Performance review and reengineering of the protection diodes of the LHC main superconducting magnets

The LHC main superconducting circuits are composed of up to 154 series-connected dipole magnets and 51 series-connected quadrupole magnets. These magnets operate at 1.9 K in superfluid helium at a nominal current of 11.85 kA. Cold diodes are connected in parallel to each magnet in order to bypass the current in case of a quench in the magnet while ramping down the current in the entire circuit. Both the diodes and the diode leads should therefore be capable of conducting this exponentially decaying current with time constants of up to 100 s. The diode stacks consist of the diodes and their heat sinks, and are essential elements of the protection system from which extremely high reliability is expected. The electrical resistance of 24 diode leads was measured in the LHC machine during operation. Unexpectedly high resistances of the order of 40 μΩ were measured at a few locations, which triggered a comprehensive review of the diode behaviour and of the associated current leads and bolted contacts. In this paper the thermal and mechanical analysis of the critical parts and bolted contacts is presented, and the results are discussed. Due to a lack of mechanical rigidity and stability, the bolted contacts between the diode leads and the busses of the quadrupole magnets have been redesigned. The consolidated design is described, as well as the dedicated tests carried out for its validation prior to implementation during the long shut down of the LHC machine that is scheduled between March 2013 and December 2014