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In a cold test of the JT-60SA CS1 module, which is composed of six octa-pancake coils and one quad-pancake coil wound with Nb3Sn CIC conductors, the measurement of self-magnetic fields generated by the CS1 module was conducted. As a result, the decay of the self-magnetic field was observed after the CS1 module was degaussed. The time constant of the decay was from 90 s to 269 s. The measurement results indicate that loop currents with long time constants occurred in the CS1 module due to coil energization. The loop currents with long time constants, which are also called ‘supercurrents’, are irregular coupling currents that are an unexpected phenomenon from the conductor design. Loop currents with long time constants in CIC conductors have only been observed in large superconducting coils so far. To verify an occurrence of loop currents with long time constants in a CIC conductor without coil winding configuration, self-magnetic fields were measured using a short straight CIC conductor where a multi-strand cable was connected electrically at the conductor ends. As a result, loop currents with long time constants ranging from 57 s to 105 s occurred in the short straight CIC conductor, the length of which is approximately 1 m. The measurement results indicate that the conductor length is not always related to loop currents with long time constants. The occurrence of loop currents with long time constants in the CIC conductor depends on the presence of current paths at the conductor ends.

Publisher Summary This chapter describes the liquefaction control method of 10 kW class cryogenic system. It also discusses the test results of the refrigeration power of each turbine. It demonstrates the automatic liquefaction operation, and measuresthe refrigeration power of each turbine are in good agreement with that of design values. The cryogenic system has an equivalent design capacity of 9.1 kW refrigeration at 4.4 K, and refrigerates all sets of superconducting coils, their supporting structures and superconducting bus-lines for the fusion experimental device Large Helical Device (LHD). The refrigerator is controlled by two sequential programs; for the start-up control of the eight compressors in sequence, and for the driving control of the seven turbines and the liquefaction control of the cryogenic system. The results of the refrigeration power of each turbine are––the setting-up time of all compressors with full load is about 1.5 hours, all turbines rotate stable and the rate of speed fluctuation is less than ±50 rps at the steady state, and the averaged refrigeration powers show good agreement with that of the designed values.

Publisher Summary The Large Helical Device (LHD) is a heliotron fusion experimental device in which magnetic fields are controlled exclusively by superconducting magnets: a pair of helical coils and three pairs of poloidal coils. A 10 kW class helium refrigerator has been constructed for LHD at National Institute for Fusion Science. During the last twelve months between 1995 and 1996, the refrigerator has been put to the training operations without connecting to any of the superconducting magnets of LHD. As one of its series report, this chapter reviews the paper on significant events and major observations experienced during that period and makes a brief discussion about several interesting topics on preliminary base. The observations revealed that there are two insufficient control system designs that forced the operator to pull the shut-down button. One is the external intervention to the engineering workstation through the integrated services digital network (ISDN) line between the manufacturer and the operator. The other is due to the insufficient computer program proof inspection before loading on the VME.