Achievement of 1 MA Discharges in Hydrogen-Boron Plasmas on EXL-50U

Abstract: One mega ampere (MA) plasma discharges were achieved on the EXL-50U spherical torus (ST) in hydrogen-boron (p-B) plasmas with toroidal fields up to 1 T at major radii up to 0.6m. A key innovation in these experiments was the use of a boron-rich fueling strategy, incorporating a high-concentration boron-containing gas mixture and real-time boron powder injection during the discharges. The boron content in the fueling reached 10%, representing the first publicly reported MA-class hydrogen-boron plasma with such a high boron concentration. Non-inductive plasma startup was achieved using electron cyclotron resonance heating (ECRH), and a rapid current ramp-up to 1 MA was realized through the synergistic use of ECRH and the central solenoid (CS). With 800 kW of ECRH power, core electron temperatures of up to 3 keV were attained. These results demonstrate the feasibility of producing high-performance hydrogen-boron plasmas in a spherical torus configuration and offer important physics and engineering insights for future reactor-scale applications - particularly in the areas of low loop voltage current start-up and real-time boronization techniques.