Theory of advanced stellarators

The main features of advanced stellarators are the optimized coil system wh ich can be extrapolated to reactor size, improved properties of plasma equi librium with reduced Shafranov shift and a stability limit which is suffici ent to meet the requirements of a commercial fusion reactor. The key elemen t to achieve these goals is the proper shape of the magnetic surfaces and m inimization of the geodesic curvature of magnetic field lines. As a consequ ence, Pfirsch-Schluter currents and radial particle transport are reduced. Neoclassical transport becomes small enough so that ignition is not endange red. The theoretical basis of the advanced stellarator is a systematic proc edure to compute plasma equilibria with reduced Pfirsch-Schluter currents. The close relation between neoclassical transport and viscous damping indic ates that a reduction of geodesic curvature not only reduces radial transpo rt 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 stellarato rs. The paper discusses the various aspects of advanced stellarators and descri bes their basic properties.