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

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.

💡 Research Summary

The paper reports the first achievement of mega‑ampere (MA) class discharges in hydrogen‑boron (p‑B) plasmas on the EXL‑50U spherical torus (ST). By adopting a boron‑rich fueling scheme—mixing 30 % diborane with 70 % hydrogen for the entire discharge and injecting boron powder in real time—the authors raised the boron content of the working fuel to at least 10 %. This high‑boron mixture dramatically increased the plasma current ramp‑up rate from 1.9 MA s⁻¹ (hydrogen‑only) to 3.4 MA s⁻¹, a 78 % improvement, while reducing the central solenoid (CS) flux consumption.

Non‑inductive startup was performed with electron cyclotron resonance heating (ECRH) using a 50 GHz, 230 kW gyrotron that was turned on 0.2 s before breakdown. During the ramp‑up and flat‑top phases, four additional 28 GHz gyrotrons were sequentially switched on, allowing the total ECRH power to be ramped up to 800 kW. This staged power increase prevented excessive unabsorbed power in the early phase, limiting impurity generation and avoiding premature plasma termination.

The synergy between ECRH and the CS enabled rapid current rise while keeping the loop voltage low. At a toroidal field of 1 T and major radius of 0.6 m, the device reached a peak plasma current of 1.03 MA, with an elongation of ≈1.5 and an edge safety factor qₐ≈4 in a limiter configuration. Core electron density was around 1×10¹⁹ m⁻³ and core electron temperature reached 3 keV (up to 3.5 keV in lower‑density shots). Magnetic reconstruction confirmed the prescribed current and position waveforms, and the current ramp‑up rate was deliberately reduced after 70 ms to avoid uncontrolled plasma expansion and intense plasma‑wall interactions on the low‑field‑side limiter.

A notable engineering breakthrough is the implementation of real‑time boronization on a full‑metal‑wall ST. The simultaneous use of diborane‑hydrogen gas and boron powder creates a boron‑rich coating on the wall during the discharge, mitigating wall erosion and impurity influx—issues that are central to ITER’s wall‑conditioning program and to future ST reactors such as EHL‑2 and STEP.

The authors conclude that the combination of low‑loop‑voltage non‑inductive startup, efficient CS‑ECRH current drive, and in‑situ boronization provides a viable pathway for high‑performance p‑B plasmas in future reactor‑scale devices, including superconducting tokamaks (e.g., ITER). Planned upgrades to increase the toroidal field to 1.2 T in 2026 are expected to push the current limit further, while higher elongation scenarios could improve confinement. Overall, the work delivers critical physics and engineering insights for the commercialization of fusion energy, demonstrating that MA‑level p‑B discharges are feasible in a compact, metal‑walled spherical torus.