Influence of Thermal Cycles on Critical Current and Mechanical Loss in Superconducting Coil Having Negative Thermal Expansion Bobbin

We have studied the vibration of superconducting windings during AC excitation of coils. Our previous study revealed that mechanical loss due to the vibration of a coil that had an expansion bobbin during cool-down was smaller than that of a coil having a contraction bobbin. However, critical currents of the coil with the expansion bobbin decreased as the number of thermal cycles increased between room and cryogenic temperatures. Hence, excessive expansion of the bobbins causes degradation of coil performance. In this study, we fabricated superconducting coils whose bobbins expanded a little, and measured the mechanical losses of those coils. We present here the expansion properties of the bobbins, test results of the critical currents and mechanical losses, and the dependence of the thermal cycles on those results.