Digital Control and Power Systems for the Pegasus-III Experiment

The PEGASUS-III experiment is a solenoid-free, low aspect ratio spherical tokamak that will serve as a dedicated U.S. platform for comparative nonsolenoidal tokamak plasma startup studies. Approximately 175 megavolt-ampere (MVA) of reconfigured and expanded programmable power systems, 7 MJ of new stored energy, and new digital control and protection systems for the facility are being commissioned to support PEGASUS-III upgrades. These include: increased toroidal field (0.15–0.6 T); new divertor and poloidal field coils; increased pulse length; local and coaxial helicity injectors for solenoid-free plasma initiation; radio frequency (RF) systems for heating and current drive; and a diagnostic neutral beam (DNB). A new real-time digital control system implements 16 proportional-integral differential (PID) feedback controllers with 25 kHz loop rates to control the electromagnets and helicity injectors. The poloidal field coils, helicity injector arc currents, and toroidal field are driven by 36 3.6 MVA (4 kA, 900 V) insulated-gate bipolar transistor (IGBT) buck converters. Helicity injector bias voltage and current will be provided by a set of four 10.8 MVA multi-level buck converters (MLBCs). Each is comprised of an 1800 V integrated gate-commutated thyristor (IGCT) stage and a ±900 V IGBT stage in series, providing controllable $I_{\mathrm {inj}}\le 4$ kA at $V_{\mathrm {inj}}\le 2.7$ kV. A field programmable gate array (FPGA)-based digital fault protection system multiplexes controller commands to individual power semiconductors in these supplies, monitors their operational status, and executes shutdown sequences within $10~\mu \text{s}$ of fault detection. An 80 kV, 4 A zero-voltage-switching (ZVS) resonant converter with < 1% output ripple is under development for the DNB and is being evaluated as a topology to drive RF sources.