Operation of the Pegasus-III Experiment at 0.6 T for Nonsolenoidal Startup Development

Developing nonsolenoidal plasma initiation techniques for spherical tokamaks in particular, and tokamaks in general, provides an attractive option for designing fusion energy systems without relying on induction from a central solenoid. The Pegasus-III Experiment is a newly upgraded facility dedicated to developing nonsolenoidal tokamak startup through magnetic helicity injection and microwave injection techniques. The main driver of the upgrade is a new toroidal field (TF) coil and power supply to enable startup demonstrations under conditions similar to larger facilities. Operating the TF coil up to 0.6 T requires adding 10 kW of water cooling to the center stack conductors and monitoring the TF bundle temperature and strain during the current pulse. The chilled water system is designed to minimize copper corrosion from low-conductivity water. Strain gauges and thermocouples are mounted at key locations on the TF coil structure. Their measurements agree with electrothermal heat transfer and stain calculations. The new facility has begun experiments at 0.6 T, demonstrating the startup of a tokamak plasma without a central solenoid.