AC loss and eddy current loss are inevitable in AC HTS (High Temperature Superconductor) magnets and even in DC HTS magnets under varying magnetic field or current. To secure thermal stability, especially in the conduction cooling, reduction of contact thermal resistances (CTR) is one of the crucial factors. To avoid the delamination problem, epoxy is not usually used between turns of ReBCO wires while thin epoxy layers between coil surfaces and metallic bobbins are used to enhance thermal conduction and remove the heat loss in conduction cooling applications. Therefore, appropriate thermal modeling of the HTS coil is necessary to ensure the stable operation with an appropriate overall thermal resistance value, which includes the contact thermal resistances between turns of the coil, in transverse direction of the coil. This paper describes the measurement of the overall thermal resistances and thermal contact resistance in transverse direction in the coils. Three test coils are fabricated with ReBCO wires: 1) a coil without conduction plates except a metallic center form connected to a cryocooler; 2) two coils with conduction plates filled by epoxy (Stycast 2850 FT, CTD 521) layers. The overall thermal resistance is measured for the coil without conduction plates from 10 to 50 K. The CTR, which can be used as a typical value of other coils, is also obtained by investigating the measured overall thermal resistance, the known transverse thermal conductivity of the wire and the epoxies. The overall thermal resistances of the coils with the conduction plates and epoxy layers are also measured and they are compared with an analysis model results that includes the conduction plate, epoxy layer, and the measured CTR. The results validate the measured CTR, which can be applied for the estimation on the overall thermal resistance of other coils with given ReBCO wires.
|Submitters Country||Republic of Korea|