Enhancement of Neutral Beam Heating Efficiency by Beam Fraction Control in VEST

Neutral beam injection (NBI), one of the most effective heating methods for tokamak plasmas, requires carefully controlled operation to minimize losses and maximize heating efficiency. In this study, enhanced NBI heating efficiency is demonstrated by adjusting the neutral beam fraction in the Versatile Experiment Spherical Torus (VEST). NUBEAM simulation results suggest that reducing beam energy may lead to improved heating efficiency for typical VEST ohmic plasmas. To achieve this, rather than simply reducing beam energy, the beam species fraction is adjusted by modifying the NBI operational conditions to maintain a high neutral beam current. As a result, two distinct beams were produced, exhibiting different power levels and species compositions: one with about 140 kW (H^+:H_2^+:H_3^+ = 67:12:21) and the other with about 110 kW (H^+:H_2^+:H_3^+ = 50:10:40). In a high-current (>200 kA) and high-density (>1.5×1019 m-3) target plasma conditioned by high-dose boronization for NBI heating, the high H_3^+ beam increased the core electron temperature by 23 eV without causing edge cooling due to impurity influx from the chamber wall. These results demonstrate that by tuning discharge conditions, a single NBI system can achieve various energy distributions, offering a promising approach for enhancing heating and current-drive capabilities in VEST and other devices.