This paper is a summary of some of the major results from the Wendelstein 7-AS stellarator (W7-AS). W7-AS [G. Grieger et al., Phys. Fluids B 4, 2081 (1992)] has demonstrated the feasibility of modular coils and has pioneered the island divertor and the modeling of its three-dimensional characteristics with the EMC3/EIRENE code [Y. Feng, F. Sardei et al., Plasma Phys. Controlled Fusion 44, 611 (2002)]. It has extended the operational range to high density (4×1020m−3 at 2.5T) and high ⟨β⟩ (3.4% at 0.9T); it has demonstrated successfully the application of electron cyclotron resonance heating (ECRH) beyond cutoff via electron Bernstein wave heating, and it has utilized the toroidal variation of the magnetic field strength for ion cyclotron resonance frequency beach-wave heating. In preparation of W7-X [J. Nührenberg et al., Trans. Fusion Technol. 27, 71 (1995)], aspects of the optimization concept of the magnetic design have been successfully tested. W7-AS has accessed the H-mode, the first time in a “non-tokamak” and has extended H-mode operation toward high density by the discovery of the high-density H-mode (HDH), characterized by H-mode energy and L-mode-level impurity confinement. In the HDH-mode quasisteady state operation is possible close to operational limits without noticeable degradation in the plasma properties. High-β phases up to tpulse∕τE=65 have been achieved, which can already be taken as an indication of the intrinsic stellarator capability of steady-state operation. Confinement issues will be discussed with emphasis on the similarities to tokamak confinement (general transport properties, H-mode transition physics) but also with respect to distinct differences (no confinement degradation toward operational boundaries, positive density scaling, lack of profile resilience, no distinct isotope effect, H-mode operational window). W7-AS turned out to be an important step in the development of the Wendelstein stellarator line towards an independent fusion power plant concept.

The xenon spectrum, excited in ECR- and NBI-heated plasmas with central electron densities of around 10(20) m(-3) and central electron temperatures from 0.7 to 2.5keV, has been studied photoelectrically with a multichannel grazing-incidence spectrometer. Besides numerous well-known lines of Zn- and Cu-like Xenon, more than 50 additional lines which have not yet been published in the literature have been found and partly identified,.

During the last shutdown the Stellarator W7-AS underwent two major modifications: First, the limiters were replaced by ten divertor modules, and the diagnostic set associated with the plasma boundary and target plate regions was greatly expanded. Secondly, the previously counter tangential neutral beam injector box was shifted to a co-position. Thus, the heating efficiency should be considerably increased at low magnetic fields and high densities. After resuming experiments these improvements will be used to test the boundary island divertor concept and further expand operational boundaries during the remaining experimental time until permanent shutdown in 2002. The present operational boundaries are reviewed with respect to the stability of high β and density limit discharges. Discharges with good confinement properties will be discussed where further progress was achieved after installing control coils to modify the size and properties of vacuum field islands. In contrast to the usual net-current free mode, W7-AS also allows operation at large toroidal currents. In this way disruption-like events in the presence of rather large external poloidal fields can be produced.

Electron cyclotron resonance heating (ECRH) plays a key role in stellarator research, because it provides net current free plasma start up and heating toward reactor relevant plasma parameters. ECRH was extensively used and investigated in the stellarator experiments at IPP Garching, i.e., the W7-A and the W7-AS stellarators. These experiments provide a solid physics and technological basis for the 10 MW, CW ECRH system, which is under construction for the superconducting next step stellarator W7-X and will become operational in 2005. We briefly describe some of the major stellarator specific physics results on ECRH and electron cyclotron (EC)-current drive from W7-A and W7-AS. The scientific goals and the design of W7-X are outlined together with the demands for the ECRH system, which is the main heating system in the first stage of the experiment. The present status of the ECRH engineering design including the gyrotrons, all auxiliary systems, the transmission line, and the launching system are presented.

