Design, fabrication, and persistent current operation of the REBCO floating coil for the plasma experimental device Mini-RT

High-temperature superconductor technology has become remarkably advanced, and components such as current leads and magnets have been explored for their applicability to fusion. Especially, REBa₂Cu₃O_7-δ (RE = rare-earth: REBCO) tape has been developed in research fields other than fusion, because of superior performance of REBCO in high magnetic fields. Here, we fabricated a REBCO coil with a radius of 150 mm and a total coil current of 55.2 kA, to be applied as an internal floating coil in Mini-RT, a plasma experimental device. This newly fabricated REBCO coil has replaced the Bi₂Sr₂Ca₂Cu₃O₁₀ (BSCCO) coil developed in 2003, and has improved coil performance and plasma operation. A REBCO tape of width 4.3 mm was employed, and a 0.1 mm-thick copper laminate was attached for the countermeasure at the quench. In addition, the REBCO tape was wrapped in a polyimide sheet for electric insulation, increasing the total thickness of the REBCO tape to 0.27-0.28 mm. A persistent current switch was constructed from a small-turn REBCO winding with a heater formed from a stainless steel sheet. Solder joint of two REBCO tapes over a 30 mm length were employed, by overlapping their copper laminates. Persistent current, with a decay time ∼320 h, has been successfully achieved at 25 K operation temperature. This decay time corresponds to a resistance of ∼125 nΩ, roughly consistent with the total resistance of the seven joint sections in the persistent current circuit. (author)