Classical Cross-Field Self-Diffusion Due to Finite Larmor Radius

It has been considered that the classical diffusion of plasma particles across the magnetic field is determined only by collisions between different species. Taking account of the finite Larmor radius rho of an ion, the random walk of its guiding center (step size rho per collision time tau) can result from collisions even with the same species. A resultant "self-diffusion" coefficient is D-perpendicular to approximate to rho(2)/2 tau. When there exists a radial electric field, the step size becomes asymmetric, and an "electric-field induced collisional displacement" of the guiding center is generated. In an inhomogeneous plasma, the collision time for an ion is varied during a gyration, and a "self-friction force" is induced. We propose these three collisional responses to be included to the ion fluid equations. We discuss that the ion cross-field self-diffusion becomes important in the edge plasma, where electrons are mainly lost along the magnetic field.