Large-current capacity High-Temperature Superconducting (HTS) conductors have been developed for the helical fusion reactor and the next-generation helical experimental device. Three kinds of HTS conductors, STARS, FAIR, and WISE, have been proposed with different internal structures. For these conductors, the common target is to achieve a high current density of 80 A/mm2 at a magnetic field of >10 T and a temperature of 20 K. The STARS (Stacked Tapes Assembled in Rigid Structure) conductor was originally developed to be applied to the helical fusion reactor FFHR with a 100-kA current capacity. A prototype conductor sample with a total 3-m length achieved 100 kA, having a 300-mm portion under a 5-T magnetic field with 20-K temperature control by a heater. The STARS conductor uses simply-stacked REBCO HTS tapes embedded in a copper (Cu) stabilizer and stainless-steel (SS) reinforcement jacket. The conductor is mechanically robust without having any deformation in REBCO tapes due to non-twisting and non-transposing. Recently, a 20-kA-class STARS conductor has been developed to apply to the next-generation helical experimental device. A 3-m-long sample was fabricated with laser beam welding in the SS jacket. The sample was tested in the large superconductor testing facility by applying a magnetic field using a split coil over a 300-mm section (like the previous testing). Still, a uniform helium gas cooling was done for the straight sample of 2-m length. A 20-kA current was stably transported at 8 T, 20 K. Then, another STARS conductor sample was fabricated with internal electrical insulation between the Cu stabilizer and SS jacket. The critical current was observed in liquid nitrogen to show the same value as the former sample with no internal insulation. A 6-m long sample of the internally-insulated type STARS conductor was then fabricated with a 600-mm diameter and a coiled structure of 3 turns. It was tested in 8 T, 20 K using a facility equipped with a 13-T, 700-mm bore solenoid coil. This time, a magnetic field was applied uniformly to the whole conductor of 6-m length, and temperature control was also done for the whole sample length with indirect cooling by supplying helium gas in a separate tube mechanically attached to the conductor. A stable operation up to 18 kA was confirmed, together with 100 times of repetitive excitations with a 1 kA/s ramp-up and down rate. For the success of this experiment, the low-resistive mechanical lap joints between the current feeders and the conductor terminals played an important role by ensuring no temperature increase. The measured magnetic field suggests a slight movement of the whole sample due to the electromagnetic force. A residual magnetic field is also observed after the transport current becomes zero, which might be caused by circulating current among the simply-stacked REBCO tapes due to the imbalance of self-inductance among them. Despite the non-uniform current distribution, the STARS conductor with a simple stacking technique is considered promising for fusion magnets.
Keywords: HTS, fusion, STARS, helical