Methods to detect faulty splices in the superconducting magnet system of the LHC

The incident of 19 September 2008 at the LHC was caused by a faulty inter-magnet splice of about 200 nΩ resistance. Cryogenic and electrical techniques have been developed to detect other abnormal splices, either between or inside the magnets. The existing quench protection system can be used to detect internal splices with R>20 nΩ. Since this system does not cover the bus between magnets, the cryogenic system is used to measure the rate of temperature rise due to ohmic heating. Accuracy of a few mK/h, corresponding to a few Watts, has been achieved, allowing detection of excess resistance, if it is more than 40 nΩ in a cryogenic subsector (two optical cells). Follow-up electrical measurements are made in regions identified by the cryogenic system. These techniques have detected two abnormal internal magnet splices of 100 nΩ and 50 nΩ respectively. In 2009, this ad hoc system will be replaced with a permanent one to monitor all splices at the nΩ level.