Relation between vapor Cs and adsorbed Cs in H^- ion source

The relation between gaseous Cs density and the coverage of Cs on the plasma grid surface is determined in the existence of the effect of Cs deposit on the cold surface. An equation for the deposit of Cs on the cold surface in the ion source is obtained by considering the saturation of the deposit. The Cs coverage is expressed as a function of gaseous Cs density in the volume of the ion source by considering the relation τ_θ ≪ τ_a, where τ_θ is the time scale of the Cs adsorption to the plasma grid surface and τ_a is that of the Cs adsorption to the cold surface. The coverage varies with the slow time scale through the variation of the gaseous density related to the deposit of Cs on the cold surface.