Combined use of superconducting magnets and suitable paramagnetic or ferromagnetic materials makes possible the utilization of the magnetocaloric effect to efficient refrigeration cycles starting as high as room temperature. The present article describes the fundamental principles of three basic magnetic refrigeration cycles corresponding to the reversed Carnot, Stirling and Ericsson cycle in gas refrigeration. A brief review of the present status of research and possible applications of magnetic refrigerators are made.
In designing a cryogenic storage system with low heat leak, a cautions consideration must be paid on the selection of a support structure and materials connecting the inner vessel with the outer vessel. In view of the structural and thermal requirements on a cryogenic storage system, support materials must exhibit high tensile yield strength accompanied by low thermal conductivity, and these properties must be retained even at cryogenic temperatures. From these points of view, measurements were carried out on mechanical and thermal properties of candidate materials for cryogenic structural support down to liquid helium temperature. The experimental results show that in metallic materials, Titanium Alloys displayed a large ratio of tensile yield strength to thermal conductivity. On the other hand, non metallic materials such as glass- or carbon-fiber reinforced plastics (GFRP, CFRP) exhibited higher strength/conductivity ratios than metallic materials. One particular result is that CFRP showed the extra low thermal conductivity in the temperature range 77K to 4.2K. On the contrary, in the temperature over 77K, GFRP showed lower thermal conductivity than CFRP. Thus, the combined use of GFRP and CFRP will be recommended for cryogenic structural support system.
Several organic insulators expected to be used in the construction of superconducting magnets in fusion reactors are irradiated in a fission reactor at about 5K and the effect of radiation on their mechanical properties has been investigated at liquid nitrogen temperature. Breaking stress and yield stress of epoxy resins hardened by aromatic amine and aliphatic amine are reduced after irradiation of about 1×109 rad. Polyimide and fibre reinforced epoxy have high radiation resistance.
It is useful for designing superconducting magnets to find the relation between the stored energy and the composite current density: Here we discuss the relation using the requirements for the full stabilization and energy dumping. Comparing our arguments with the performance of the magnets built before, we find that the relation can be devided into following three regions; (1) Energy Dumping Limit, where the current density is inversely proportional to the square root of the stored energy. (2) Full Stability Limit, where the current density is almost independent of the stored energy. (3) Superconductor Limit, where magnets can be wound without the liquid helium cooling channels, and the current density is determined by the critical current density of the conductor. Then we figure out the problems to be solved to develop large-scale superconducting magnets.