Equilibrium Diffusion Rate in a Toroidal Plasma at Intermediate Collision Frequencies

It is shown that a consistent equilibrium must include an azimuthal electric field. The ambipolar diffusion rate differs in several respects from earlier estimates, although these differences are not primarily due to the azimuthal field. The new rate tends to be lower. In a stellarator field an ambipolar condition may occur in which the electron diffusion due to the toroidal field variation balances the ion diffusion in the helical variation.