we want to do this interlock on synchrotron light measurement, the contact would be Federico Roncarolo. Belen comments that Fabian Titz performed some simulations in which the beam was offset vertically, and a clear increase on the signal of the simulated BLMs was observed due to the synchrotron radiation partially missing the absorbers. If the chosen BLM detectors are sensitive to synchrotron radiation, they would probably be sensitive to this failure scenario. 
  • Action —> Investigate how to measure and interlock on the synchrotron radiation missing the absorber