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.

Stellarators have the intrinsic property of steady state operation. However, on present-day stellarators the pulse length is usually not only limited due to technical reasons, but also by physical problems. Lack of density control and a subsequent radiation collapse terminate the discharges quite often at high densities. To improve the control of the plasma–wall interaction, the island divertor concept was developed for optimized stellarators. To test this divertor concept on W7-AS, all limiters were removed and replaced by ten divertor modules. In subsequent divertor experiments a promising new plasma operational regime has been discovered which is termed `high density H-mode' (HDH-mode). During the transition into that regime a clear reduction of ELM-like events and turbulent fluctuations is observed. The HDH-mode combines good energy confinement with very low impurity confinement resulting in low core radiation, but high edge-localized radiation. Consequently, stationary discharges at densities of typically 2×1020 m−3 can be performed within the accessible pulse length of about 1 s. At densities above 3×1020 m−3 a controlled transition from attached to partially detached plasmas is observed. The still edge-localized radiation reaches 90% of the heating power so that the power load onto the divertor target plates is further reduced. At a lower toroidal field of 0.9 T average β-values could be raised from earlier 2% to more than 3% in magnetic field configurations with rather smooth flux surfaces at the plasma boundary. The recently obtained results render excellent prospects for W7-X, the larger superconducting successor experiment of W7-AS.

After ten years of operation many experimental goals of W7-AS have been reached and many problems solved as far as possible. Among the set of criteria used to design the further optimized W7-X the feasibility of modular coils, the reduction of the Pfirsch-Schlueter currents and the resulting reduction of the Shafranov shift were demonstrated on the partly optimized W7-AS. The neoclassical transport in the long mean free path regime and the anomalous transport are still being investigated. High average beta values of 2% were obtained, the beta limit will be studied further after increasing the neutral beam power. ICRH experiment were continued with a new double strap antenna. In addition, the experiments on W7-AS point out the necessity of a divertor. A first test of the island divertor concept foreseen for W7-X is one the major remaining goals. After installation of control coils to change the size and properties of boundary islands their influence on the plasma/ limiter interaction and on the transport was investigated. In a second step, divertor moduls will be mounted in the torus vessel, and the necessary divertor diagnostics added. 1. NEOCLASSICAL TRANSPORT Neoclassical transport is an important subject in optimized stellarators because the threedimensional magnetic field configuration is unavoidably connected with magnetic mirrors where particles can be trapped. Thus, neoclassical transport as the transport enhanced by trapped particle effects can be rather large in the long mean free path collisionality regime and needs to be optimized. The tool to calculate the neoclassical transport in 3D geometry is the DKES code [1]. Consequently, the investigation of the neoclassical transport in the partly optimized W7-AS was a major objective. The essential result is that the heat transport is well described by the DKES code if the important radial electric field effects are included. Thus its predictions for the further optimized W7-X should be reliable. Radial electric fields usually reduce the neoclassical heat transport losses [2]. Thus, in the case of the ion root (negative Er(r)) high ion temperatures could be obtained in W7-AS, and the electron root feature (positive Er(r)) leads to very high central electron temperatures [3]. Statements on particle transport are usually much more difficult. Neoclassical particle fluxes are assumed in the DKES code to determine selfconsistently the radial electric field for each flux surface from the local ambipolarity condition Γe = Γi. They are roughly consistent with the experimental particle fluxes derived by integrating the NB particle deposition, and also the calculated Er(r) values agree quite well with measured ones. Only at outer radii, where recycling results in strong particle sources an additional anomalous particle transport mechanism is needed [2]. In electron root discharges an interesting particle transport phenomenon should occur because a neoclassical outward drift of impurities is predicted for the center where the strong positive Er(r) is found. Measurements by active CXRS on He and N lines show somewhat hollow profiles and confirm this prediction at least partly. An anomalous particle diffusion as typically found for low density ECRH discharges by Al laser blow off seems to reduce the effect as compared with a simplified neoclassical transport model [4]. Generally, particle transport on W7-AS as derived from impurity transport studies improves with increasing electron density and deteriorates with heating power as is typical for an anomalous transport scaling. Compared with neoclassical predictions impurity transport in low density ECRH discharges is found to be higher. In high confinement discharges (high density, but rather low heating power) diffusion coefficients seem to be even smaller than simplified neoclassical estimates predict [5]. In these discharges a stationary state in the bolometer radiation and in impurity line measurements is not reached within pulse lengths of up to about 2 s. 2. ANOMALOUS TRANSPORT There are many indications that basically the same transport mechanisms are responsible for the anomalous transport in tokamaks and stellarators, which also means that the anomalous transport is essentially not understood in both cases. On the other hand, the large flexibility of W7-AS can be used to obtain at least some additional hints and to study e.g. the role of magnetic shear and the influence of high order rational magnetic surfaces on the transport. In the meantime, the wellFig. 1: Dependence of the diamagnetic energy content and of the impurity radiation on the rotational transform at the plasma boundary. Discharge parameters are: Bo = 2.5T, 350 kW ECRH, constant average electron density of about 2⋅1013 cm-3 known dependence of the diamagnetic energy content Wdia on the rotational transform ι in W7-AS (Fig. 1) is at least partly understood, i.e. it is understood why in the standard mode of net-current free operation and at rather low beta optimum confinement is usually found only close to low order rational values of ι like 1/3 and 1/2 where high order rational values are almost absent. Evidently these high order rational ι values seem to enhance the local transport as long as the shear is not too large. Increasing the magnetic shear for such degraded situations e.g. by an ohmic current, continuously improves confinement back to the optimum. The radial region dominated by neoclassical transport continuously expands towards the boundary due to the increase of neoclassical transport with temperature and a simultaneous reduction of anomalous transport. Thus there seems to be no essential difference between optimum confinement at low shear and the confinement at high shear, and this result is independent on the sign of shear [6]. In this way the dependence of Wdia on ι in net current free discharges can roughly be modelled di am ag ne tic e ne rg y [k J] 0 0.30 0.40 (a) ι 0.50 0.60 2 4 6

