Analysis and Design of the CMS Magnet Quench Protection

The CMS experiment (Compact Muon Solenoid) is a general-purpose detector designed to run at the highest luminosity at the CERN Large Hadron Collider (LHC). Its distinctive features include a 4 T superconducting magnet with 6 m diameter by 12.5 m long free bore, enclosed inside a 10000 ton return yoke. The magnetic field is achieved by a 4-layer superconducting solenoid made of a reinforced Rutherford type superconductor with a hybrid configuration and wound inside an external aluminum cylinder. The coil is indirectly cooled at an operating temperature of 4.5 K by a thermosiphon, and it is designed to run at a nominal current of 20 kA. The corresponding maximum stored magnetic energy reaches 2.6 GJ with an E/M ratio of 12 kJ/kg. The quench protection is achieved by discharging the magnet in an external resistor in fast mode, with a peak voltage in the coil kept at 600 V and a voltage to ground of plusmn300 V during the discharge. The coil design takes benefit of the quench back effect to limit the temperature gradients inside the winding pack in case of quench. The design and the characteristics of the quench protection system are described. The computational results of the quench protection for several operating currents are presented. The behavior of the magnet in case of faulty conditions on the protection system is analyzed