Neoclassical transport in a toroidal plasma with finite ion orbits is studied, including for the first time the self-consistent radial electric field. Using a low-noise $\delta f$ particle simulation, we demonstrate that a deep electric-field well develops in a region with a steep density gradient, because of the self-collision–driven ion flux. We find that the electric field agrees with the standard neoclassical expression, when the toroidal rotation is zero, even for a steep density gradient. Ion thermal transport is modified by the electric-field well in a way which is consistent with the orbit squeezing effect, but smoothed by the finite orbits.

• Received 1 March 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.055002