A stability criterion for cryocooler-cooled HTS coils

The magnet technology of a high-temperature superconductor (HTS) with a conduction cooling system by a GM cryocooler has been advanced. It is very important to understand the thermal and electromagnetic phenomena in HTS tapes for HTS coil design. We have measured the quench characteristics of HTS tapes with a conduction cooling system of a GM cryocooler and developed a computer program based on the finite element method (FEM) to investigate the thermal and electromagnetic behaviors within HTS tapes. From these experimental and analytical results, the validity of the computer program has been confirmed. In this paper, we discuss a stability criterion for cryocooler-cooled HTS coils by using the computer program. The current at which "thermal runaway" occurs, which depends on the relationship between the cooling power of the GM cryocooler and the heat generation within HTS coils, is chosen as a stability criterion. We also investigate the transient thermal and electromagnetic behavior in HTS coils.