Status Update on the Characterization of Simply-Stacked HTS STARS Conductor - Compatibility With Non-Uniform Current Distribution -

The High-Temperature Superconducting (HTS) Stacked Tapes Assembled in Rigid Structure (STARS) conductor has been developed for application in fusion magnets. The STARS conductor employs a simple stacking of Rare-Earth Barium Copper Oxide (REBCO) tapes to fabricate a large current and high-current-density conductor. Despite allowing for a non-uniform current distribution among the stacked REBCO tapes, stable operation at the rated current of 18 kA with a fast ramp rate of 1 kA/s has been successfully confirmed in the two experimental campaigns with a 6-m conductor coiled sample tested in a uniform 8 T background magnetic field and at a temperature of 20 K. The residual magnetic field observed after the total sample current was ramped down to zero indicates the existence of circulation currents among tapes, which was investigated using a numerical simulation. The applicability of the STARS conductor to large-scale coils, such as ITER-TF coils, is also discussed from the viewpoint of compatibility between stable operation and non-uniform current distribution. A new configuration of the STARS conductor is proposed to accommodate AC operation, with a preliminary analysis on the coupling losses.