Neoclassical transport considerations for advanced tokamaks

Abstract

Interest in attaining high beta has stimulated the study of strongly shaped tokamak configurations. The influence of strong shaping on neoclassical transport, specifically on the ion thermal flux and the diffusion induced (or 'bootstrap') current, has been investigated. Axisymmetric magnetohydrodynamic equilibria have been computed for elliptical, D, bean and crescent shapes. The geometric data have been used in the Drift Kinetic Equation Solver code of the Oak Ridge National Laboratory to compute the resulting fluxes under a range of conditions. It is found that neoclassical transport generally improves with advanced shaping, as the larger plasma current reduces the ion thermal flux. Optimization of both neoclassical transport and beta presents conflicting demands, however, because 'omnigenous' reductions in neoclassical fluxes can be obtained only at the expense of reducing the local shear, which causes a deterioration in the critical beta.