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Description
Thermal stability is one of the important parameters for the conduction cooled superconducting coil. In the conduction cooled superconducting coil, the only cooling source for the coil is the cold head of the refrigerator, thus a good thermal contact between the coil and the cold head is required. In general, an impregnation is inserted between the wires in the coil. For example, epoxy and greases are used in the superconducting coil as the typical impregnations. However, voids are easily caused in the impregnation during the coil fabrication because these impregnations are high viscosity, and these voids can cause the performance degradation of the coil. In addition, these impregnations are difficult to remove after the coil fabrication, thus the superconducting wire cannot be recycled. In this study, we proposed a new method using ionic liquids as the impregnation. The ionic liquids have many unique properties. For example, the ionic liquids are low viscosity at the room temperature and the crystalline or glassy at lower temperature. Hence, by using ionic liquids, the fabrication of the coils without voids in the impregnation can be handle easier than by using epoxy. In addition, the vapor pressure of the ionic liquid is very low under the high vacuum. Furthermore, the superconducting wires can be recycled because the ionic liquids are low viscosity at room temperature. We prepared the epoxy and two types of ionic liquids as the impregnation. The thermal conductivities of the epoxy and two ionic liquids were measured by a steady-state heat flow method, when the temperature of the impregnation was varied from 40 to 90 K. As a result, the thermal conductivity of ionic liquid is higher than that of epoxy, which shows that ionic liquid may be applied as a heat drain material for a conduction cooled coil. The results provide fundamental data for improving the thermal stability of conduction cooled superconducting coils.
