Design, construction and tests of 20 kA current leads for the CMS solenoid

The CMS experiment (Compact Muon Solenoid) is a general-purpose proton-proton detector designed to run at the highest luminosity at the LHC. Distinctive features of the CMS detector include a high-magnetic-field solenoid (4T) coupled with a multilayer muon system, a fully active scintillating-crystal electromagnetic calorimeter, a tile hadronic calorimeter, and a powerful inner tracking system. The two 20 kA current leads for the CMS electrical circuit have been designed, manufactured and tested by CEA Saclay. Their design, with reliability as prime goal, is based on the use of a pure-copper braid, having an RRR of 130, placed inside a conduit and cooled by evaporating helium gas. Their length is of 3.3 m to cross the return yoke and their conductive cross-section has been fixed at 1800 mm², slightly above the optimal section. An important specification is the behavior in case of lack of coolant: the current leads are able to hold the maximal current during 5 minutes followed by a fast discharge, time constant of 190 s, without any damage. They are fully instrumented with sensors and diagnostics (temperature, voltage and helium flow) for safety and control. In case of discharge, they are submitted to a high voltage and then must ensure an insulation of 3 kV. The tests will include insulation, mechanical and electrical tests (at nominal current, with and without coolant).