NIGUN: A two-dimensional simulation program for the extraction of H− ions

Abstract

A new theory for a multiparticle plasma sheath in the case of H− extraction recently provided the basis to modify the well-established two-dimensional (2D) computer program IGUN.© As in the case of IGUN, stability of convergence is obtained for the very nonlinear sheath region by directly inserting the analytical space charge according to the Poisson equation for each mesh point above the wall potential. For three-component plasmas (e, p, and H−) only the relation of proton birth potential to electron energy and H− energy to electron energy is needed to obtain all parameters. For four-component plasmas (e, p, H−, and Cs+) the density and energy ratios of protons and Cs+ ions are required as input for the calculations. Up to 10 different positive ion species may be declared, also requiring the input of the corresponding ratios for density and energy. The actual field strength in the extraction gap is used to determine the proper densities of all particles by the program. The numerical “proof” will be given that, as predicted by theory, an angle of 45° between the beam boundary and the wall electrode will provide rectilinear flow once the density matches the field in the extraction gap.