Electron cyclotron resonance heating (ECRH) at the W7-AS stellarator is performed two systems of comparable heating power at 70 GHz (0.8 MW, 3 s) and 140 GHz (0.9 MW, 0.4 s). Experiments with 140 GHz open a new parameter window with high density operation up to 1.1 × 1020 m−3. H mode transitions were observed in high density discharges at 2.5 T and at lower density at 1.25 T. The operational window for the stellarator H mode with emphasis on the density and power threshold and the influence of gas puffing on the H transitions are discussed. Perturbation experiments with modulated ECRH power were performed to determine the heat transport and the power deposition profile. Density control is achieved in combined heating with neutral beam injection (NBI) despite the beam particle fuelling, whereas with NBI alone a steady density rise is observed. The impact of combined heating on the impurity confinement was investigated. Electron cyclotron current drive (ECCD) was studied in different magnetic field configurations and the influence of trapped particles on the ECCD efficiency was examined experimentally and compared with theory.

For a limiter defined SOL, as given by small values of the rotational transform, the edge topology of W7-AS is characterized by large flux bundles distributed regularly over the poloidal angle. These bundles are poloidally decoupled to a good approximation, as indicated by 2D resolved Langmuir probe data. Therefore, for iota less-than-or-equal-to 0.4, a 1D radial plasma model is a reasonable first step to provide a qualitative description of the transport in the W7-AS boundary layer. The toroidal variation of the flux bundle geometry is taken into account by averaging the transport equations along the corresponding field lines. The effects of the neutral gas sources and the sensitivity of the diffusion coefficient to the unknown T(i) profile are discussed. A iota scan of the diffusion coefficient shows a minimum at iota congruent-to 0.35, where smooth magnetic surfaces exist throughout the SOL. This suggests that topological effects related to perturbations at the ''natural'' 5/m resonances may be responsible for the observed iota dependence of the diffusion coefficient.

Optimum confinement is realized in WENDELSTEIN 7-AS (low shear modular stellarator, R = 2 m, a congruent-to 0.18 m) by wall conditioning and by properly adjusting the parameters determining the magnetic field configuration. In particular low order rational values of the rotational transform have to be excluded from the confinement region or sufficient shear must be established by internal currents. The effective heating of net current free plasmas by ECRF (P less-than-or-similar-to 0.8 MW, 70 GHz) and neutral beam injection (NBI, P less-than-or-similar-to 1.5 MW, 45 kV) involves different plasma parameters and transport regimes. Stationary plasmas are generally produced by ECRF, whereas density and impurity control is a severe problem during NBI. This has initiated different kinds of impurity and particle control scenarios (carbonization, boronization and edge cooling). Thus, < beta > less-than-or-similar-to 1.1% (1.25 T) could be achieved. An extended parameter range with electron temperatures of 200 eV less-than-or-equal-to T(e) less-than-or-equal-to 3 keV, ion temperatures of 100 eV less-than-or-equal-to T(i) less-than-or-equal-to 0.7 keV and electron densities of 10(19) less-than-or-equal-to n(e) less-than-or-equal-to 3 . 10(20) m-3 was accessible. The characteristics of the energy confinement (confinement times up to 35 ms are observed in low power/low density ECRF heated and up to 25 ms in high power/high density NBI heated plasmas) and the particle and impurity transport are described and related to the specific heat and particle sources. The investigations comprise the analysis of electron and ion heat conductivity, particle transport modelling based on H-alpha measurements at relevant locations around the torus and impurity transport studies by laser blow-off experiments. The influence of the ambipolar electric field is discussed.

WENDELSTEIN VII-A has been operated for ten years. It is a low-shear, high-aspect-ratio device. The confinement properties have been thoroughly studied for both ohmically heated and net-current free plasmas. For the latter case, NBI- and ECF-maintained plasmas were of particular importance. It was found that under optimized conditions the core of high-pressure, net-current free plasmas is mainly governed by collisional effects.

Both in Ohmically and beam-heated L-mode discharges of ASDEX, the electron-temperature (Te) profile shape can be varied over a wide range by the choice of the safety factor qa. The power-deposition profile, on the contrary, has no effect on the Te profile shape. In current-free W-VII-A stellarator plasmas, no such invariance property is found. An independent constraint seems to fix the current distribution j(r) of the tokamak, which defines the conditions of electron heat transport.Received 4 February 1986DOI:https://doi.org/10.1103/PhysRevLett.56.2187©1986 American Physical Society