Effect of Coulomb collision between surface produced H^- ions and H⁺ ions on H^- extraction mechanism and beam optics in a Cs-seeded H^- ion source by 3D particle in cell model

Recently, in Cs-seeded H^- ion sources for fusion applications, the experiments have shown that the surface produced H^- ions are mainly extracted from the bulk plasma region, that is, 1-2 cm inside from the beam aperture. The purpose of this paper is to clarify the extraction mechanism of the surface produced H^- ions from the bulk plasma region in Cs-seeded H^- ion sources for fusion applications by our 3-Dimensional Particle in Cell model for the vicinity of the beam aperture. In the present model, Coulomb collisions between surface produced H^- ions and H⁺ ions (CC H^--H⁺) are taken into account by the Binary Collision Model. By our model, the trajectories of extracted surface produced H^- ions and beam profiles have been investigated for the case with and without CC H^--H⁺. From the results of trajectories for extracted surface produced H^- ions, it has been shown that the surface produced H^- ions extracted from the bulk plasma region are drastically increased due to CC H^--H⁺. Thus, our results have shown that the surface produced H^- ions are extracted from the bulk plasma region due to Coulomb collision with H⁺ ions. Moreover, our results of the beam profiles have shown that CC H^--H⁺ causes the increase in the beam convergence component. From these results, Coulomb collisions between surface produced H^- ions and H⁺ ions are shown to be very important for the modeling of the extraction mechanism and for the beam optics in Cs-seeded H^- ion sources for fusion applications.