Hideharu Yanagi

Effect of impurity content in stainless steel on resolidified surface condition after disruption load

Plasma disruptions in tokamaks may cause melting of the surface layer of first walls. If the layer resolidifies without any movement, the same layer repeatedly melts and protects the rest of the wall. A number of stainless steel samples were irradiated by a neutral beam injector. After irradiation tests with the same condition, some had wavy resolidification surfaces with dents, while some had smooth surfaces. In the former samples, the amplitude of the roughness increased by repetitive heat loads. The melt layer moved to the convex areas, which deepened the dents on each occasion. Sulphur was found to be one of the harmful elements which promotes the formation of the wavy surface. It is recommended to be kept below 0.005 wt% for obtaining a uniform resolidification surface. Even if samples have a low sulphur content, a high oxygen content also makes the surface rough. Other impurities have some effects, but they have not been clarified yet.

Spatial distribution of radicals produced by fast neutron irradiation

Abstract The linear energy transfer (LET) of ∼0.5 MeV fast neutron in organic compounds is known to be more than 100 times larger than that of 60Co γ-ray. The local concentration of radicals produced by irradiation with two different kinds of radiation were evaluated. Microwave power saturation of electron spin resonance (ESR) spectra of radicals formed by fast neutron irradiation in a fast neutron source reactor “YAYOI” at 77K has been measured. Although ESR spectra of irradiated cyclohexane, polymethylmethacrylate and polyethylene show no large difference between neutron and γ-irradiations, power saturations of the ESR spectra for the two irradiations are very different. From the saturation characteristics, the spin-spin relaxation time in fast neutron irradiated samples was evaluated to be 3∼7 times smaller than that of γ-irradiation. In other words, the local concentration of radicals formed by fast-neutron irradiation is 3∼7 times higher than that formed by γ-irradiation. The spatial distributions are discussed in connection with LET of the radiations and structures of spurs in the matrix.

Development of a highly reliable helium refrigeration system––R&D of a highly reliable helium refrigeration system (oil-free type)

Abstract The present paper reports the development of the highly reliable refrigeration system (three types), which is applied for 70 MW class superconducting generators. R&D of the oil-free turbo type: In the turbo type of advanced refrigeration system, a complete oil free 80 K cold compressor was developed. Moreover, the cold compressor driven by a coaxial turbine was developed to enhance the system efficiency and to save space. R&D of the oil-free screw type: In the oil-free screw rotors, taper rotors were planned to maintain a radial clearance between male and female rotors and the casing, to compensate their thermal deformation because of the heat of compression. Moreover, a seal technique was established that used a magnetic fluid and gas seals to prevent lubricating oil from bearings and the timing gear chamber into the rotor housing, under a higher rotational speed of 25,000 rpm.

Basic cryogenic heat transfer research on superconducting magnet cooling

The cryogenic heat transfer research program at UTBLT is devoted primarily to the fundamental heat transfer characteristics involved in the development of large superconducting magnets for fusion reactors. This includes steady and transient heat transfer, flow instability phenomena, stabilization of superconductors and advanced refrigeration systems. It also contains a study on magnet system design and optimization. Extensive experimental and analytical research has been conducted on various interesting and challenging problems. In this paper a few aspects of the program will be reviewed.