Quench behaviors of parallel-wound no-insulation high temperature superconductor coils

No-insulation (NI) high-temperature superconducting (HTS) coil wound with parallel-stacked tapes emerges as a prospective choice for high-field fusion magnets owing to lower inductance and faster ramping rate. The parallel stacked-tape structure leads to new current redistribution among stacked tapes in each turn during local quenches, which also considerably changes the current redistribution behavior through inter-turn contacts. Therefore, quench behaviors of parallel-wound no-insulation (PWNI) coil should differ from its counterpart wound with single tape, which are still unknown. This study is to illustrate quench behaviors of PWNI HTS coils induced by local hot spot. A multi-physics model integrating an equivalent circuit network, a FEM heat transfer module, and a FEM T-A model is developed to analyze the electromagnetic and thermal characteristics of PWNI HTS coils during quench. Results show that the transport currents are mainly redistributed among parallel-stacked tapes through terminal resistances when a local hot spot happens on one tape, while being less dependent on turn-to-turn electrical contacts. It leads to a coupling current within PWNI coils that is not present in NI coils wound with single tape (single-wound no-insulation (SWNI) coil), resulting in a highly non-uniform transport current distribution among parallel-wound tapes. The reduced terminal joint resistances further enhance the coupling current, potentially leading to an extra overcurrent quench risk in PWNI coils. Moreover, the current redistribution between parallel-stacked tapes inhibits the turn-to-turn current redistribution in the PWNI coil, thus significantly reducing its magnetic field degradation under a high heat disturbance, which can be almost less than half of the SWNI counterpart in this study. These results offer important theoretical guidance to safety operation and robustness improvement of high-field HTS magnets wound by PWNI technique.