First Cool-down and Test at 4.5 K of the ATLAS Superconducting Magnet System Assembled in the LHC Experimental Cavern

The four superconducting magnets (Barrel Toroid, two End-Cap Toroids and Central Solenoid) of the ATLAS detector have been tested individually at 4.5 K in the LHC underground experimental cavern. Subsequently, as foreseen in the final configuration, they have been cooled in parallel at 4.5 K and powering up to their nominal current (20.5 kA) is in progress, prior to the first LHC proton beam. In order to fulfill all the cryogenic scenarios required for the cooling of these magnets which have a total cold mass of 680 tons, two separate helium refrigerators and a complex helium distribution system have been implemented. This paper summarizes the basic design principles and the results obtained so far for the cool-down, steady-state operation at 4.5 K and energy fast dump