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Trapped field characteristics are one of the key factors in the application of high-temperature superconducting bulk, such as magnetic levitation systems, motors, bulk magnets and so on. The immutable trapped field is required in superconducting bulk applications as a quasi-permanent magnet, however, the trapped field is influenced and changed by a time-varying external magnetic field in a realistic operational environment of electrical devices. This means that shielding current distribution within the bulk is changed by the time-varying magnetic field and the transient magnetic flux movement results in temperature rise and finally reduction of the trapped field. In this study, we observed the abrupt reduction of the trapped field for the external AC magnetic field in the range of frequency up to 50 Hz. And we also performed the numerical simulation using a newly developed computer program based on the finite element method (FEM). In the simulation program, the voltage-current (E-J) characteristics, which depend on the irreversibility temperature are taken into account. From these results, it can be considered that the abrupt reduction of the trapped field is caused by the temperature rise due to AC loss.

The Large Helical Device (W) mQr constnrtion at NIFS is a nuclear fusion experimental device consisting of two superconducting helical coils andthree pails of poloidal coils. A pairof innervertical (IV) coils with a combined stored energy of 161 MJ have been deliveredto NIFS. They were the 4 m outside dameter coils wound fmm cable-in-conclit conductors composed of 486 strands with a copper matrix. We conducted supercritical helium cooling and excitation tests to confirm the cooling and superconducting characteristics of one of the IV coils before its assembly into the LHD cryostat in 1997. It has been assembled into the SHe test facility at the Toki site of NIB, and was cooled down andenergizedto 2.2 kA in Febnary and iWmh 1995. The test facility will be repairedin part, and the coil will be tested to confirmthechatactenstics at the rated current of 20.8 kA within the year.