An ohmic field power supply based on a modified IGBT H-bridge for Sino-UNIted Spherical Tokamak

Air core solenoids are widely used in spherical tokamaks (STs), since their central stacks are quite compact and difficult to insert iron cores. However, much higher current are required to generate the same amount of flux in the solenoids with air core than in the ones with iron cores. Therefore, the power supplies of ohmic fields in STs are more challenging than those in conventional tokamaks with similar size. In this paper, a power supply based on insulated gate bipolar transistors (IGBTs) for the ohmic field coils in the Sino-UNIted Spherical Tokamak (SUNIST) is described. This power supply utilizes a modified H-bridge circuit, in which the high current switches are made by 10 IGBTs (rating 3.3 kV/1.2 kA) connected in parallel. A high voltage capacitor bank (1.2 kV/60 mF) and a low voltage capacitor bank (600 V/500 mF) store the energy required in different discharging stages. An adjustable low inductance high power resistor (0-0.3 Ω) made by stainless steel foils is used to help the circuit to generate a high current derivative in the ohmic field coils, which are required for gas breakdown, plasma current initiation and ramp up. The small loop voltage required for maintaining the plasma current is realized by the low voltage capacitor bank. For each tokamak discharge, all the switches of this power supply only need to be switched on and off one time with an interval of several milliseconds. Such slow operations of the IGBTs lower the risk of failure and minimize the technical difficulties of dynamical current sharing. The power supply is compact and reliable, and has discharged for more than 5000 times successfully.