Quench Protection of Stacks of No-Insulation HTS Pancake Coils by Capacitor Discharge

No-Insulation (NI) technology for HTS coils has been around for a decade and is a proven method to protect small pancake coils in case of a quench. Without turn-to-turn insulation, excess current can bypass a sudden resistive part, thereby possibly preventing damaging the conductor locally. Unfortunately, having a very low turn-to-turn insulation does not automatically translate into coil protection when the magnets are larger and their coil energy density reaches values in the order of tens of kJ/kg. During a quench of such magnets, the quench propagation behavior is often driven by electromagnetic diffusion due to inductive effects, rather than thermal diffusion. This may locally increase the current and energy density far above nominal values and thereby also increase the Lorentz force density locally on the conductor. In that case, a coil's mechanical and thermal limits become a much more stringent factor in the survival of such coils and thus force, stress and temperature management and the possible addition of an active quench protection system are a key component in their design. In this paper a new active quench protection concept is proposed for high-field HTS solenoids that comprise a stack of many NI pancake coils. This protection concept is based on a capacitor discharge into the NI magnet and offers a fast and homogeneous superconducting to normal state transition of the entire coil without the need for any additional internal components, which makes it a very robust protection system.