Diffusion of charged particles in a stochastic magnetic field

A doubly stochastic model is studied with test guiding centres experiencing stochastic collisions on stochastic field lines. A nonlinear differential equation is derived for the plasma cross-field diffusion. It appears as a singular perturbation problem which is solved analytically by matched asymptotic expansions. The asymptotic behaviour confirms the numerical result of a purely diffusive regime, with an effective diffusivity which has the same scaling as classical results but which shows a much better agreement with experimental data. (author)