The theoretical and experimental development of stellarators has removed some of the specific deficiencies of this configuration, viz., the limitations in β, the high neoclassical transport, and the low collisionless confinement of α particles. These optimized stellarators can best be realized with a modular coil system. The W7-AS experiment [Plasma Phys. Controlled Fusion 31, 1579 (1989)] has successfully demonstrated two aspects of advanced stellarators, the improved equilibrium and the modular coil concept. Stellarator optimization will much more viably be demonstrated by W7-X [Plasma Physics and Controlled Fusion Research, Proceedings of the 12th International Conference, Nice, 1988 (IAEA, Vienna, 1989), Vol. 2, p. 369], the successor experiment presently under design. Optimized stellarators seem to offer an independent reactor option. In addition, they supplement, in a unique form, the toroidal confinement fusion program, e.g., energy transport is anomalous in stellarators too, but possibly more easily understandable in the frame of existing theoretical concepts than in tokamaks.

The electron energy balance is analyzed for equivalent low-density electron cyclotron resonance heated (ECRH) discharges with highly peaked central power deposition in the stellarators W7-A [Plasma Phys. Controlled Fusion 28, 43 (1986)], L-2 [Proceedings of the 6th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Berchtesgaden, 1976 (International Atomic Energy Agency, Vienna, 1977), Vol. 2, p. 115] and W7-AS [Proceedings of the 9th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Baltimore, 1982 (International Atomic Energy Agency, Vienna, 1983), Vol. 3, p. 141]. Within the long mean-free path (LMFP) collisionality regime in stellarators, the neoclassical electron heat diffusivity χe can overcome the ‘‘anomalous’’ one. The neoclassical transport coefficients are calculated by the dkes code (Drift Kinetic Equation Solver) [Phys. Fluids 29, 2951 (1986); Phys. Fluids B 1, 563 (1989)] for these configurations, and the particle and energy fluxes are estimated based on measured density and temperature profiles. Neoclassical transport in the LMFP regime is minimum in W7-A and maximum in L-2, the standard configurations in W7-AS are in between. The radial electric field is estimated from the ambipolarity condition of only neoclassical particle fluxes. For these types of discharges in the quite different stellarator configurations, only the ‘‘electron root’’ exists in the innermost region, and, at the outer radii, only the ‘‘ion root.’’ In the region where both roots are found, a rather narrow shear layer in the poloidal plasma rotation is expected. Especially for W7-AS, a significant improvement of the neoclassical confinement is predicted in the ‘‘electron root’’ region. On the ‘‘ion root’’ side of the predicted ‘‘shear layer,’’ both the neoclassical energy and particle fluxes agree quite well with the experimental findings. At outer radii, the neoclassical fluxes are much lower. The predicted improvement for the ‘‘electron root’’ region is not found experimentally.

A review of experiments with ECRHand NEiLheated plasmas in W7-AS is given. Global results of Wl-AS are summarized. Particular emphasis is put on electron cyclotron currenl drive, comparative analysis of electron heat msport derived from power balance and perNrbative studies, and p d c k transport under combined ECRHand NBIheating conditions. The role of the plasma boundary field configuration as a necessary condition for the existence of the H-mode in Wl-AS is discussed and first observations of coherent global Alfv6n eigenmodes and turbulent temperature fluctuations in the plasma core are reported.

The local electron and ion heat transport as well as the particle and impurity transport properties in stellaratoa are reviewed. In this context, neoclassical theory is used as a guideline for the comparison of the experimentd results of the quite different confinement concepts. At sufficiently high temperatures depending on the speci6c magnetic configuration, neoelsssical predictions are mfimed by experimental findings. The confinement properties in the LMFP collisionality regime are discussed with respect to the next stellarator generation, for which at higher temperatures the neodassicd transport is expected to become more important.