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4.4.4 Ionic diffusion

The coefficient of diffusion D is given by Fick’s (1855) law J = −Dn where n is the ion density and J is the flux density of the ions, i.e. number of ions flowing in unit time through unit area. J = nv, where v is the velocity. At low ion densities, i.e. non-plasma conditions, and low E/N, ionic diffusion occurs uniformly in all directions and the diffusion coefficient D is given by the Einstein relation D = μkT/q where μ is the ionic mobility, k is Boltzmann’s constant, q is the charge on the ion and T is the temperature. At higher values of E/N the coefficients for diffusion parallel and perpendicular to the electric field are not the same and separate measurements of the two quantities must be made. Values of ND as a function of E/N are given in the literature (see for example Ellis et al., 1978, 1984). For information on the effect of magnetic fields see Biagi (1989).

J.W. Leake

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