Design, Construction and Tests of 20 kA Current Leads for the CMS Solenoid

Abstract

Abstract-The CMS experiment (Compact Muon Solenoid) is a general-purpose proton-proton detector designed to run at the highest luminosity at the LHC [1]. Distinctive features of the CMS detector include a high-magnetic-field solenoid (4 T) 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 mm 2 , 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).

Status of the Construction of the CMS Magnet

“…Those for CMS are 3 m long to cross the barrel yoke and they have been designed to stand the nominal current for 7 min to allow for a safe fast discharge. Better results have been obtained on the finished units [19].…”

Cryogenics Activities at the Institute of Research into the Fundamental Laws of the Universe (Irfu)

“…On the basis of a collaboration with CERN since 1996, Irfu (previously named Dapnia) was involved in the overall design, production and testing of the superconducting solenoid for the CMS (Compact muons solenoid) experiment installed at CERN on the LHC (Large Hadron Collider) 1) .The LCSE was in charge of the thermal design of the proximity cryogenics and the tests of its different components such as the phase separator/reservoir, the currents leads 2) , the titanium support rods and a large thermosiphon loop reproducing the cryogenic loop of the CMS magnet 3) . The concept of cooling such a magnet with a two-phase thermosiphon at 4.5 K was proposed by LCSE and accepted by CERN.…”

Magnetic Tests of the CMS Superconducting Magnet

“…They are made of watercooled copper bars for the lines which are continuously powered and of aluminum bars for those which are only used for a short while (dump circuit); -current leads: the two 20 kA current leads are made of a high purity copper braid (to reduce their section because of their long length, 3.3 m, as they must cross the return yoke) and have been designed to hold a current of 20 kA during 5 minutes followed by a fast discharge without any damage with no cooling [7]; -grounding circuit: the grounding circuit is connected across the solenoid terminals and fix its potential though a 1 resistor, dividing by two the potential to ground ( 300 V). A short circuit to ground can be detected by measuring the leak current through a 40 grounding resistor; -electrical joints: all electrical connectors between layers and modules (21 in total) are done at the outer radius of the solenoid.…”