Effect of Thermal Cycles on Critical Current and AC Loss for Superconducting Coils Having Positive or Negative Thermal Expansion Bobbin

We investigated the effect of thermal cycles between room and liquid-nitrogen temperatures on the critical current (Ic) and AC loss of two superconducting coils: one with a bobbin that expands during cooling from room temperature to cryogenic temperature and one with a bobbin that contracts during the cooling. After 100 cycles, neither bobbin suffered degradation. The Ic of the contraction-bobbin coil did not decrease, and that of the expansion-bobbin coil decreased due to the repeated thermal strain. The expansion coil´s AC loss was significantly smaller than the contraction coil´s loss at the first cycle; however, it increased with the thermal cycles and eventually surpassed the contraction coil´s loss because the Ic of the expansion coil decreased due to the thermal fatigue. These results indicate that a moderate expansion in the size of the bobbin effectively decreases AC loss and that excessive expansion reduces the Ic and increases the AC loss.