Advanced High Voltage Switching Technology for Fusion Reactor Power Systems

Commercial fusion energy reactors will employ Electron Cyclotron Heating (ECH) and Neutral Beam Injector (NBI) systems for current drive and plasma heating and fueling. ECH utilizes megawatt-class gyrotrons and NBI uses a specialized ion accelerator. The gyrotron cathode is typically pulsed at a voltage of −100kV and currents up to 100A and similar for the NBI accelerators. Also required is precise voltage regulation to achieve the specific electron or ion beam energy. Prior technique was to use very large vacuum tetrodes to regulate the output from large, DC power supplies. Vacuum tetrodes were later replaced by silicon insulated-gate bipolar transistor (IGBT) modules with thirty or more arranged in series-connected “stacks”. The “buck regulator” circuit topology was used where the series switch was pulse-width-modulated (PWM) to regulate the output voltage. A sufficiently high frequency modulation is used to reduce the size of reactive components and to reduce the stored energy of the system. An important feature of the IGBT switch was its ability to rapidly interrupt fault current (e.g., cathode arc), thus eliminating complex crowbar systems. Power systems based on large Si-IGBTs have reached a practical limit of installed power density, efficiency and performance. With the advances in SiC semiconductor devices, comes the promise of substantial gains in thermal, volumetric, and operational efficiencies that can be achieved over the present state-of-art. The low-loss, high-speed switching, and ease of control of this new generation of devices will also allow development of other power system topologies for high voltage DC and fast magnet coil control. We consider the present state-of-art of SiC VJFET devices to be a “transformational” enabling technology that will positively affect the designs and performance of power systems needed for fusion energy plants. Other commercial, industrial, and defense applications will greatly benefit from this technology as well. This new configuration, the SiC VJFET “Supercascode”, is modularized based on commercial 3. 3kV rated SiC JFETs which in turn are combined into “building blocks” to form higher voltage assemblies. Recent results of SiC VJFET Supercascode switch development by Tetra and UNC will be presented.