Conceptual Design of Switch Network Unit for CFETR Coil Power Supply Systems

The China Fusion Engineering Test Reactor (CFETR) device is a milestone in the roadmap for the utilizing of fusion energy in China, whose final goal of plasma current is up to 10.02 MA. As the key system to generate a high enough voltage (20 kV) required for gas breakdown and plasma initiation, the Switch Network Unit (SNU) should have the ability to transfer 80 kA direct current by inserting resistor (0.25Ω ) to the superconducting coil circuit in 12 milliseconds. SNU topologies applied in other fusion devices are mostly based on the utilization of oscillation zero-point circuit, or combination of mechanical switch and semiconductor switch and so on, which bring stricter requirements for the components in the circuit and raise the cost due to the series and parallel connection of multiple devices if used in 80 kA/ 20 kV application. Based on the combination of solid-state switch (SSS) and auxiliary capacitor, this paper proposes a novel hybrid switch scheme. The SSS consists of insulated gate bipolar transistors (IGBT) group and thyristors group connected in series. The combination of added auxiliary capacitor and SSS can significantly reduce the number of series-connected IGBTs. This paper introduces the operation principle, design methods and simulation analysis of proposed scheme in detail. Coinciding with the analysis, the simulation results of SNU demonstrates the feasibility and reliability of the proposed scheme, which suggests its promising applications in fast switches with tens of kilovolts and hundreds of kilo-amperes.