Transport theory in stellarators

A comprehensive numerical treatment has been developed for analysing neoclassical transport in general three-dimensional plasmas. It is used to study the assumptions underlying previous analytic approximations and to develop accurate formulae for transport coefficients in stellarators. In the plateau regime, a resonant enhancement factor for diffusion that agrees with analytic results is found. In the lower collisionality regimes, the structure of the particle distribution function in phase space is computed to examine the validity of boundary conditions employed in analytic calculations. A diffusion coefficient D in the collisionless detrapping regime is computed and found to have the form D ≅ νεt3/2 G(εh, εt)T2/(M2Ω2r2ΩE2). Here, G(εh, εt) is a weak function of εh and εt, in contrast with previous analytic results. A resonance between parallel and ExB motions, which is neglected in bounce-averaged theories, is found to increase (decrease) diffusion in the ν (1/ν or plateau) regime. The improved transport properties of configurations with helical wells localized at the inside of the torus, which have been previously noted for the 1/ν regime, are found to persist into the lower collisionality regimes. This tendency is confirmed by comparison with Monte Carlo results. (author)