Solenoid-Free Startup in the Pegasus-III Experiment

The Pegasus-III Experiment (A ~ 1.2, I_p ≤ 0.3 MA, B_T ≤ 0.6 T) is a spherical tokamak dedicated to solenoid-free startup research, which is critical to next-step STs and may benefit larger-A tokamaks. The Pegasus-III facility was upgraded to quadruple B_T at R₀ up to 0.6 T to study local helicity injection (LHI), coaxial helicity injection (CHI), and ECH/EBW heating and current drive with a 28 GHz microwave system in a regime that overlaps with larger ST facilities. Initial physics campaign replicated prior LHI scenarios at 0.15 T and extended them to the 0.3 T, I_p > 0.2 MA regime with a new injector system, consistent with expectations from global Taylor relaxation and helicity balance limits. Studies of T_e scaling, impurities, and electron Bernstein wave emission during LHI have begun, supported by new and upgraded diagnostics. Experiments at 0.6 T are in progress. Near-term plans include testing a new noncircular injector that may optimize the Taylor and helicity limits; operation of a dual-electrode CHI system that is in fabrication; and characterizing EBE to understand microwave coupling physics in the dynamic HI environment.