Study on the current bypassing and mechanical properties of no-insulation HTS coil with metallic protection ring

We have been studying no-insulation (NI) high temperature superconducting (HTS) coils that do not use turn-to-turn electrical insulation. The no-insulation winding technique can provide high stability of the HTS coils against the quenching, but it cannot prevent mechanical deterioration of the HTS coil due to electromagnetic force and multiple cooling. To solve these problems, we propose the installation of metal protection ring on the outermost turn of NI HTS coil to improve thermal, mechanical, and electrical stability. In this study, the current bypass characteristics in the transverse direction within an NI test coil wound with 37 turns of REBCO wire with/without copper protection ring were experimentally investigated for various amounts of heating by a heater and various heater positions. Eight repeated cooling experiments with liquid nitrogen proved that the proposed copper protection ring was effective against shape deformation of the NI test coil and deterioration of contact resistance between turns due to thermal stress.