Effect of Surface Produced H- Ion on the Plasma Meniscus in Negative Hydrogen Ion Sources

To extract intense ion beams with good beam optics from ion sources, controlling the distance d(eff) between the plasma meniscus (i.e., beam emission surface) and the beam extraction grid is important. This study conducts a novel investigation into the dependence of the effective distance d(eff) on the amount of surface H- production SH-. For this purpose, a 3D PIC (three dimensional Particle-in-Cell) simulation is conducted to obtain a model geometry of the extraction region for a H- ion source with SH- as a parameter. Based on results, d(eff) significantly depends on SH- and the H--electron density ratio (alpha = n(H)-/n(e)) in front of the extraction aperture for the same plasma density; as SH- increases, d(eff) decreases. The results suggest that SH- is critical for controlling d(eff) and the resultant beam optics extracted from the negative ion source. (C) 2023 The Japan Society of Plasma Science and Nuclear Fusion Research