New Evaluation Method of Superconductor Characteristics for Realizing the Large Helical Device

"It is necessary to apply extra large scale superconducting magnets for a fusion device like Large Helical Device (LHD). Because of the relatively larger current (10kA or more), new evaluation method to determine the fundamental characteristics of superconductors have to be developed, which are already established for thin superconducting wires. In this paper, a measuring method of critical current, stability margin, current distribution are mainly discussed. Bath-cooled composite type and supercritical helium force-cooled type NbTi conductors have been chosen for helical coils and poloidal coils of LHD, respectively. A critical current of a conductor was measured as follows. Two 2 m long parallel conductors, which were connected each other at the bottom, were inserted in a 9 T split coil. The surface of the conductor was covered 50% by spacers to simulate heat transfer conditions. The time dependent transition phenomena between normal and superconducting state are observed by voltage distribution along the conductor. On study of forced flow cooled conductors, the current transfer from a quenching strand to another strand was measured by pick up coils on a conduit. We measured its stability margin by a pancake-type coil as a long sample test. The major measurement results of bath- and forced-cooled LHD conductors using this system have given enough fundamental data of high current multistrand conductors for the LHD.