Performance of the LHD cryogenic system during cooling and excitation tests

Performance of the LHD cryogenic system in the first year's operation was described making importance on the recovery process after the normal transition of the helical coils. During the excitation tests of the LHD superconducting coils up to 2.75 T, the normal zone propagation was observed in the helical coil and the emergency shut-off of the coil power supplies was carried out. 2,700 l of liquid helium evaporated from the helical coils. The coils and the helium refrigerator were separated automatically and the helium refrigerator could keep its steady state operation. After the pressure and flow rate of the recovery gas from the helical coils were settled down to the normal state, the coils were connected to the helium refrigerator and the cooling was restarted. The system could return to the steady state in which coil excitation is enabling, by only three and a half hours.