Achievement of High Availability in Long-term Operation and Upgrading Plan of the LHD Superconducting System

"The Large Helical Device (LHD) that has been demonstrating high performance of heliotron plasma is the world\u27s largest superconducting system. Availability higher than 98% has been achieved in a long-term continuous operation both in the cryogenic system and in the power supply system. It will be owing not only to the robustness of the systems but also to efforts of maintenance and operation. One big problem is shortage of cryogenic stability of a pair of pool-cooled helical coils. Composite conductors had been developed to attain the sufficient stability at high current density. However, it was revealed that a normal-zone could propagate below the cold-end recovery current by additional heat generation due to the slow current diffusion into a thick pure aluminum stabilizer. Besides, a novel detection system with pick-up coils along the helical coils revealed that normal-zones were initiated near the bottom of the coil where the field is not the highest. Therefore, the cooling condition around the innermost layers, the high field area, will be deteriorated at the bottom of the coil by bubbles gathered by buoyancy. In order to raise the operating currents, methods for improving the cryogenic stability have been examined, and stability tests have been carried out with a model coil and small coil samples. The coil temperature is planned to be lowered from 4.4 K to 3.5 K, and the operating current is expected to be increased from 11.0 kA to 12.0 kA that corresponds to 3.0 T at the major radius of 3.6 m.