Recent progress of superconducting magnet system design for Large Helical Device (LHD) of National Institute for Fusion Science (NIFS) is described. LHD is one of large scale nuclear fusion devices which will be completed in the 20th century, and fully superconducting coil system to create magnetic field. LHD is composed of a plasma vacuum vessel, superconducting helical and poloidal coils, cryogenic supporting structures, and an outer vacuum vessel, and surrounded by utilities of coil power supply, helium refrigerator and nitrogen supply, and vacuum pumping system. Cryogenic mass of the system is estimated as 860tons including coil and supporting structure. Cooling method of the superconducting coil is decided as bath cooled of liquid helium for the helical coil, and force cooled of supercritical helium for the poloidal coil. Major and minor radii of the helical coil are 3.9m and 0.975m, respectively. The conductor material of Nb-Ti alloy is used for both helical and poloidal coils from the point of strain effect. Conductor maximum field will be 6.9T for the plasma center field of 3T at the first phase with saturated boiling helium and 9.2T for 4T at the second phase with superfluid helium, Electromagnetic force and electrical insulation of the coil are the most important items to design the coil. Research and development work is intensively continued now, including conductor stability, helical and poloidal winding, elasticity of the coil, and cryogenic handling.
In measuring strain and displacement at cryogenic temperatures, the strain-gauge measurement system is particularly noted for high accuracy and ease of handling. When measuring at cryogenic temperatures, it is important to grasp in advance the fundamental characteristics of a strain gauge, such as the thermal effects on the apparent strain and gauge factor. There are a variety of trial strain-gauge instruments under examination for measuring displacement. They are roughly classified either in the rod type or the clip-on type, which respectively meet specific measurement purposes. Their fundamental characteristics depend on the strain gauges being used. They serve in various fields to measure displacement by taking advantage of distinctive strain gage features.
Detailed knowledge on the steady-state and transient heat transfer from solid surfaces in He I and He II is important as a database for the analysis of the influence of local thermal disturbances on the stability of He I or He II cooled large superconducting magnets. In this paper, an overview of the transient heat transfer characteristics on solid surfaces in He I and He II caused by various large stepwise heat inputs, such as the quasi-steady nucleate boiling with a certain lifetime in He I and the quasi-steady Kapitza conductance heat flux with a certain lifetime in He II, are presented in comparison with their steady-state heat transfer characteristics.