Screening of the dust grain charge in a nonequilibrium plasma with two positive ion species

The screening of the grain charge in a nonequilibrium plasma is studied with allowance for electron and ion fluxes to the grain surface and the bulk processes of production and loss of charged particles in an argon plasma. The objective of the paper is to investigate how the conversion of monatomic Ar⁺ ions into diatomic Ar₂⁺ ions influences the screening of the dust grain charge in a plasma produced by an external gas ionization source. It is found that the conversion of positive ions leads to the onset of a second ion species in the plasma and, as a consequence, to a three-exponential screening of the grain charge; moreover, in a certain range of plasma parameters, all three screening constants are of the same order of magnitude. Analytical results are compared with the data of numerical simulations carried out based on the drift-diffusion approximation.