Fundamental study for the establishment of the design principle of conduction-cooled oxide superconducting magnets

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Paying attention to the conduction cooling to cool oxide superconducting magnets in place of the conventional bath-cooling with liquid helium, we studied the stability and the protection method of oxide superconducting magnets. In order to get rid of the design principle for the conventional low-Tc superconducting magnets, which consists of the stability theory based on the cooling characteristics of liquid helium and the protective condition of superconducting magnets, e.g. the recovery-condition of stored energy, we aimed at constructing the new design principle of conduction-cooled oxide superconducting magnets operated at over 20K, including the fabrication method of oxide superconductors with a large current capacity. First we observed the electromagnetic and thermal properties of a 1T cryocooler-cooled oxide superconducting coil. It was shown that the thermal runaway of the coil depended on the local thermal balance between the heat generation due to the flux-flow loss and the cooling due to thermal conduction inside the winding, not on the balance between the total heat generation of the coil and the cooling capacity of the cryocooler. Next we made numerical expressions of the temperature dependencies of the heat generation of oxide superconducting wires due to ac loss and flux-flow loss with large anisotropy and the thermal conductivity inside the winding. We also developed a numerical simulation program of the behavior of the conduction-cooled coil and made it possible to discuss the dependences of the thermal runaway of the coil on temperature and transport current. As a result, we showed the design principle of conduction-cooled oxide superconducting magnets clarifying the difference between the conventional bath-cooled superconducting magnets and conduction-cooled oxide superconducting magnets.