Study on the Mechanism of Preventing Degradation in the Performance of REBCO Coils

After the commercialization of REBCO-coated conductors, a number of REBCO coils have been fabricated targeted toward various magnet applications. One of the major technological problems is degradation in coil performance such as in the critical current, due to delamination of the REBCO-coated conductors in epoxy impregnated windings. A number of effective methods to prevent such degradation have been proposed by several groups including our laboratory. However, detailed mechanisms of those methods have not been definitively clarified and thus they have not been technologically categorized in REBCO coils technology. In this paper, we discuss these mechanisms, focusing on the effects of different mixture ratios of curing agent and epoxy prepolymer on the performance of epoxy-impregnated coils. Also, a direct interaction between the YBCO layer and epoxy at room temperature was investigated for short wire samples. The results showed that a REBCO coil impregnated in epoxy prepolymer has no degradation in critical current. The degradation of the coil was induced by the addition of the curing agent, and the critical current was decreased by the increase in the amount of the curing agent. The direct effect shows a decrease in the critical current with the increase of the impregnating duration. The optimum conditions with the impregnating duration were investigated to protect the wire from the direct interaction.