Operations of the Helium Subcooling System for the LHD Helical Coils during Ten Plasma Experimental Campaigns

The Large Helical Device has a helium subcooling system with two cold compressors for helical coils to enhance the magnetic fields and improve the cryogenic stability of the coils by lowering the coil temperature. The system was installed in 2006 and then it has stably supplied 3.2 K subcooled helium at the nominal mass flow rate of 50 g/s to the coils during ten plasma experimental campaigns. The running time of the cold compressors exceeds 30,000 hours and the total time of subcooling operations exceeds 20,000 hours. In the system, the supplied helium is subcooled in a heat exchanger of a saturated helium bath. The bath pressure and temperature are reduced by a series of two centrifugal cold compressors with gas foil bearing. In the steady state subcooling operation, the bath temperature is stabilized within range of 0.02 K with automatic flow control of helium gas through the cold compressors by a heater in the bath. The control method is also useful to protect the system by mitigating large disturbance of the pressure and the mass flow rate. In the present study, the thermal hydraulic behavior of the system and the operational performance during ten plasma experimental campaigns are reported. (C) 2018 The Japan Society of Plasma Science and Nuclear Fusion Research