Operation and Control of Helium Subcooling System of LHD Helical Coils During Change of Rotational Speed of Cold Compressors

Helical coils of the Large Helical Device (LHD) are large scale superconducting magnets for fusion plasma experiments. The cooling system was upgraded in 2006 to improve the cryogenic stability of the coils by subcooling supplied helium. After the upgrade, the subcooled helium of designed mass flow rate of 50 g/s at designed temperature of 3.2 K has been supplied to the coils stably during steady state subcooling operations. The supplied helium is subcooled at a heat exchanger in a saturated helium bath. A series of two centrifugal cold compressors are utilized in the subcooling system to reduce the bath pressure and temperature. It is necessary to operate the cold compressors within a range of appropriate mass flow rate in order to escape from surge and choke.When the rotational speed of the cold compressors increased/decreased, the mass flow rate of the cold compressors varied. Thus, it is important to comprehend the transient performance of the cold compressors in those cases and to control the mass flow rate for the purpose of the safe operations. In the present study, the characteristics of the system were investigated and automatic operating methods with a heater in the bath and a bypass valve were developed during the change of the rotational speed of the cold compressors. Consequently, the reliable subcooling operations of the system have been achieved over 6,000 hours for three years.