A demonstration of magnetic field optimization in LHD

The behaviors of trapped particles in helical ripples are complicated and would enhance the radial transport of energetic particles and thermal plasma (neoclassical transport) in heliotrons. Thus the improvement of energetic particle confinement and the reduction of the neoclassical transport are key issues for a future reactor based on the helical system. On the other hand, recent LHD and CHS experimental results have shown good plasma performances in the 'inward shifted' configurations, in which the ideal MHD stability analysis predicts instability. These facts suggest that the MHD stability problem is not a severe one for plasma confinement in heliotrons and makes it reasonable to consider shifting the magnetic axis further inwards in LHD where further improvement of the neoclassical transport and energetic particle transport can be expected. In this paper, we show an optimized configuration of LHD to a level typical of so-called 'advanced stellarators' and demonstrate experimentally the effect of magnetic field optimization on the energetic particle confinement and the thermal plasma transport (neoclassical transport). (author)