Mechanisms and scalings of energetic ion transport via tokamak microturbulence

The turbulent ExB advection of energetic ions in three-dimensional tokamak geometry is investigated both analytically and numerically. It is shown that orbit averaging (leading to a significant reduction of the diffusivity) is only valid for low magnetic shear. At moderate or high magnetic shear, a rather slow decrease of the diffusivity is found, proportional to (E/T-e)(-1) or (E/T-e)(-1.5) for particles with a large or small parallel velocity component, respectively. The decorrelation mechanisms responsible for this behavior are studied and explained in detail. Moreover, it is found that resonances between the toroidal drift of the particles and the diamagnetic drift of the turbulence can lead to an enhancement of the fast ion transport.