The main parameters of W7-X are shown in table 1 [2,3]. The magnet system consists of 50 superconducting Non-Planar-Coils (NPC), 20 superconducting Planar Coils (PC) and the mechanical structure, which is based on the Central Ring and the intercoil support structure. The NPC and the PC are supported by the Central Ring through the Central Support (CS) elements, two for each coil (Fig.1). The Narrow Supports (NS) and the Lateral Supports (LS) connecting adjacent NPC casings in the inner and outer region of the machine respectively (Fig.1) and the Planar Supports connecting the PC to the NPC are the elements of the intercoil support structure. The coils are arranged toroidally in five equal modules, each one consisting of two flip symmetric semi-modules. One semi-module includes 5 differently shaped NPCs and 2 PCs. The Plasma Vessel (PV) has to closely follow the twisted shape of the plasma and it is

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.

A thin foil Faraday cup array is being built to measure the loss of 3.5 MeV alpha particles and MeV ion cyclotron heating (ICH) tail ions on JET. It will consist of nine detectors spread over five different poloidal locations and three radial positions. They will measure the poloidal distribution and radial scrape off of the losses. The detectors will be comprised of four layers of thin (2.5 micron) Ni foil, giving some resolution of the lost particle energy distribution as different ranges of energies will stop in different layers of the detector. One detector will utilize eight thinner (1.0 micron) foils to obtain a better resolved energy distribution. These detectors will accept particles incident up to 45{sup o} from the normal to the foils.

The miniaturization of receiver arrays permits the arrangement of numerous poloidally staggered radial sightlines of an electron cyclotron emission (ECE) diagnostic for the measurement of the electron temperature and its fluctuations, making two-dimensional (2D) imaging of the electron temperature and its fluctuations caused by plasma turbulence possible. For the stellarator W7-AS, the development of a fully monolithic microwave integrated circuit 150 GHz subharmonic mixer array is under development. As a first step, a 2D ECE system for the measurement of electron temperature fluctuations using four individual horn-reflector arrangements in conjunction with multichannel heterodyne radiometers was installed and set into operation. With Gaussian beam optics and four poloidally staggered sightlines, electron temperature fluctuations could be characterized in radial and poloidal directions simultaneously. First observations in purely electron cyclotron resonance heated stellarator plasmas reveal a broadband drift-wave feature. Earlier experiments showing a decrease of the electron temperature fluctuation level with increasing heating power were confirmed. Additionally, it was revealed, using the 2D ECE correlation radiometer, that an increased velocity shear might account for the decrease of the coherence length and thus for the reduction of the electron temperature fluctuation level.

The research on divertors for stellarators is at the beginning. Extensive studies are being prepared on large helical device (LHD) and W7-X. W7-AS is now being operated with an open island divertor (ID) which serves as a test bed for the W7-X divertor. The divertor enables access to a new NBI-heated, high-density operating regime with improved confinement properties. This regime-the high-density H-mode (HDH)-displays no evident mode activity, is extant above a threshold density and characterized by flat density profiles, high-energy- and low-impurity-confinement times and edge-localized radiation. Impurity accumulation, normally associated with ELM-free H-modes, is avoided. Quasi-teady-state discharges with (n) over bar (e) up to 4 x 10(20) m(-3), edge radiation levels up to 90% and plasma partial detachment at the divertor targets can be simultaneously realized. The accessibility to other improved confinement modes in W7-AS (conventional H-mode and OC-mode) is not restricted by the divertor. The results provide a promising basis for future experiments, in particular on W7-X, and recommend the ID as a serious candidate for solving the plasma exhaust problem in stellarators. (C) 2003 Elsevier Science B.V. All rights reserved.

Recently a new improved confinement regime was found in the Wendelstein 7-AS (W7-AS) stellarator (Renner H. et al 1989 Plasma Phys. Control. Fusion 31 1579). The discovery of this high density high confinement mode (HDH-mode) was facilitated by the installation of divertor modules. In this paper, measurements of short wavelength density fluctuations in the HDH-mode using collective scattering of infrared light are presented. These measurements will be contrasted to fluctuations during normal confinement operation (NC-mode). The autopower spectra of the measurements show a consistent increase of the fluctuation level associated with the transition from NC- to HDH-mode. Correlation calculations on a 20 μs timescale between magnetic and density fluctuations lead to the result that the fluctuations are correlated in NC-but not in HDH-mode. Finally, a comparative analysis between the enhanced Dα H-mode (EDA H-mode) found in the Alcator C-Mod tokamak and the HDH-mode in W7-AS is carried out.

