First operational experience and performance optimization of the ATLAS magnet cryogenic system

The ATLAS magnet system, comprising a superconducting central solenoid and three superconducting toroids, has been successfully ramped up for the first time to the nominal operational current of 20.4 kA on 4th August 2008. Since then, new cryogenic operational challenges have been raised, like the smoothing of steady-state parameters, the enhancing of transient procedures to minimize thermal shocks on the magnet cold masses, the optimization of the complex cryogenic system in order to reduce the compressors electric consumption and finally how to avoid regular clogging of the shield refrigerator by water contamination. This paper presents the heat load identification of the various cryogenic sub-systems done at 4.5 K and how one of these loads was reduced, what was gained - in term of electrical consumption - by tuning the turbines settings of the main refrigerator and finally the first consolidation of the cryogenic system implemented in order to minimize the detector downtime during LHC beam runs.