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) 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.

There exist two classes of discharges on ASDEX wich overcome the severe confinement deterioration at high densities or heating power. The first class is characterized by unusually peaked density profiles and the second one by exceptionally broad ones. These profiles and the concomitant confinement enhancement imply a change in both the particle and heat transport. Whereas the changes in particle transport are not fully understood, transport analyses point out that the improved heat transport in the first profile class can be explained by reduced ion contribution losses coming close to the neoclassical ones. The different results for the ion transport with flat and peaked density profiles are qualitatively consistent with that expected from eta-i-modes. This conclusion is supported by momentum transport studies. The analyses, however, cannot yet explain the electron heat transport which is found to be anomalously high in all regimes.

High resolution X-ray spectroscopy was used to evaluate the fractional population of non-thermal electrons generated when electromagnetic waves in the lower hybrid frequency range are launched in the ASDEX tokamak. In complete current drive discharges a typical value of 1% is found for this parameter, in agreement with a theoretical estimate. Furthermore, it is shown that the toroidal electric field is very effective in enhancing tail electron generation, as confimed by β measurements, and that the wave penetration to the plasma centre is inhibited when interaction of the waves with plasma ions is the dominant damping mechanism.

The ASDEX Data Acquisition System is built as a distributed system to collect a large number of data from many diagnostics. The data acquisition system software GALE is adaptable to different experiments, flexible with respect to hardware configuration changes and simple to use for the experimentalist. A distributed system and a flexible acquisition software is the adequate response to fast growing demands for computer power of fusion experiments.

Use of the ASDEX divertor permits the production of stable low-density discharges (ne≳1012 cm−3) with extremely low resistivity lasting for more than 10 s. While the distribution functions of electrons and ions show suprathermal tails, runaway electrons in the megaelectronvolt range are found to disappear with decreasing density. There are indications that in these discharges the energy confinement is improved compared with ALCATOR scaling.Received 20 July 1981DOI:https://doi.org/10.1103/PhysRevLett.47.1004©1981 American Physical Society

In the first experimental phase the divertor tokamak ASDEX was run with a closed SS-limiter. The cleaning procedure for the vessel consisted of baking to 120°C, carbon removal by glow discharge in hydrogen, and oxygen removal with a continuous low power 50 Hz AC discharge. Wall contact of the plasma was reduced by carefully positioning the plasma with a feedback system. Discharges with plasma currents up to 280 kA and a disruption-free duration of up to 1 s were reliably produced with a filling pressure of 5 × 10−5 mbar and a programmed current rise. No change in start-up conditions and discharge behaviour was observed in material limiter discharges with superimposed divertor field.

The problem of scaling plasma focus devices for increased neutron output has recently been discussed by several authors. Imshennik et al. developed a similarity theiry, proceeding from the assumption that the ratio of collision mean free path to characteristic length of the device should remain constant throughout the scaling process. In the following note, it will be shown that the constancy of the Mach number to Reynolds number ratio can be argued from a kinetic treatment of the neutron production mechanism. Furthermore, the influence of turbulent resistivity will be considered throughout the similarity considerations, and for the current an equation yielding the dip due to collaps will be employed instead of using the maximum current value.