Theory of advanced stellarators

Abstract

The main features of advanced stellarators are the optimized coil system which can be extrapolated to reactor size, improved properties of plasma equilibrium with reduced Shafranov shift and a stability limit which is sufficient to meet the requirements of a commercial fusion reactor. The key element to achieve these goals is the proper shape of the magnetic surfaces and minimization of the geodesic curvature of magnetic field lines. As a consequence, Pfirsch-Schlüter currents and radial particle transport are reduced. Neoclassical transport becomes small enough so that ignition is not endangered. The theoretical basis of the advanced stellarator is a systematic procedure to compute plasma equilibria with reduced Pfirsch-Schlüter currents. The close relation between neoclassical transport and viscous damping indicates that a reduction of geodesic curvature not only reduces radial transport but also leads to a reduction of viscous damping, and thus, facilitates the rotational spin-up of the plasma. For this reason achievement of shear flow and reduced anomalous transport may be expected in advanced stellarators. The paper discusses the various aspects of advanced stellarators and describes their basic properties.