The medium size divertor tokamak ASDEX Upgrade (major and minor radii 1.65 m and 0.5 m, respectively, magnetic-field strength 2.5 T) possesses flexible shaping and versatile heating and current drive systems. Recently the technical capabilities were extended by increasing the electron cyclotron resonance heating (ECRH) power, by installing 2 x 8 internal magnetic perturbation coils, and by improving the ion cyclotron range of frequency compatibility with the tungsten wall. With the perturbation coils, reliable suppression of large type-I edge localized modes (ELMs) could be demonstrated in a wide operational window, which opens up above a critical plasma pedestal density. The pellet fuelling efficiency was observed to increase which gives access to H-mode discharges with peaked density profiles at line densities clearly exceeding the empirical Greenwald limit. Owing to the increased ECRH power of 4 MW, H-mode discharges could be studied in regimes with dominant electron heating and low plasma rotation velocities, i.e. under conditions particularly relevant for ITER. The ion-pressure gradient and the neoclassical radial electric field emerge as key parameters for the transition. Using the total simultaneously available heating power of 23 MW, high performance discharges have been carried out where feed-back controlled radiative cooling in the core and the divertor allowed the divertor peak power loads to be maintained below 5 MW m(-2). Under attached divertor conditions, a multi-device scaling expression for the power-decay length was obtained which is independent of major radius and decreases with magnetic field resulting in a decay length of 1 mm for ITER. At higher densities and under partially detached conditions, however, a broadening of the decay length is observed. In discharges with density ramps up to the density limit, the divertor plasma shows a complex behaviour with a localized high-density region in the inner divertor before the outer divertor detaches. Turbulent transport is studied in the core and the scrape-off layer (SOL). Discharges over a wide parameter range exhibit a close link between core momentum and density transport. Consistent with gyro-kinetic calculations, the density gradient at half plasma radius determines the momentum transport through residual stress and thus the central toroidal rotation. In the SOL a close comparison of probe data with a gyro-fluid code showed excellent agreement and points to the dominance of drift waves. Intermittent structures from ELMs and from turbulence are shown to have high ion temperatures even at large distances outside the separatrix.

/pdf/overview-of-asdex-upgrade-results-3vw1cm6jwx.pdf

Overview of ASDEX Upgrade results

Ion Bernstein wave heating (IBWH) utilizes the ion Bernstein wave (IBW), a hot plasma wave, to carry the radio frequency (rf) power to heat the tokamak reactor core. Earlier wave accessibility studies have shown that this finite‐Larmor‐radius (FLR) mode should penetrate into a hot dense reactor plasma core without significant attenuation. Moreover, the IBW’s low perpendicular phase velocity (ω/k⊥≊VTi≪Vα) greatly reduces the otherwise serious wave absorption by the 3.5 MeV fusion α particles. In addition, the property of IBW’s that k⊥ρi≊1 makes localized bulk ion heating possible at the ion cyclotron harmonic layers. Such bulk ion heating can prove useful in optimizing fusion reactivity.In another vein, with proper selection of parameters, IBW’s can be made subject to strong localized electron Landau damping near the major ion cyclotron harmonic resonance layers. This property can be useful, for example, for rf current drive in the reactor plasma core. IBW’s can be excited with loop antennas or with a lowe...

/pdf/ion-bernstein-wave-heating-research-4rvnqub6sa.pdf

Ion Bernstein wave heating research

The interaction between the ion cyclotron resonance heating and the plasma edge is a vast and interlinked domain: the edge affects the radio frequency heating and the radio frequency heating affects the edge. This interaction between the radio frequency heating and the edge is-in view of the relation between the edge and overall plasma properties and performance-of utmost importance. The paper gives an overview of the topic, emphasizing basic concepts and recent results. The identification of the sheath rectification mechanism as the main cause for the influence of the RF on the edge, has permitted a better understanding of this domain and significant progress in the avoidance of the effects.

https://iopscience.iop.org/article/10.1088/0741-3335/35/11/001/pdf

The interaction between waves in the ion cyclotron range of frequencies and the plasma boundary

High-power millimeter wave sources are a key enabling technology in fusion energy research. The present state of the art of application of these sources to the areas of heating, current generation, and scattering for diagnostic purposes in fusion plasmas is reviewed. The extrapolation of these applications to future devices and the requirements which they place on sources and transmission lines are also discussed.

Applications of high-power millimeter waves in fusion energy research

Nonlinear interactions of ion cyclotron range of frequency (ICRF) waves with fusion plasmas are reviewed. Although the linear theory of ICRF waves, including fast waves (FWs), high-harmonic fast waves and ion Bernstein waves (IBWs), is widely applicable, nonlinear effects can still be important, especially in the edge plasma or for novel core applications. Here the topics of flow drive, ponderomotive forces, radiofrequency (rf) sheaths, parametric decay and related interactions with the edge plasma are considered. Primary emphasis is placed on the basic underlying physics and tokamak applications. For FW antennas, the parallel electric field near launching structures is known to drive rf sheaths which can give rise to convective cells, interaction with plasma 'blobs', impurity production and edge power dissipation. In addition to sheaths, IBW waves in the edge plasma are subject to strong ponderomotive effects and parametric decay. In the core plasma, slow waves can sometimes induce nonlinear effects. Mechanisms by which these waves can influence the radial electric field and its shear are summarized and related to the general (reactive-ponderomotive and dissipative) force on a plasma from rf waves. Standard ICRF codes have begun to incorporate the nonlinear topics described here. Further progress in integrated simulation should allow new predictive modelling capabilities.

/pdf/nonlinear-icrf-plasma-interactions-4mjjju6omh.pdf

Nonlinear ICRF-plasma interactions

Two types of parametric decay instabilities were observed in the scrape-off layer of ASDEX during hydrogen second harmonic heating, in a single as well as in a two ion species plasma. The first type was identified as decay into an ion Bernstein wave and an ion cyclotron quasi-mode, and the second as decay into an ion Bernstein wave and a low frequency electron quasi-mode. The parametric decay processes are due to the high electric fields near the fast wave antennas. Theoretical growth rate calculations predict a threshold for the electric fields near the antennas. In the relevant experiments the electric fields, estimated from a full-wave code, indeed exceeded the thresholds. These parametric decay processes, as well as the harmonics of the generator frequency also seen in the scrape-off layer, provide a mechanism that might contribute to the observed direct energy deposition in the edge plasma and to ICRF induced impurity production.

/pdf/parametric-decay-in-the-edge-plasma-of-asdex-during-fast-41u2tei1im.pdf

Parametric decay in the edge plasma of ASDEX during fast wave heating in the ion cyclotron frequency range

In ASDEX Upgrade ECRH and ECCD with a power of up to 2 MW has been used. With counter ECCD in discharges with an internal transport barrier we achieved an electron temperature of ≈13 keV. In low density ohmic plasmas we obtained an rf driven current of ≈80% both in co- and counter directions. Neoclassical tearing modes have been completly stabilised by driving a current on the resonant q-surface. Electron heat transport has been studied in standard L- and H-mode plasmas and can be described by a dependence on a critical electron temperature gradient.

ECRH Experiments in ASDEX Upgrade

Status and Planning for ICRF at ASDEX Upgrade

Variation of the Sawtooth Activity with ICRF in ASDEX Upgrade

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Parametric decay in the edge plasma of ASDEX during fast wave heating in the ion cyclotron frequency range

ECRH Experiments in ASDEX Upgrade

Status and Planning for ICRF at ASDEX Upgrade

Variation of the Sawtooth Activity with ICRF in ASDEX Upgrade