Reliable Long-Term Operation of the Cryogenic System for the Large Helical Device

Abstract

Reliability is discussed during long-term operations of the cryogenic system for the Large Helical Device (LHD). The cooled objects of LHD are the helical coils, the poloidal coils, the supporting structure and the superconducting bus lines. The cryogenic system, which permits complicated cooling schemes for each cooled object, was designed to focus on the reliable long-term operation. Impurities in the helium gas should be controlled carefully to prevent a blocking of filters, malfunction of control valves and deterioration of heat exchanger performance. The first cycle operation of LHD started on February and ended on June in 1998, and the second cycle operation started on August and continued to the end of 1998. The performance during the first and second cycle long-term operations and the precise impurity measurement technique are reported.