Technical Innovation Assessment of SiC Varistors for Quench Protection of DEMO Toroidal Field Superconducting Coils

This paper investigates the feasibility and technical advantages of utilizing silicon carbide (SiC) varistors for the energy extraction systems of the toroidal field coils (TFC) in case of a quench in the present European DEMO fusion reactor design. Varistors offer a non-linear voltage-current response that provides faster discharge times and voltage limiting behavior compared to traditional linear resistors. Using the Metrosil 9000 series varistors as a baseline, comprehensive discharge simulations were conducted for both 8 and 16 Fast Discharge Unit (FDU) configurations. The study integrates electrical transient simulations to assess clamping voltages and advanced thermal modeling to evaluate the corresponding stress on the superconducting cables. Specifically, the quench propagation and the hotspot temperature evolution were investigated using both conservative adiabatic calculations and detailed transient thermal-hydraulic modeling (via the 4C code). Results indicate that, in normal operation mode, while voltage limits are successfully managed, the Joule integral limit is currently exceeded. Consequently, the thermal-hydraulic analysis reveals that jacket hotspot temperatures surpass the 150 K acceptance limit with varistor-based discharges, inducing higher thermal stresses than traditional resistors. This highlights a dual requirement: the need for varistor material optimizations in the case of DEMO TFCs-specifically targeting a lower non-linearity parameter (β < 0.3)-, in order to fully realize the benefits of SiC varistors for high-energy fusion applications, and a potential reassessment of the conductor design, in terms of both copper stabilizer amount and cross-section geometry, to improve current sharing during the discharge and conductor stability against small disturbances.