Basic mechanism of self-healing from thermal runaway for uninsulated REBCO pancake coils

• We reveal the mechanism of “self-healing” effect of uninsulated REBCO coils.
• Uninsulated REBCO pancake coil shows “self-healing” from thermal runaway.
• Current mode transition from “multi-turn” mode to “single-turn” triggers this effect.

This paper clarifies the basic mechanism of self-healing from natural thermal runaway for uninsulated REBCO pancake coils. Based on the numerical simulation and experimental results, it is demonstrated that current flow patterns for an uninsulated REBCO pancake coil sequentially changes with the overcurrent value. Due to natural thermal runaway above the coil critical current, the current flow changes from “multi-turn-coil” mode to “single-turn-coil” mode, reducing the conductor current density and Joule heating, and thus the thermal runaway shrinks and the coil is self-healed; i.e. the REBCO coil is self-protected. For higher overcurrents, however, transverse currents across both electrodes become dominant, here called “terminal-to-terminal current” mode. It is dangerous for REBCO coils as the winding volume connecting both electrodes is extremely overheated, damaging the conductor winding.