Refrigeration system for W7-X

The refrigeration system for the W7-X superconducting magnet and the divertor cryo-vacuum pumps is presented. In total, five main helium cooling circuits have to be supplied by the refrigerator—four for the magnet including auxiliary equipment like support structure, thermal shield and current leads, and one for the cryo-pumps. For the shields of the latter, an additional LN2—cooling circuit is required. The lowest operating temperature is 3.3 K. It will be provided by evacuating a sub-cooler bath using a cold or warm compressor. Three of the helium cooling circuits use altogether four identical cold circulators. Apart from the current leads which are supplied with the coolant from a LHe storage tank, the peak reserve power required is equal to 7 kW at 4.5 K entropy equivalent. However, this potential maximal demand occurs continuously for periods of only a few hours at most, and altogether for less than 1% of annual time. The refrigerator thus will be designed for 5 kW continuous power at 4.5 Kequiv. corresponding to 1.5 MW compressor connected rating. The reserve peak power will be covered, if necessary, by using the latent heat and vapour enthalpy of LHe from a storage tank. This supporting LHe stream is added to the phase separator and fed subsequently to the low pressure return stream at the cold end of the cold box. LN2-pre-cooling equipment of the cold box—which is installed for W7-X cool-down anyway—can also be used to increase refrigeration power. The LHe required for maintaining reserve refrigeration power as well as for running the current leads is generally produced overnight when W7-X is in idle current mode.