Nonlinear Perpendicular Diffusion in Strong Turbulent Electromagnetic   Fields
  A nonlinear description for perpendicular particle diffusion in strong electromagnetic fluctuations is developed by using the fundamental Newton-Lorentz equation. Although not based on the same approach, the recently presented nonlinear guiding center (NLGC) theory is recovered as a special case. The approach used here is rather based on the argument that a well defined particle gyromotion does not exist in strong fluctuations than on the assumption that the particle gyrocenter follows magnetic field lines which themselves separate diffusively. The assumption of a guiding center motion and the diffusive separation of magnetic field lines is absolutely central to the NLGC theory. It is argued that the NLGC result should provide most accurate results for strong fluctuations. Furthermore, as a direct consequence of the particle equation of motion, it is shown that particle diffusion in one perpendicular direction is governed by the fluctuations in the other normal direction. This results contradicts the NLGC result, where perpendicular diffusion is triggered by fluctuations in the same direction. This is of particular interest for anisotropic perpendicular diffusion in a non-axisymmetric turbulence. Future numerical simulation results for non-axisymmetric magnetic turbulence and their comparison with the approach presented here and the NLGC theory have to provide an answer whether particle diffusion in one perpendicular direction is governed by the fluctuations in the other normal direction or by fluctuations in the same direction. 
O. Stawicki