Diffusion near nonequilibrium steady state

The diffusion equation describing the response of a system in a steady nonequilibrium state to a nonuniform external perturbation is derived from the Boltzmann equation. For transient processes, the diffusion equation governs the slow (final) stage of the process, irrespective to the form of the initial distribution. The initial velocity distribution becomes the local steady-state distribution during the first fast stage of the process. This fast initial relaxation affects only the initial condition for the diffusion equation. Some principal features of the nonequilibrium diffusion constant are discussed. As an illustration, the diffusion constant of hot electrons is calculated in the electron temperature approximation.