Collisional Diffusion in an Axisymmetric Torus

Collisional cross-field diffusion of plasma in a general axisymmetric torus is calculated using the Fokker-Planck collision term, and a variational principle is found for the diffusion coefficient. Anomalously large transport, as found by Galeev and Sagdeev, occurs where the effective collision frequency is of order or less than the “bounce” frequency of trapped particles. Only unlike-particle collisions contribute, and the ion and electron diffusion coefficients are equal and independent of any radial electric field; these conclusions differ somewhat from those of Galeev and Sagdeev.