Achievement of 1 MA discharges in hydrogen–boron plasmas in the EXL-50U

One mega ampere (MA) plasma discharges were achieved in the EXL-50U spherical torus (ST) in hydrogen–boron (p–B) plasmas with toroidal fields of up to 1 T at major radii of up to 0.6 m. 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 fuel reached 10%, representing the first publicly reported MA-class p–B plasma with such a high boron concentration. Noninductive 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). At 800 kW of ECRH power, core electron temperatures of up to 3 keV were attained. These results demonstrate the feasibility of producing high-performance p–B plasmas in a spherical torus configuration and offer important physics and engineering insights for future reactor-scale applications—particularly in the areas of current start-up at low loop voltage and real-time boronization techniques.