Subcooling system for LHD helical coils and its operation

The Large Helical Device (LHD) is an experimental devise for heliotron plasma. The helical coils, which are large scale superconducting magnets, had been cooled with saturated helium at 4.4 K until 2005. However, normal zone propagations were observed 17 times at the innermost block of lower portion of the coils, where the cryogenic stability was degraded by geometric factor. Thus, the cooling system of the coils was upgraded in 2006 to improve the cryogenic stability of the coils by using subcooled helium as the coolant. A nominal mass flow rate and temperature of the subcooled helium at the inlet of the coils are 50 g/s and 3.2 K, respectively. The supplied helium is subcooled at a heat exchanger in a saturated helium bath. The bath pressure and temperature are reduced by a series of two centrifugal cold compressors with gas foil bearing. The helium flow through the compressors is automatically regulated by a heater in the bath to stabilize amount of the evaporated helium. In the present study, performance of the upgraded cooling system was investigated and optimization of the operating method was performed. Consequently, the designed mass flow rate and temperature of subcooled helium could be supplied to the coils and stable long-term operations have been achieved. The improvement of the cryogenic stability was also confirmed and the maximum average current of 3 blocks of the coils reached up to 11.833 kA. (author)