Monte-Carlo simulation of neoclassical transport in stellarators

Abstract

Neoclassical transport coefficients are computed by Monte-Carlo simulation over a wide range of mean free paths in the approximation of small-gyroradius, mono-energetic-particle distribution, and vanishing electric field for several stellarator fields. Pfirsch-Schlüter, plateau, and ripple transport coefficients are obtained. The transport coefficients for ℓ = 2 stellarators in the ripple regime can be described by a single coefficient depending on aspect ratio, rotational transform, and number of periods. There are stellarator configurations in which transport is reduced by a factor of two to four in all three regimes as compared with a tokamak with equal aspect ratio and effective ripple, and by a factor of up to eight as compared with an equivalent ℓ = 2 stellarator. Transport in actual stellarators and in those being designed is only moderately worse than in the equivalent ℓ = 2 stellarator.