This paper is a summary of some of the major results from W7-AS stellarator and addresses - electron Bernstein wave heating - beach-wave heating - aspects of the partial optimisation of the magnetic design - the island divertor operation and the modelling of the results with the EMC3/EIRENE code - confinement issues with emphasis on the differences to tokamaks (isotope effect, density scaling, lack of profile resilience) - some of the H-mode findings (operational windows, transition physics) - the characteristics of the High-Density-H-mode - stability issues (GAEs, ELMs), and - operational limits. After nearly 14 years with 56953 discharges W7-AS has suspended operation. The device is now mothballed. The development of the Wendelstein stellarator line will continue with the W7-X device. The major goals of W7- AS were - to test the modular coil concept, - to demonstrate the effectiveness of the first steps toward an optimised stellarator design, and - to develop an exhaust concept based on the natural island chain which forms the plasma boundary. W7-AS has contributed to all areas of stellarator and fusion research. This paper gives a brief summary of some of the most relevant results.

Introduction: The High Density H-Mode (HDH) /1-3/, discovered and exploited over a wide range of conditions on the W7-AS stellarator, combines optimal core behavior along with edge parameters necessary for successful operation of an Island Divertor. Potentially this bodes well for the larger W7-X device (a/R = 0.16/2m vs. 0.5/5.5m) which represents the next step in the Wendelstein line of development. Still, the question remains if the HDH properties of high energy confinement with low impurity retention and radiation localized at the edge under steady-state conditions (ELM-free) at high densities (to 4 10 20 m -3 ) and heating powers (to 1.7MWm -3 ) can be realized on W7-X or other stellarators? Early investigations revealed that the differences in energy confinement between normal confinement- (NC) and HDH-modes were related to the much broader density profiles of HDH, in contrast to the peaked profiles of NC /1/. Further, the diminished residence time of laser-ablated aluminum in HDH compared to NC could be modeled in an ad hoc fashion by a smaller inwards impurity pinch in the core plasma /1, 4/. Neoclassical calculations indicated that temperature screening together with the flat n e -profiles could be the physical factors for this reduced pinch. However, the inwards convection associated with steep density gradients at the plasma edge for HDH would nonetheless still contrive to confine impurities within the core plasma. A postulation of enhanced impurity diffusion at the plasma edge was necessary in order to reproduce the impurity flushing features of HDH /5, 6/. It is of note that the Enhanced D H-mode (EDA) of the C-Mod tokamak has properties similar to HDH, e.g. flat n e -profiles and steady-state, ELM-free operation without impurity accumulation. A quasi-coherent (QC) mode at the plasma edge is thought to provoke impurity flushing /7-9/. In contrast, ELM-free H-modes without QC exhibit accumulation. This paper reports on dedicated experiments performed on W7-AS to compare HDH with standard ELM-free discharges (H*) ‐ which typically suffer radiation collapse as a result of impurity accumulation. A back-to-back comparison offers an excellent basis for pinpointing essential operative elements of HDH physics. Experiment: A discharge was tailored in an island-divertor configuration with P abs ~1.4MW whereby the density was ramped up until attainment of H*, and then after 60ms increased again until the HDH mode was solidly established (Fig.1). The entrance into H* is indicated by the cessation of ELM activity (seen in H of Figs. 1&2), followed by a rapid increase in

This paper is a summary of some of the major results from W7-AS stellarator and addresses electron Bernstein wave heating beach-wave heating aspects of the partial optimisation of the magnetic design the island divertor operation and the modelling of the results with the EMC3/EIRENE code confinement issues with emphasis on the differences to tokamaks (isotope effect, density scaling, lack of profile resilience) some of the H-mode findings (operational windows, transition physics) the characteristics of the High-Density-H-mode stability issues (GAEs, ELMs), and operational limits.

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

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.

Radiometry of electron cyclotron emission (ECE) is established as an electron temperature diagnostic in fusion plasmas since many years. By applying correlation techniques, also temperature fluctuations otherwise completely burried in thermal or wave noise of the ECE can be extracted. Crossed sightline correlation measurements were done on W7-AS, detecting electron temperature fluctuations with a level down to 0,1% of thermal noise [3]. With improved poloidal resolution parameter studies were performed [4]. The single sightline decorrelation, was used for temperature fluctuation measurements on TEXT [5]. These techniques proved the applicability of ECE diagnostics for measurements of electron temperature fluctuations. Both, crossed sightline and single sightline decorrelations, are proven to give the same results [6]. On the plasma edge two-dimensional (2D) characterisations of fluctuating structures are carried out by the use of 2D Langmuir probe arrays. Theoretical models were able to describe the measurements [7]. Recent development in mixer technology makes two-dimensional imaging also possible for the ECE. 2D arrays of microwave antennas are used to detect the fluctuating structures in both, the radial and the poloidal direction. Using specially designed millimeterwave optics, each antenna can detect different radial positions in the plasma core along its specific poloidal sightline. According to the principle of the diagnostic, it is called ECE imaging. Two-dimensional measurements were successfully applied at TEXT-U [8]. The system was later installed at RTP [9]. Measurements of this kind are also planned for W7-AS. In the following an outline of the system planned for W7-AS and a characterisation of the system formerly used at RTP will be given.