Numerical study of the plasma meniscus shape and beam optics in RF negative ion sources

In order to extract intense ion beams with good beam optics from hydrogen negative ion sources, it is important to control the shape of the plasma meniscus (i.e. beam emission surface). Recently, it is suggested experimentally that the degradation of beam optics in the RF negative ion sources may be due to the fluctuation of the distance d _eff between the meniscus and the extraction grid caused by the fluctuation of the plasma density n_p . The purpose of this study is to make clear the dependence of d _eff on n_p in the presence of a large amount of surface produced H^- ions in order to understand such fluctuation of beam optics in RF sources For the purpose, 3D electrostatic PIC simulation was conducted taking the bulk plasma density as a parameter, investigating the extraction region of a H^- ion source. A large amount of the surface H^- production on the PG has been taken into account under the assumption that the H^- production rate is proportional to the bulk plasma density. The result shows that the effective distance d _eff is proportional to n_p^-1/2 even for a large amount of surface H^- production. This dependence suggests that the bulk plasma density n_p is the key parameters to control d _eff and the resultant beam optics extracted from the negative ion source.