Neoclassical transport optimization of LHD

Abstract

Neoclassical transport is studied for large helical device (LHD) configurations in which the magnetic axis has been shifted radially by determining the mono-energetic transport coefficient and the effective helical ripple. With respect to the transport in the long mean-free-path collisionality region- the so-called 1/v transport-the optimum configuration is found when the magnetic axis has a major radius of 3.53 m, which is 0.22 In inward shifted from the 'standard' configuration of LHD. In the optimized case, the effective helical ripple is very small, remaining below 2% inside 4/5 of the plasma radius. This indicates that a strong inward shift of the magnetic axis in the LHD can diminish the neoclassical transport to a level typical of so-called 'advanced stellarators'.