Helium II calorimetry for the detection of abnormal resistive zones in LHC sectors

Following the incident on a LHC sector due to an electrical arc on the main dipole bus-bar circuit, postmortem analysis of previous current plateaus has shown abnormal temperature drift in the helium II baths of some magnets in the concerned area. In order to identify other possible risky areas, a detection system based on calorimety using available precision cryogenic thermometers has been first validated by applying calibrated heating in the magnet cold-mass and then implemented in the different sectors. On the 3-km long continuous helium II cryostat of each LHC sector, this method allows detecting abnormal dissipation in the W-range, i.e. additional resistive heating due to abnormal resistance of about 40 nΩ at 7 kA and less than 15 nΩ at the nominal current of 12 kA. The paper describes the principle and the methodology of this calorimetric method and gives the results obtained on the LHC sectors.