High-temperature superconducting coil option for the fusion reactor FFHR and development of indirectly-cooled HTS conductors

Large current-capacity high-temperature superconducting (HTS) conductors using YBCO wires are being considered as an option for the fusion energy reactor FFHR [1, 2]. Typical required current for such conductors in FFHR is 100 kA at about 13 T magnetic field. The operating temperature for HTS magnets in FFHR is about 20 K with conduction cooling using an indirect cooling method. Since the specific heats of the materials at 20 K are significantly higher as compared with 4.2 K, which is the typical operating temperature of low temperature superconducting (LTS) magnets, the HTS magnets are operated much more stably and reliably against any mechanical or electromagnetic disturbances, which is one of the biggest advantages over LTS magnets. Another advantage is the reduction in required refrigeration power as the HTS magnets are operated at the elevated temperatures. Or, using HTS conductors, it is also considered to be viable to assemble the continuous helical coils in segments with a number of joints for conductors, as we can allow additional heat generation utilizing the surplus refrigeration power [3]. Two options of aluminum-alloy and SUS jacketed HTS conductors have been considered. The maximum strain in the winding has been estimated to be less than 0.3%, which is less than the critical strain of ~0.5% for YBCO conductors. As the first step towards the HTS conductor developments, a 10 kA-class conductor has been fabricated using Bi-2223/Ag tapes, by simply stacking and soldering them inside a copper sheath. The critical currents of the conductor have been measured at 4.2 K and at elevated temperatures up to 30 K. The stability margin experiments have also been done. The HTS conductor was found to be highly stable. In near future, the YBCO coated-conductors are planned to be used for next HTS conductor sample, and the stability and quench characteristics will be compared with those obtained for the present one.