The Large Helical Device (LHD) is a heliotron-type toroidal fusion experimental device which will provide useful and reliable datasets for high-temperature helical plasmas with an equivalent Q value of 0.1 to 0.35. One of the crucial tasks of LHD is to demonstrate steady-state operations by taking the advantage of its magnetic configuration with currentless plasmas as well as built-in divertors. In this respect, the coil systems are fully superconducting (SC); consisting of a pair of helical coils, three pairs of poloidal coils and nine bus lines. All the SC coils have been successfully fabricated with high accuracy and they are presently in the final stage of assembly in a cryostat vessel along with outside preparations of plasma heating devices, diagnostic equipment and control systems. The first plasma operation is scheduled for the end of March 1998. The technological development of SC magnets through this project played a key role in LHD, and will also be useful for constructing future fusion reactors.
The Large Helical Device (LHD) under construction at the NIFS is a plasma physics experimental device consisting of two helical coils wound from aluminum-stabilized composite superconductors, three pairs of poloidal coils wound from cable-in-conduit conductors, a cryostat, and a plasma vacuum vessel. The design concepts of the helical and poloidal coils are especially described. Successful IV-L coil tests showed the validity of the poloidal coils. The LHD superconducting coils, cryostat, and plasma vacuum vessel will be assembled at the end of 1997, and trial tests will be carried out in March 1998.
A pair of helical coils for LHD, the largest pool-cooled superconducting coil system with a magnetic energy of 1.6GJ. In order to produce a fine magnetic surface, highly accurate manufacture within ±2mm and small deformation below 3.4mm against electromagnetic forces at 4T operation are required. Besides high current density, more than 53A/mm2 is necessary to keep sufficient distance between the coil and the plasma vacuum vessel. The helical coils are designed to satisfy the cryostable criterion by optimizing the wetted surface fraction of each conductor. Against large electromagnetic forces, the conductors were packed into thick cases supported by an outer shell structure. In order to suppress the maximum tensile stress in the conductors below the yield stress, the average compressive Young's modulus of the coils should be high. We used an insulator with a high compressive modulus of 30GPa, and we planned to maintain the average gap between layers within 65μm while winding. Whole conductors 36km long had been wound on-site by day-and nighttime work over 16 months. We have successfully managed average gap and position error within 50μm and ±1.5mm, respectively. After winding, the top cover of the coil cases were set up on the coils. By inserting machined spacers and controlling the shrinkage of welding, the top covers were successfully installed without any additional gaps and without deforming the accurate shape.