Diffusion of Charged Particles in Magnetized Low-Density Plasmas

The diffusion phenomena in plasmas with low charged-particle densities, in which the quasi-neutrality condition cannot be applied, are investigated with the aid of diffusion equations and the Poisson equation. From the asymptotic analysis in the limits of high and low densities, it is found that the transition between the ambipolar diffusion and the free diffusion is classified into three types. In each type, numerical solutions are obtained and the variations of the column thickness, the wall potential, the leak current to the wall, etc. with the change of the electron density are shown. When ions diffuse faster than electrons in a strong magnetic field, it is found that the electron diffusion is most enhanced in the transitional state. It is also shown that this phenomenon has influence upon the relation between the longitudinal current and field in discharge column.