抄録
For the purpose of the stability analysis of a pool boiling superconductor, many studies on the heat transfer of LHe have been conducted. There are a number of variables that affect heat transfer. It is well-known that to variables are surface orientation and surface treatment. Surface orientation of a superconductor is varied by magnet winding. The change is associated with the variation of gravitational force on the surface, thus causing heat transfer characteristics to change. Usually, the surface of a superconductor is treated to improve heat-transfer characteristics; for example, oxidation. During the winding process, the winding machine may strip off the treatment at some locations. The resulting damage may change the heat-transfer characteristics and degrade the stability of the superconducting magnet. In this study, the heat transfer of polished Cu, oxidized Cu and partially oxidized surfaces were measured as a function of orientation. The critical and minimum heat fluxes depend on the area fraction of oxidation. The calculation method for the heat flux of a partially oxidized surface was established. Heat-transfer measurements of a prototype superconductor with polished surfaces were also conducted to change the surface orientation. The heat-transfer characteristics with the prototype superconductor were degraded as compared with those of the Cu surface. It became clear that heat transfer for a stability analysis must be measured using a prototype superconductor. Next, the recovery current for a large-sized pool boiling superconductor, which is a helical coil superconductor, was calculated according to the experimental results of heat transfer. The dependence of the recovery current on surface orientation and the area fraction of surface treatment was estimated. The calculated results were compared with the measured recovery currents of the short sample. The calculated recovery current agreed well with the experimental result.
