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.

In W 7-AS the H mode has been observed for the first time in a currentless stellarator plasma. H modes are achieved with 0.4 MW electron cyclotron resonance heating at 140 GHz at high density. The H phases display all characteristics known from tokamak H modes including edge localized modes (ELMs). The achievement of the H mode in a shear-free stellarator without toroidal current has consequences on //-mode transition and ELM theories.

ECR heating at Bo = 2.5T has been extensively used in the 1990 experimental period of the W VII-AS stellarator.As it is a low-shear experiment the magnetic configuration (especially details of the rotational transform profile) depends sensitively on plasma currents (pressure driven, ohmic, E C driven, Ohkawa current) which in turn have a strong influence on energy and particle confinement properties. For the stationary phase a transport analysis has been performed, yielding the profiles of the electron heat conduction and the ion particle diffusion coefficients. The former was subjected to a statistical analysis resulting in phenomenological expressions for xe and TE. First experiments using neutral beam injection (ECRH target plasma) as well as combined heating (NBI+ECRH) will also be discussed.

Parameter scans in density, heating power and isotope mass have been carried out in W7-AS. ECRH at a frequency of 140 GHz has allowed to study the density scaling of the energy confinement time of ECRH plasmas up to densities of 1020 m−3. In power scans it has been tried to relate the power degradation of the energy confinement to a local plasma parameter. Transport analyses using power balance an heat wave techniques indicate that the transport coefficient does not depend on the electron temperature or related parameters. This observation can be reconciled with power degradation if the transport coefficient is formally allowed to vary with changes in the heating power on a faster than the diffusive time scale. Such a transport process describes also the observations in the dynamic phases following large changes in the heating power.

The Wendelstein 7-X experiment is a concept test for properties of reactor relevant plasmas in advanced stellarators. Prominent features include a modular superconducting coil assembly, a fivefold toroidal symmetry, and a helical magnetic axis. Due to the optimization process, W7-X is characterized by a vacuum magnetic field configuration with smooth magnetic surfaces, improved equilibrium properties with a weak dependence of rotational transform and shear on the plasma pressure β, good magneto-hydrodynamic stability properties due to magnetic well stabilization, reduced neoclassical transport losses and negligible bootstrap current in the long mean-free-path regime, good collisionless α-particle confinement in an equivalent reactor, and, as a technical aspect, good feasibility of the superconducting modular coils. W7-X will be heated by continuous electron cyclotron resonance heating and pulsed neutral beam injection and ion cyclotron resonance heating. The envisaged parameters are Te⩽10 keV, Ti⩽6 keV central densities ⩽3×1020 m−3 with an averaged 〈β〉⩽5%. Despite the complicated geometrical structure, all basic diagnostics are compatible with W7-X. Generally, diagnostic methods and applications in a stellarator are not different from those in tokamaks. However, special efforts are being made to equip the experiment with those diagnostics necessary to measure the quantities directly related with the optimization of the machine: the verification of the predicted magnetic topology and characterization of the configuration throughout the entire parameter range, the identification of equilibrium and stability, and the determination of the confinement properties. The article describes the strategy developed which assures that the detailed measurement needs of the W7-X experimental program can be met.