Fifteen Years of Operation of the Compact Muon Solenoid Detector Superconducting Magnet

The Compact Muon Solenoid (CMS) detector magnet has been in operation since 2008 at CERN's Large Hadron Collider (LHC). It will have to operate until the end of the High-Luminosity LHC run, beyond 2040. The CMS magnet comprises a large superconducting solenoid coil providing a magnetic field of 3.8 T with a free bore of 6 m in diameter and a length of 12.5 m. The coil is constructed with an aluminium stabilized Rutherford Nb-Ti/Cu cable and operates at 4 K with indirect conduction cooling in thermosiphon mode with boiling helium. The magnet reached 4 T and a record stored energy of 2.6 GJ when it was commissioned in 2006 in the surface hall at CERN Point 5. It was then transferred in 2007 to the underground experimental area, where it was recommissioned and successfully operated at a nominal field of 3.8 T since then. A summary of the magnet operating data is presented in this paper along with the observed progressive change of the Residual Resistivity Ratio (RRR) of the pure aluminium conductor stabilizer as a function of operating cycles and magnet warm-ups. The technical problems encountered, and the solutions implemented with the cryogenics and the vacuum pumping of the cryostat are described, as well as the upgrades carried out during the LHC shutdown periods on the control system, the cryogenics and the powering circuit where a freewheel thyristor system has been implemented.