Analysis of Silicon Carbide Varistors for Fast Discharge Units of DEMO Toroidal Field Superconducting Magnets in Case of a Quench

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This work investigates innovative technologies for DEMOnstration power plant (DEMO) Fast Discharge Units (FDUs), in comparison with traditional temperature-dependent discharge resistors. The protection system for the DEMO Toroidal Field (TF) superconducting magnets is designed to respond to a quench, which is a rapid transition from the superconducting state to normal conduction, or other faults. This system helps prevent severe damage to the TF magnets, which can experience significant thermal and voltage stresses during a quench, due to the intervention of the FDUs. These stresses can damage the insulation and compromise the magnet's integrity. FDUs play a crucial role in rapidly discharging the magnetic energy stored in the magnets. Silicon Carbide (SiC) varistors within the FDU, with their nonlinear behavior, can significantly reduce the voltage peak across the TF magnet at fast discharge with respect to normal resistors. In the high current configuration, the DEMO TF magnet system is composed of 16 TF coils, with: a total inductance of 22.56 H, an operating current of 105 kA and a total magnetic energy stored of about 124 GJ. Maximum voltage limits for TF coils are 6 kV terminal-to-terminal and 3 kV terminal-to-ground. Performance of technologies are compared considering voltage and hotspot temperature.

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