Abstract
An extensive acceptance test program for the Wendelstein 7-X (W7-X) confinement coils is presently being carried out in the CEA Saclay cryomagnetic test facility. Over half of the 50 non-planar coils and 20 planar coils have already been subject to a cool-down to liquid helium temperature, allowing current tests in the superconducting state. This paper presents a description of the cooling protocol observed at CEA. In-depth background information about the helium refrigeration technology limitations, the coil active cooling procedure and its control are given. The cryogenic power extraction is estimated through mass flow rate and enthalpy balance of the winding and casing helium circuits, which are derived from various coil and facility sensors. Coils geometry and material thermal properties are given, as well as a simple modelling of the coil cooling. The observed coil thermal behaviour can help to better understand the role of the casing cooling loop on the cooling inertia and on the total cooling time. Finally, the cooling down process is projected into the future operation of the W7-X stellarator under construction. More generally, data and experience gained from the cooling tests operated at the CEA Saclay cryomagnetic test facility provide practical knowledge to foresee the thermal behaviour and cryogenic challenges of other large magnets for thermonuclear fusion.
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