Significant progress has been made on ASDEX Upgrade during the last two years in the basic understanding of transport, in the extension of the improved H-mode in parameter space and towards an integrated operating scenario and in the development of control methods for major performance limiting instabilities. The important features were the understanding of particle transport and the control of impurity accumulation based on it, the satisfactory operation with predominantly tungsten-clad walls, the improved H-mode operation over density ranges and for temperature ratios covering (non-simultaneously) the ITER requirements on ν*, n/nGW and Te/Ti, the ELM frequency control by pellet injection and the optimization of NTM suppression by DC-ECCD through variation of the launching angle. From these experiments an integrated scenario has emerged which extrapolates to a 50% improvement in n T τ or a 30% reduction of the required current when compared with the ITER base-line assumptions, with moderately peaked electron and controllable high-Z density profiles.

The ICRF system at the ASDEX Upgrade tokamak is in operation since May 1992. Following some modifications of which the major one was the installation of 3dB couplers it has become a reliable additional heating system. The maximum power coupled into the plasma has been raised up to 7.2MW (90% of the installed RF power) for short pulses and up to 6.2MW for pulses several second long with energy of up to 29MJ. A power of 5MW is delivered on a regular basis to replace two NBI sources.

The coupling characteristics of the ion cyclotron range of frequencies antenna during type I edge localized modes (ELMs), asymmetries in the reaction of toroidally distributed antennas to ELMs and arcing phenomena following ELMs are studied for ASDEX Upgrade. The maximum change of the antenna resistance due to ELMs increases with triangularity δL in the on-axis minority heating. In the cases with off-axis heating, the change of the resistance is less sensitive to δL. Time-resolved RF measurements show asymmetries in times of response of the toroidally distributed antennas to the rising edge of ELMs and in the further evolution of this response. Toroidal propagation of ELM perturbations in counter-current direction with times ≈50μs for one toroidal turn is observed. ELM-induced arcing is observed during ELMs and with delays after ELMs. Conditioning takes place as the arc number during ELMs decreases and time delay between ELM and arc increases.

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.

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.

Recent experiments at ASDEX Upgrade have achieved advanced scenarios with high βN (>3) and confinement enhancement over ITER98(y, 2) scaling, HH98y2 = 1.1–1.5, in steady state. These discharges have been obtained in a modified divertor configuration for ASDEX Upgrade, allowing operation at higher triangularity, and with a changed neutral beam injection (NBI) system, for a more tangential, off-axis beam deposition. The figure of merit, βNHITER89-P, reaches up to 7.5 for several seconds in plasmas approaching stationary conditions. These advanced tokamak discharges have low magnetic shear in the centre, with q on-axis near 1, and edge safety factor, q95 in the range 3.3–4.5. This q-profile is sustained by the bootstrap current, NBI-driven current and fishbone activity in the core. The off-axis heating leads to a strong peaking of the density profile and impurity accumulation in the core. This can be avoided by adding some central heating from ion cyclotron resonance heating or electron cyclotron resonance heating, since the temperature profiles are stiff in this advanced scenario (no internal transport barrier). Using a combination of NBI and gas fuelling line, average densities up to 80–90% of the Greenwald density are achieved, maintaining good confinement. The best integrated results in terms of confinement, stability and ability to operate at high density are obtained in highly shaped configurations, near double null, with δ = 0.43. At the highest densities, a strong reduction of the edge localized mode activity similar to type II activity is observed, providing a steady power load on the divertor, in the range of 6 MW m−2, despite the high input power used (>10 MW).

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.

Radially peaked density profiles were found in discharges heated purely with ECH in the currentless W7-AS stellarator. Changes in the central particle source are not significant and do not explain the density peaking. This is evidence for inward convection in W7-AS. The existence of the inward convection is confirmed with two independent transient transport studies. Flatness of the temperature profiles is essential for the density peaking, and an anti-correlation was found between the density and temperature gradients. The density peaks as the ECH-deposition is moved outwards. The density peaking parameter was found to decrease with increasing heating power. No clear density dependence was found. Strong gas puffing, or the corresponding high edge density prevents the formation of the peaked profiles.

A newly designed ten channel microwave interferometer for line integrated electron density measurements at the experiment W7-AS stellarator is described. The probing signals in the frequency range of 160–162 GHz are generated by Gunn oscillators with subsequent frequency doublers. Double conversion heterodyne receivers with 2 MHz bandwidth are used for detection. All oscillators involved are phase locked. Use of each Gunn oscillator is twofold: as a source for the probing signal and as a local oscillator for heterodyne detection. The dynamic range of the millimeter wave part is about 70 dB. The instrument operates up to a peak density of 7×1019 m−3 with a temporal resolution of 5 μs. This article gives a detailed technical description of the system and demonstrates its usefulness by means of an experimental example.

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