Analysis of neoclassical transport in the banana regime with the DKES code for the Large Helical Device

Abstract

Neoclassical transport in the banana regime has been analysed with the DKES (Drift Kinetic Equation Solver) code for the Large Helical Device (LHD). It is found that in the 1/ν regime the diffusion coefficients change by one order of magnitude for various LHD configurations, depending on the structure of the helical magnetic ripple. The neoclassical transport calculated with the DKES code is quantitatively in good agreement with a multi-helicity theory formulated by Shaing and Hokin (1983). Incorporating the multi-helicity effect into the diffusion coefficient, the authors propose an interpolation formula for the diffusion coefficient in the 1/ν and ν regimes. When the ion temperature is increased at a fixed density of n = 10²⁰ m^-3, the ions undergo a transition from 1/ nu neoclassical transport to the nu regime when T_i > 3 keV, with a radial electric potential e comparable to the ion temperature (e/T_i ≈ 1). For the optimized configuration, the ion thermal diffusivity has a maximum value of ≈ 3.5 m²/s at a minor radius r/a ≈ 0.5. The bootstrap current has also been studied and the results have been compared with the theory. At the collisionless limit with a moderate radial electric potential of e/T_i ≈ 1, the DKES calculations evaluated for various LHD configurations support the theoretical formula given by Shaing and Callen (1983). For collision frequencies between the plateau regime and the banana regime, where the analytical theory is not applicable, the bootstrap current might become larger (by a factor of about two) than in the collisionless limit, depending on the radial electric field