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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 3 dB couplers it has become a reliable additional heating system. The maximum power coupled into the plasma has been raised up to 7.2 MW (90% of the installed RF power) for short pulses and up to 6.2 MW for pulses several second long with energy of up to 29 MJ. A power of 5 MW is delivered on a regular basis to replace two NBI sources.

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 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 ≈50 μs 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.

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.

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

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. Nuhrenberg 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-to...

Both in ohmically and beam-heated L-mode discharges of ASDEX, the electron-temperature (T/sub e/) profile shape can be varied over a wide range by the choice of the safety factor q/sub a/. The power-deposition profile, on the contrary, has no effect on the T/sub e/-profile shape. In current-free WVII-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.

Plasma build-up and heating of net-current-free plasmas in W VII-A was investigated by ECRH. Experiments were performed at two ECR-frequencies (28 and 70 GHz) and different heating scenarios were investigated such as first harmonic ordinary mode heating and second harmonic extraordinary mode heating. The basic effects predicted by theory, i.e. localized wave absorption and optical thickness of the plasma were verified. The electron heat conduction was found to be governed by neoclassical losses in the plasma core for high enough temperatures, whereas enhanced losses have to be assumed in the outer plasma regions. Generation of a target plasma with sufficient parameters to allow further heating by NBI was successfully demonstrated. Configuration studies showed a beneficial influence of small shear on the confinement, where internal currents have to be taken into account.

A high density current-free deuterium plasma withs(o) > 0,5 %, n e (o) > 10 14 cm -3 , T e , Ti > 350 eV can be maintained in W VII A stellarator by injection of 600 kW of H° at 27 kV. The plasma energy content rises linearly with power, and in first experiments with 1.1 MW of neutral power 3.5 kJ have been achieved. The plasma shows no major instabilities but the energy balance is dominated by radiation losses, which are attributed to an increase of impurity ions in the plasma center, predominantly oxygen. The observed heating efficiency in particular for the plasma ions, seems to be higher than predicted by the beam deposition code. Possible mechanisms for this are discussed. It appears that radial electric fields as low as kT e /a improve the computed efficiency as well as the ion heating considerably. Despite the uncertainty about actual input power, it can be shown that the plasma confinement improves during injection.

Recent experiments at ASDEX Upgrade have achieved advanced scenarios with high β N (>3) and confinement enhancement over ITER98(y, 2) scaling, H H98y2 = 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, β N H ITER89-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, q 95 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).