Diffusion coefficients of ions in lighter gases in an electric field

Abstract

Abstract The diffusion theory for ions in single lighter gases (and in mixtures of lighter gases), in moderately low electrostatic fields, is formulated in the so-called quasi-Rayleigh limit, starting from a proper approximate kinetic equation previously derived by the author. In this way new simple approximate expressions for transverse and longitudinal ion temperatures and diffusion coefficients are obtained. Their dependence on the ion-neutral interaction law is investigated in the simple case of an inverse-power force law. Moreover, the results are compared with the previous ones of the literature. In particular, agreement is found with Wannier's results in the Maxwell model and with the results of the first Chapman-Enskog approximation. On the contrary, some discrepancies with the one-temperature formulation of the moment method are encountered as regards the transverse ion temperature and the transverse diffusion coefficient, but this appears to be due to the questionable computational criteria used in the above method. Finally, the limits of validity of the present formulas are discussed.