The Pegasus-III Spherical Tokamak

The Pegasus-III experiment is a solenoid-free spherical tokamak dedicated to comparative non-solenoidal tokamak startup studies. Major upgrades to the magnets and power systems will increase B_T four-fold to 0.6 T for up to 100 ms with improved shape control while maintaining low A~1.2. This increased field directly supports the mission of researching non-solenoidal power plant-relevant startup techniques such as local and coaxial helicity injection and RF wave injection. These techniques will be contrasted and possibly combined. The Ohmic solenoid has been removed and the toroidal field system now is comprised of a new 24-turn center rod and outer coil system, torque assemblies with crossover finger joints, and integrated divertor coils. Approximately 175 MVA of programmable power systems and 7 MJ of stored energy are available to support the four-fold toroidal field increase, new divertor and poloidal field coils, local and coaxial helicity injectors, RF systems, and a diagnostic neutral beam. A new digital control system uses FPGA technology and real-time computing to control the electromagnets (IGBT buck converters) and helicity injectors (multi-level buck) while providing fault detection and protection.