Digital Control and Power Systems for the Pegasus-III Experiment

Abstract

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 realtime 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 multilevel 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 inj ≤ 4 kA at V inj ≤ 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 µs 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.

A Coaxial Helicity Injection System for Nonsolenoidal Startup Studies on the PEGASUS-III Experiment

“…Initiation of S-CHI discharges will use the T-CHI bank. S-CHI sustainment will be powered by the 2 MJ multilevel buck convertor power supplies used for the LHI bias [3]. This system can deliver up to 16 kA and 2.7 kV of controlled power for up to 25 ms.…”

“…To address the startup issue, the PEGASUS-III Experiment (see Fig. 1) will provide a dedicated U.S. platform for solenoid-free startup development [2], [3]. This is a major upgrade to the PEGASUS facility located at the University of Wisconsin at Madison, Madison, WI, USA, that is presently underway.…”

The New PEGASUS-III Experiment

“…In addition to this new center column, the upgraded TF coil set is comprised of outer return legs, interconnects between the center rod and outer legs, and a torque assembly to counter forces on the system. This new coil set has an inductance of 1.04 mH and is powered by an insulated-gate bipolar transistor (IGBT) driven power supply with 900-V capacitor banks for stored energy and is capable of ramping to full current in 50 ms with a 100-ms flat top [8].…”

“…Additional upgrades to the power supplies to accommodate the higher TF operation as well as increased total power and pulselength of the helicity injection drive systems are also being performed as discussed in a separate paper [8]. Upgrades to install a CHI system are also discussed separately [9].…”