Toshiyuki Mito

Stability Characteristics of the Aluminum Stabilized Superconductor for the LHD Helical Coils

Stability tests have been carried out on the aluminum stabilized composite-type superconductors developed and used for the pool-cooled helical coils of the Large Helical Device (LHD). The longitudinal voltage of a normal zone shows a short-time rise before reaching a final value, which seems to be explained by the rather slow diffusion process of transport current into the pure aluminum stabilizer and the copper housing. The propagation velocity has a finite value even below the recovery current, and it differs depending on the direction of transport current. A numerical analysis dealing with transient thermal and electromagnetic relaxation processes in the aluminum stabilizer well explains the experimentally observed results.

Design and Construction of Cryogenic Components for LHD

Publisher Summary This chapter describes the current status of the construction of certain cryogenic components for Large Helical Device (LHD) in the aspect of thermal performances. The Large Helical Device (LHD) is a heliotron fusion experimental device in which magnetic fields are controlled exclusively by superconducting magnet: a pair of helical coils and three pairs of poloidal coils. The refrigerator for the 3 Tesla experiments (phase I) has already been constructed and is being put to trial operations without LHD magnets. On the other hand, construction of other key cryogenic components has progressed as well. The chapter reviews their design and heat load measurements obtained in NIFS experiments. It describes the developments on some new technologies. The results are encouraging and are expected to contribute to the operation of LHD cryogenic system significantly.

Construction Report of 10 kW Class Helium Refrigerator for LHD

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.

Proposal of a New ‘Low Loss-High Stability Type’ Rutherford Cable

A new structure of Rutherford cables is proposed in which low resistive paths, enabling transfer of transport currents from one strand to another when a normal quench occurs, do not induce increase in coupling losses. This cable is designed so as to have a relatively longer strand twist pitch in relation to cable twist pitch and good crossover contacts between stands located near each edge of the cable’s cross-section. The inter-strand coupling current is reduced inside the cable in changing transverse magnetic fields with a face-on orientation, which qualitatively results in decrease in coupling losses. In order to confirm quantitatively the expected loss property, two-dimensional finite element method analyses are carried out. It is found that the coupling loss of this cable can be reduced to between 1/5 and 1/10 the amount calculated in some previous cable designs in spite of the existence of the low crossover resistances between -ands, required for ensuring high stability.

Liquefaction Control of 10 kW Class Cryogenic System for the LHD

Publisher Summary nThis 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.