Dissipation by thermal forces in plasmas

Abstract

This paper deals with non-isothermal plasmas in which each species is described by Grad's thirteen-moment approximation. A theoretical framework, which includes a generalized Ohm's law and an ambipolar diffusion law, is used to treat energy dissipation resulting from ‘Motional’ interactions between the species. The frictional forces consist of a momentum relaxation force together with a ‘thermal force’ that occurs, in the presence of heat flow, partly because of the dependence of the collision frequencies on temperature. Detailed results are obtained for binary plasmas and for partially and fully ionized ternary plasmas. Our formalism is then compared with the technique used by Demetriades & Argyropoulos to study dissipation in thirteen-moment plasmas. The effects of thermal forces are illustrated by considering situations in which the drift contribution to the electronic relative thermal flux vector predominates over the thermal flux vector itself. Then, for binary plasmas and for ternary plasmas that are not too lightly ionized, the thermal forces increase the resistivity by a factor of about 5/2.