ABSTRACT
The sparkler analogy captures the essence of Fick's law, and gives insight into the significance of the mean-free path. But it cannot model many other transport properties. Transport theory describes the behavior of a plasma that is close to equilibrium. It predicts the plasma response to so-called thermodynamic forces, which characterize small departures from equilibrium. Collisional transport of a heavy species necessarily involves like-particle collisions, and the corresponding operator is too complicated for analytic solution in closed form. The collisional bilinear form provides the crucial tool for calculating transport coefficients when the mathematical complexity of like-species collisions cannot be evaded. Parallel transport is an exceptionally simple context for Onsager symmetry because the magnetic field does not enter. The perpendicular particle flow is along the temperature gradient, rather than opposite to it, because perpendicular transport is abetted by collisions, rather than impeded as in the parallel case.
