Test particle transport in stochastic magnetic fields modeled by stochastic processes

The theory of stochastic processes is applied to analyze the effect of magnetic‐field perturbations on the motion of classical charged particles. For arbitrary field fluctuations along the path of a particle, an approximate diffusion equation is obtained by means of the cumulant technique. The limit of white noise, in which the model is exact, and the difference between adiabatic and nonadiabatic motion are discussed. These cases relate results obtained for astrophysical plasmas and tokamak plasmas. It is shown that large longitudinal perturbations give rise to a new regime for diffusion perpendicular to the mean magnetic field.