Persistent Current Switch for a Superconducting Synchronous Motor Using REBCO Tapes Without Copper Plating

A superconducting synchronous motor designed for operation in persistent current mode (PCM) is being developed. A persistent current switch (PCS) is required for PCM operation to connect the field coil in parallel. We propose a PCS constructed from REBa<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub>Cu<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub>O<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">y</sub> (REBCO) tapes without copper-stabilized layers. We fabricated and tested a PCS using these REBCO tapes to evaluate magnetic field decay during PCM operation. The results confirm that the PCS does not degrade. The fabrication process encountered challenges owing to the absence of stabilized layers, including potential degradation from silver diffusion into the solder and moisture penetration into the superconducting layer. Therefore, copper plating was selectively applied to the soldering area, and the PCS was encapsulated in epoxy resin. The effectiveness of these measures was verified by measuring the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V-I</i> characteristics of the copper-plated REBCO tape over several days, along with the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V-I</i> characteristics of the PCS. Neither the copper-plated REBCO tapes nor the PCS showed degradation after ten days. A double-pancake coil designed for PCM operation was fabricated using REBCO tapes with stabilized layers, considering 1% magnetic field decay over 10 min. During PCM operation, the time constant of the PCS aligned with the calculated value, and no degradation was observed.