Abundance ratio of multiple velocity distribution components in a single negative ion beamlet produced by a cesium-seeded negative ion source

Well focused negative ion beams are required for neutral beam injection systems for heating and current drive in magnetically confined fusion plasma experiments. The control of a single negative ion beamlet divergence is a significant challenge with the use of a cesium-seeded negative ion source, where negative ions are mainly produced by conversion of hydrogen or deuterium atoms on a cesiated surface of a plasma grid. The single negative ion beamlet was found to be made by three-Gaussian components in our previous work. The origins of such multiple components are considered to be related to dynamics in the ion source and extraction processes of negative ions. This work has demonstrated a measurement of the abundance ratio of the three components (41%, 40%, and 19%) based on a full picture of their transverse velocity distributions, which is a powerful technique to investigate the origins of individual components and will contribute to improve the divergence of negative ion beamlet.