Numerical simulation of thermal properties of a cryocooler-cooled Bi2223 superconducting pulsed coil in AC operation

We fabricated a cryocooler-cooled Bi2223 superconducting pulsed coil and experimentally studied the thermal run-away in dc or ac operation. We carried out numerical simulation of thermal properties of the coil in order to explain the thermal runaway of the coil. Firstly, we analyzed the total heat generation of flux-flow losses and ac losses inside the winding from the experimental results of the external field losses and the E - J characteristics for the Bi2223 strands. Secondly, we numerically simulated the thermal properties by using a 2-dimensional heat conduction equation with axial symmetry. The numerical simulation shows the relation between the initiation of thermal runaway and the temperature distribution with highly concentrated heat source in the winding. We have a semi-quantitative agreement between the numerical results and the experimental ones for the condition of the thermal runaway.