In order to test the superconducting magnet performance for Comprehensive Research Facility for Fusion Technology (CRAFT) project of China, a power supply converter needs to be designed. In this paper, short circuits in upstream and downstream of direct current (DC) reactor are analyzed, and the thyristor style and the parallel number are determined by the limit analysis of junction temperature and fault current I2t. On this basis, the over current and voltage verification of fast fuse are finished to protect the thyristor at fault cases by considering the short circuit of bridge arm. Then, the resistor and capacitor parameters of thyristor snubber are committed to decreasing the reverse over voltage. These analysis results will be used as preliminary design of high power magnet converter for CRAFT.

The performance of power semiconductor devices near fusion reactor may be changed by the fast neutron irradiation in nuclear fusion. In order to investigate the reliability of thyristors used in Toroidal Field Fast Discharge Unit (TFFDU), two thyristors KPD5000-52 with almost the same parameters were selected to be irradiated by 14.9 MeV neutrons in the range from $\rm 0.403\times 10^{5} n/(cm^{2}\times s) to 1.38\times 10^5 n/(cm^2 \times s)$ for 28 hours. Metal of thyristor was activated to 3 mSv/h after irradiation, and it decreased to 360 nSv/h after 40 days. The electrical parameters of thyristor such as the forward /reverse leakage current and on-state voltage were measured under normal junction temperature (25°C) and high junction temperature (125°C) respectively. The irradiated thyristors cannot be fully conducted until the gate trigger strength is enhanced to 10 A. Moreover, these devices turn off automatically with a small delay after the gate signal is removed. According to the irradiation theory, this paper also presents the decrease of carrier lifetime, carrier concentration and mobility, which lead to thyristor failure.