Neoclassical diffusion in an l = 3 stellarator

Neoclassical diffusion in a stellarator is investigated in detail by introducing a consistent expansion in λ1/3 (where λ is the inverse aspect ratio of the torus) for the magnetic fields and particle distribution function. As a result of the expansion, the various characteristic bounce and transit frequencies of the particle orbits appear in different orders, and various regimes of collision frequency (analogous to the “banana”, and “plateau” regimes of the axisymmetric analysis) are considered separately. It is found that over most of the range of collision frequencies the diffusion is the same as that of an equivalent axisymmetric configuration with the sole role of the helical fields being that of supplying the rotational transform. At sufficiently low collision frequencies particle trapping in the helical field modulations becomes important and localized particle diffusion dominates. Expressions for the diffusion flux and ion heat flux are derived in this limit.