Neoclassical Transport Analysis in the Banana Regime on Large Helical Device (LHD) with the DKES Code

"Neoclassical transport in the banana regime has been analyzed with the DKES (Drift Kinetic Equation Solver) code for the Large Helical Device (LHD). It is found that in a 1/nu regime, diffusion coefficients change by one order of magnitude for various configurations of LHD (-0.2 m leq Delta leq 0m, 0% leq Bq leq 200%, -0.1 leq alpha leq 0.1), depending on the structure of the helical magnetic ripple. The neoclassical transport calculated with the DKES code is quantitatively in good agreement with multi-helicity theory formulated by Shaing and Hokin. Incorporating the multi-helicity effect into the diffusion coefficient, we have proposed an interpolation formula between the l/nu and nu regimes. When the ion temperature is increased at a fixed density of n = 10^20 m^-3, the ions undergo a transition from l/nu neoclassical transport to the nu regime when their temperature Ti becomes > 3 keV with radial electric potential e phi comparable to the ion temperature (e phi/Ti approx1). For the optimized configuration (Delta = -0.2 m, Bq = 100%), the ion thermal diffusivity chi i has a maximum value of chi i approx 3.5 m^2/s at a minor radius of r/a approx 0.5. The bootstrap current has been also studied, and the results have been comprehensively compared with the theory. At the collisionless limit with a moderate radial electric potential of e phi/Ti approx I , the DKES calculations evaluated for various configurations of LHD have supported the theoretical formula given by Shaing and Callen. At the collision frequency between the plateau and the banana regimes, where the analytic theory is not applicable, the bootstrap current might become larger than in the collisionless limit (by a factor of about two), depending on the radial electric field.