Density control problems in large stellarators with neoclassical transport

With respect to the particle flux, the off-diagonal term in the neoclassica l transport matrix becomes crucial in the stellarator long-mean-free-path r egime. Central heating with peaked temperature profiles can make an active density profile control by-central particle refuelling mandatory. The neocl assical particle confinement can significantly exceed the energy confinemen t at the outer radii. As a consequence, the required central refuelling may be larger than the neoclassical particle fluxes at outer radii leading to the loss of the global density control. Radiative losses as well as additio nal 'anomalous' electron heat diffusivities further exacerbate this problem . In addition to the analytical formulation of the neoclassical link of parti cle and energy fluxes, simplified model simulations as well as time-depende nt ASTRA code simulations are described. In particular, the 'low-' and 'hig h-mirror' W7-X configurations are compared. For the W7-X 'high-mirror' conf iguration especially, the appearance of the neoclassical particle transport barrier is predicted at higher densities.