Ambipolar Diffusion in a Nonuniorm Plasma Containing Different Positive Ion Components

A plasma of low degree of ionization consisting of electrons and several positive ion components cannot be described by a set of diffusion equations of the ions where the diffusion terms are given by coefficients of ambipolar diffusion and laplacians of the ion densities. This simple case is limited to proportional density distributions of all particles; in the general case, the "ambipolar" electric field couples the spatial derivatives of the ions with those of the electrons. The conditions following from the proportionality assumption either restrict the usefulness of such a description to a few cases, the more important one being the fundamental mode of diffusion or impose nonphysical correlations on the mobilities or coefficients of recombination of the ion species. Computer calculations of one-dimensional axially symmetric density distributions show appreciable differences in the wall currents and absolute densities between the general and simplified "proportional" model especially for ions with low concentration. The interpretation of mass-spectroscopic effusion measurements can be greatly affected by the choice of the description of the plasma.