Initiation of thermal runaway in a 1Â T cryocooler-cooled oxide superconducting pulsed coil in AC operation

We experimentally studied thermal runaway in a Bi2223 pulsed coil cooled by a cryocooler. It was found the total heat generation of the coil in thermal runaway was remarkably lower than the cooling capacity of the cryocooler. We simulated the total heat generation of flux-flow loss and AC loss in the winding from the experimental results of the external field losses and the E-J characteristics for the Bi2223 strands in order to estimate the thermal runaway in AC operation. The simulation shows that the thermal runaway is induced by the flux-flow loss localized in both coil sides, which is enhanced with increasing the coil temperature.