Increase of Operating Current of the LHD Helical Coils by Upgrading the Cooling System

The Large Helical Device (LHD), one of the world's largest superconducting systems, has been operated for the research of fusion plasma since 1998. The toroidal field of almost 3 T is made by a pair of pool-cooled helical coils. When they were cooled with saturated liquid helium at 4.4 K, a normal-zone had been induced several times in their innermost layer at the higher current than 11.0 kA. In order to improve the cryogenic stability by lowering temperatures, an additional cooler with two-stage cold compressors was installed at the inlet of the coil. The inlet and outlet temperatures of the coils are successfully lowered to 3.2 K and 3.8 K, respectively, with a mass flow of 50 g/s. Stable operation of the subcooling system has been established, and the operating current for the plasma experiments can be increased up to 11.4 kA.