Japan fusion breakthrough may triple plasma measurement precision

Researchers at Japan’s National Institute of Fusion Science have significantly improved the precision of plasma potential measurements in the Large Helical Device (LHD), the world’s largest superconducting plasma confinement device. By introducing an innovative “electrostatic lens” technique through optimized voltage distribution in the existing multistage accelerator, they overcame a major limitation caused by the space-charge effect that previously caused ion beam expansion and loss. This advancement doubled or tripled the efficiency of the Heavy Ion Beam Probe (HIBP) system, which uses a high-energy gold ion beam to measure internal plasma potential crucial for fusion reactor performance. The improved beam focusing increased the injected Au⁻ beam current by two to three times, resulting in a stronger Au⁺ beam inside the plasma and extending the measurable electron density range to 1.75×10¹⁹ m⁻³. This enhancement enabled clearer detection of rapid changes in plasma potential linked to different heating systems, providing more detailed and reproducible data essential for future plasma control