INVESTIGATION OF NONTHERMAL PARTICLE EFFECTS AN IONIZATION DYNAMICS IN HIGH-CURRENT DENSITY ION-BEAM TRANSPORT EXPERIMENTS

Light ion inertial fusion experiments require the presence of a modera te density background gas in the transport region to provide charge an d current neutralization for a high current density ion beam. In this article, we investigate the effects of nonthermal particles such as be am ions or non-Maxwellian electron distributions on the ionization dyn amics of the background gas. In particular, we focus on the case of Li beams being transported in an argon gas. Nonthermal particles as well as thermal electrons are included in time-dependent collisional-radia tive calculations to determine time-dependent atomic level populations and charge state distributions in a beam-produced plasma. We also bri efly discuss the effects of beam ions and energetic electrons on the v isible and vacuum ultraviolet (VUV) spectral regions. It is found that the mean charge state of the gas, and hence the electron density, is significantly increased by collisions with energetic particles. This h igher ionization significantly impacts the VUV spectral region, where numerous resonance Lines occur. On the other hand, the visible spectru m tends to be less affected because the closely spaced excited states are populated by lower energy thermal electrons. (C) 1997 American Ins titute of Physics.