Solenoid-Free Startup at Increasing B_T in the Pegasus-III Experiment

The Pegasus-III Experiment (A ~ 1.2, I_p ≤ 0.3 MA, B_T ≤ 0.6 T) is a newly upgraded US facility developing the physics and technology basis of solenoid-free startup. Such methods are critical to next-step STs and may simplify the cost and complexity of tokamak fusion energy systems by reducing the demands of—or need for—a central solenoid. The Pegasus-III upgrade quadrupled B_T to demonstrate local helicity injection (LHI) at higher field, enable tests of coaxial helicity injection (CHI), and support a 28 GHz RF heating and current drive system for comparative and synergistic startup scenarios. The first 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; fielding a dual-electrode CHI system that is in fabrication for CHI scenarios; and characterizing EBE to optimize the microwave system.