The behavior of an ionized plasma is discussed in an approximation in which an individual particle is assumed to obey a Fokker-Planck equation, and where its interaction with the environment is incorporated in the coefficients of the partial differential equation. It is found that if the interaction of the test particle with the medium is divided into a "nearest neighbor" interaction (which manifests itself in "large-angle collisions") and an interaction with the rest of the medium, then the latter can be adequately treated by perturbation method. If the nearest neighbor interaction is neglected, the coefficients of successive derivatives form a rapidly decreasing sequence, provided the average kinetic energy greatly exceeds the mean potential energy (which is usually the case). Within the framework of this approximation the coefficients of damping (dynamical friction) and diffusion in velocity space are calculated and the higher (small) coefficients are estimated.

• Received 12 September 1955

DOI:https://doi.org/10.1103/PhysRev.101.922