Diffusion across a Layered Medium and Relation to Transport in Thermonuclear Plasmas

The generic characteristics of diffusion in a medium exhibiting strong variations of the diffusivity are considered. The varying diffusivity is parametrized by alternating layers of high and low diffusivity. The limiting case is considered that the insulating layers determine the net transport. Some salient features of tokamak transport, such as the fact that perturbative experiments yield a higher diffusivity than a steady-state power balance, are shown to be a natural result of the inhomogeneity. The importance of a possible inhomogeneous diffusivity for the comparison of measurements with theoretical expressions for the diffusivity is discussed. (C) 1995 American Institute of Physics.