T. Obana

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.

Large helical device, the largest superconducting stellarator, has been operated for the research of fusion plasma since 1998. The toroidal field of almost 3 T is produced by a pair of pool-cooled helical coils, in the innermost layers of which a normal-zone had been induced several times at the bottom of the coil at higher currents than 11.0 kA. Since the field is not the highest there, the local cooling conditions are probably deteriorated by bubbles gathered by buoyancy. In order to improve the cryogenic stability by subcooling, an additional cooler with two-stage cold compressors was installed at the inlet of the coil in 2006. The inlet and outlet temperatures of the coils were successfully lowered to 3.2 K and 3.8 K, respectively, with a mass flow of 50 g/s. In spite of a half charging rate to reduce AC losses, a normal-zone was induced near the top of the coil at 11.45 kA. It propagated to one side and stopped near the inner equator, where the field is the highest. In comparison with the stability tests with a model coil, the local temperatures of the innermost layers near the top is considered to have been raised up to almost the saturated temperature of 4.4 K by charging. The excitation method was revised to waiting cool-down at 11.0 kA, and the excitations up to 11.5 kA have been attained.