The superconducting magnet system of the ATLAS detector will consist of a c
entral solenoid, two end-cap toroidal magnets (ECT) and the barrel toroid m
agnet (ET) made of eight coils symmetrically placed around the central axis
of the detector. The magnets will be tested individually in a 5000 m2 expe
rimental area prior to their final installation at an underground cavern of
the LHC Collider. For the BT magnets, a dedicated cryogenic test facility
has been designed which is currently under the construction and commissioni
ng phase. A liquid nitrogen pre cooling unit and a 1200 W@4.5K refrigerator
will allow flexible operating conditions via a rather complex distribution
and transfer line system. Flow of two-phase helium for cooling the coils i
s provided by centrifugal pumps immersed in a saturated liquid helium bath.
The integration of the pumps in an existing cryostat required the adoption
of novel mechanical solutions. Tests conducted permitted the validation of
the technical design of the cryostat and its instrumentation. The characte
ristics of one pump were measured and pressure rise of 300 mbar at nominal
flow of 80 g/s confirmed the specifications.