Overall characteristics of 9 kW class helium refrigerator for experimental fusion device

The cryogenic system for the experimental fusion device, LHD has an equivalent refrigeration capacity of 9.1 kW at 4.4 K, and will refrigerate all sets of superconducting coils, their supporting structures, superconducting bus-lines and current leads. Eight sets of oil injected screw-type compressors with the massflow rate of 1100 g/s are equipped. Massflow rate to coldbox was controlled to 700 g/s, and the other of 400 g/s was bypassed. To minimize the bypass flow and to reduce the power consumption, overall operating characteristics of the helium refrigerator were investigated in this report. A power consumption of 500 kW was decreased by reducing the massflow rate of 200 g/s in the bypass circuit.