Finite gyroradius corrections in the theory of perpendicular diffusion 2. Strong velocity diffusion

Abstract

The current paper is a sequel to an article where we have started to incorporate finite gyroradius effects in the theory of perpendicular diffusion of energetic particles interacting with turbulent magnetic fields. In the previous paper we have focused on the case that velocity diffusion is suppressed and we derived corrections to the perpendicular diffusion coefficient. In the current article we focus on the limit of strong non-linear velocity diffusion. If finite gyroradius effects are not present, we derive the well-known limit where the perpendicular diffusion coefficient is directly proportional to the parallel diffusion coefficient. As in the previous paper, we employ different turbulence models as examples, namely the slab model, noisy slab turbulence, and the two-dimensional model. We show that finite gyroradius effects reduce the perpendicular mean free path in all considered cases except for the slab model where such effects do not occur.