Overview of LHD superconducting magnet system and its 10-year operation

The Large Helical Device (LHD) is one of the world's largest superconducting systems. It consists of a pair of pool-cooled helical coils, three pairs of forced-flowcooled poloidal coils, nine superconducting bus lines, a helium liquefier and refrigerator of 10-kW class, and six dc power supplies. Its stored magnetic energy reaches 0.8 GJ. Availability higher than 99% has been achieved in the long-term continuous operation since the first cooldown in February 1998 owing to the robustness of the systems and to efforts of maintenance and operation. One major problem is shortage of cryogenic stability of the helical coil conductor due to the slow current diffusion into a thick pure aluminum stabilizer. To improve its cryogenic stability by lowering the temperature, a subcooling system was installed before the tenth cooldown. The outlet temperature of the coil was successfully lowered to 3.8 K from 4.4 K of the saturated temperature, and its operation current was increased to 11.6 kA from 11.0 kA. These experiences of modification, maintenance, and operation should be useful for next large superconducting systems.