Solenoid-Free Startup in the Pegasus-III Experiment

Solenoid-free startup techniques offer the potential to simplify the cost and complexity of tokamak fusion power plants by reducing the technical requirements of, or need for, a central solenoid. Robust solutions are critical to the spherical tokamak (ST) concept as well. The Pegasus-III Experiment provides a dedicated US platform for comparative solenoid-free startup studies. Presently completing construction and beginning commissioning, this new solenoid-free ST (A ≥ 1.22, I_p ≤ 0.3 MA, B_T ≤ 0.6 T, t_pulse ~ 0.1 s) will be equipped with local helicity injection (LHI) and coaxial helicity injection (CHI) startup systems; a 28 GHz radiofrequency (RF) system for heating and current drive; and enhanced diagnostics. The research mission is centered on developing the underlying non-solenoidal startup physics basis via validated, predictive models and the needed technology to effectively implement these techniques. Its long-term objective is to facilitate the deployment of MA-class startup concepts on larger-scale STs and beyond. Initial experiments are planned to extend LHI scenarios to B_T ~ 0.3 T and I_p = 0.3 MA; develop transient CHI scenarios; and characterize electron Bernstein wave emission to optimize the RF system.