Enhanced beam transport via space charge mitigation in a multistage accelerator for fusion plasma diagnostics

Efficient transport of high-current negative ion beams is critical for accurate plasma potential diagnostics using heavy-ion beam probe (HIBP) systems in magnetically confined fusion plasmas. However, strong space-charge effects often degrade transport efficiency, particularly for heavy ions such as Au^−. In this study, we demonstrate a substantial improvement in beam transport by introducing an electrostatic lens effect through optimized voltage allocation in a multistage acceleration system. Numerical simulations using IGUN, supported by experiments with the LHD-HIBP system, show that this approach effectively suppresses spacecharge- induced beam divergence and loss. Without requiring mechanical modifications to the beamline, the optimized configuration enables a 2–3 fold increase in Au^− beam current injected into the tandem accelerator. Consequently, plasma potential measurements were extended to higher-density plasmas, reaching line-averaged electron densities up to 1.75 × 10¹⁹~m⁻³ with improved signal-to-noise ratio. This technique offers a compact, practical, and highly effective solution for transporting high-current heavy-ion beams under pace-charge-dominated conditions. Beyond its impact on plasma diagnostics, the method is broadly applicable to a wide range of accelerator systems, including those used in scientific and industrial applications where highintensity beam transport is required.