Ambipolar Diffusion in Radio-Frequency-Excited Magnetoplasmas

Abstract

A theoretical analysis of ambipolar diffusion in electrodeless, weakly ionized, collision dominated, radio-frequency-excited magnetized plasmas reveals an essential dependence of the steady-state density profile on the electron-density dependence of the net volume-ionization rate. In particular, if and only if the ionization rate is simply proportional to the electron density, the particle fluxes are ambipolar; otherwise, there exist divergenceless current cells for which the axial and radial current densities are not everywhere zero. Experimentally, it was found impossible to produce such a uniform plasma in argon, where the ionization process is two stage, and goes as n2, whereas the same method of excitation easily yielded a uniform cesium vapor plasma, for which the ionization process is proportional to n.