The diffusion of charged particles in collisional plasmas: Free and ambipolar diffusion at low and moderate pressures

Abstract

The interpretation of measurements of the properties of weakly ionized plasmas in terms of diffusion of electrons and ions is reviewed both critically and tutorially. A particular effort is made to tie together various aspects of charged particle diffusion phenomena in quiescent, partially ionized plasmas. The concepts of diffusion length and effective diffusion coefficient and the treatment of partially reflecting boundaries are developed in the limit of the space-charge-free motion of the electrons or ions. A simplified derivation of the screening length for space charge electric fields is followed by a review of the conventional derivation of diffusion in the ambipolar limit. A discussion of the scaling parameters of the ratio of the diffusion length to the screening length and the ratio of the diffusion length to the ion mean-free-path leads to a map used to correlate published models covering the complete range of these parameters. The models of measurements of the diffusion of electrons, several types of positive ions, and negative ions are reviewed. The role of diffusion in the decay of charged particle densities and wall currents during the afterglow of a discharge is then considered. The effects of collapse of the space charge field and of diffusion cooling are reviewed. Finally, the application of the diffusion models to a number of different discharges is discussed.

Cavity Formation Modeling of Fiber Fuse in Single-Mode Optical Fibers

“…However, as shown in Figure 10 and also Figure 1 in [63], the densities of Si + and O + ions and electrons are reasonably large above 1 × 10 4 K. At high charge densities, it is known that the positive and negative species diffuse at the same rate. This phenomenon, proposed by Schottky [71], is called ambipolar diffusion [72,73]. Ambipolar diffusion is the diffusion of positive and negative species owing to their interaction via an electric field (space-charge field).…”

“…The thickness 0 of the double layer is approximately 1/ , which is the characteristic length known as the Debye length. The parameter is given in terms of and as follows [73]:…”

Cavity Generation Modeling of Fiber Fuse in Single-Mode Optical Fibers

“…[66], the densities of Si þ and O þ ions and electrons are reasonably large above 1 Â 10 4 K. At high charge densities, it is known that the positive and negative species diffuse at the same rate. This phenomenon, proposed by Schottky [75], is called ambipolar diffusion [76,77]. Ambipolar diffusion is the diffusion of positive and negative species owing to their interaction via an electric field (spacecharge field).…”

“…The thickness δ 0 of the double layer is approximately 1/κ, which is the characteristic length known as the Debye length. The parameter κ is given in terms of N e and T as follows [77]: where e is the charge of an electron and ε 0 is the dielectric constant of vacuum. When T ¼ 1 Â 10 4 K, N e ¼ 2:2 Â 10 20 cm À3 .…”

A global model of cylindrical and coaxial surface-wave discharges

“…The validity of the collisionless models [18,23] is breached in the considered collisionality regime and a constant diffusion model is adopted instead [19,22,33,43,44]. The one-dimensional model is based on the drift-diffusion formulation, where the drift is due to the ambipolar electric field, and the ambipolar diffusion simply reduces to the regular diffusion for the neutrals.…”