T. Happel

Generation of blobs and holes in the edge of the ASDEX Upgrade tokamak

The intermittent character of turbulent transport is investigated with Langmuir probes in the scrape-off layer and across the separatrix of ASDEX Upgrade Ohmic discharges. Radial profiles of plasma parameters are in reasonable agreement with results from other diagnostics. The probability density functions of ion-saturation current fluctuations exhibit a parabolic relation between skewness and kurtosis. Intermittent blobs and holes are observed outside and inside the nominal separatrix, respectively. They seem to be born at the edge of the plasma and are not the foothills of avalanches launched in the plasma core. A strong shear flow was observed 1 cm radially outside the location where blobs and holes seem to be generated.

Core turbulence behavior moving from ion-temperature-gradient regime towards trapped-electron-mode regime in the ASDEX Upgrade tokamak and comparison with gyrokinetic simulation

Additional electron cyclotron resonance heating (ECRH) is used in an ion-temperature-gradient instability dominated regime to increase R/LTe in order to approach the trapped-electron-mode instability regime. The radial ECRH deposition location determines to a large degree the effect on R/LTe. Accompanying scale-selective turbulence measurements at perpendicular wavenumbers between k⊥ = 4–18 cm−1 (k⊥ρs = 0.7–4.2) show a pronounced increase of large-scale density fluctuations close to the ECRH radial deposition location at mid-radius, along with a reduction in phase velocity of large-scale density fluctuations. Measurements are compared with results from linear and non-linear flux-matched gyrokinetic (GK) simulations with the gyrokinetic code GENE. Linear GK simulations show a reduction of phase velocity, indicating a pronounced change in the character of the dominant instability. Comparing measurement and non-linear GK simulation, as a central result, agreement is obtained in the shape of radial turbulence...

Investigation of the parallel dynamics of drift-wave turbulence in toroidal plasmas

The three-dimensional structure of drift-wave turbulence is studied in the core of the toroidal low-temperature plasma in the torsatron TJ-K. The results are compared with simulations from the GEM3 turbulence code. In experiment and simulation, the dimensionless parameters are similar to those of fusion edge plasmas. Arrays with 64 probes are used to measure parallel wavenumbers, propagation velocities and the tilt of the turbulent structures with respect to the field line. A parallel wavelength of 15 m and a parallel velocity in between the ion-sound and the Alfven velocity confirm the three-dimensional nature of drift-wave turbulence. Quantitative agreement between experiment and simulation is found. The comparison with the drift-wave dispersion relation gives evidence for the coupling of the density perturbation to the shear-Alfven wave.

Scale-selective turbulence reduction in H-mode plasmas in the TJ-II stellarator

Wavenumber spectra of density turbulence in L- and H-mode plasmas have been measured in the TJ-II stellarator by means of Doppler reflectometry. A pronounced suppression of the density fluctuation level is observed in H-mode close to the radial position of maximum radial electric field (Er) shear. Furthermore, intermediate scale density turbulence is reduced preferentially. This effect can be interpreted within the framework of vortex stretching feeding energy through Reynolds stress into zonal flows, while shear decorrelation of turbulent structures might not play a central role in TJ-II. Moreover, it is shown that in both L- and H-mode, the phase velocity of density fluctuations does not depend on the structure scale.

Gyrokinetic studies of core turbulence features in ASDEX Upgrade H-mode plasmas

Gyrokinetic validation studies are crucial for developing confidence in the model incorporated in numerical simulations and thus improving their predictive capabilities. As one step in this direction, we simulate an ASDEX Upgrade discharge with the GENE code, and analyze various fluctuating quantities and compare them to experimental measurements. The approach taken is the following. First, linear simulations are performed in order to determine the turbulence regime. Second, the heat fluxes in nonlinear simulations are matched to experimental fluxes by varying the logarithmic ion temperature gradient within the expected experimental error bars. Finally, the dependence of various quantities with respect to the ion temperature gradient is analyzed in detail. It is found that density and temperature fluctuations can vary significantly with small changes in this parameter, thus making comparisons with experiments very sensitive to uncertainties in the experimental profiles. However, cross-phases are more robust, indicating that they are better observables for comparisons between gyrokinetic simulations and experimental measurements.

Experimental turbulence studies for gyro-kinetic code validation using advanced microwave diagnostics

For a comprehensive comparison with theoretical models and advanced numerical turbulence simulations, a large spectrum of fluctuation parameters was measured on the devices ASDEX Upgrade, TCV, and Tore-Supra. Radial profiles of scale-resolved turbulence levels in H-mode discharges are measured and compared with GENE simulations in the transition range from ion-temperature-gradient to trapped-electron-mode turbulence. Correlation reflectometry is used to study the microscopic structure of turbulence and GAMs in discharges where poloidal flow damping was varied by means of variations of the shape of the poloidal plasma cross-section and collisionality. Full-wave codes and synthetic diagnostics are applied for the interpretation of the data.

Applications of large eddy simulation methods to gyrokinetic turbulence

The large eddy simulation (LES) approach—solving numerically the large scales of a turbulent system and accounting for the small-scale influence through a model—is applied to nonlinear gyrokinetic systems that are driven by a number of different microinstabilities. Comparisons between modeled, lower resolution, and higher resolution simulations are performed for an experimental measurable quantity, the electron density fluctuation spectrum. Moreover, the validation and applicability of LES is demonstrated through a series of diagnostics based on the free energetics of the system.

Spatial, temporal and spectral structure of the turbulence–flow interaction at the L–H transition

The physical mechanisms behind the L-H transition have been experimentally studied in the TJ-II plasmas. The spatial, temporal and spectral structure of the interaction between turbulence and flows has been studied close to the L-H transition threshold conditions. The temporal dynamics of the turbulence-flow interaction displays a predator-prey relationship and both, radial outward and inward propagation velocities of the turbulence-flow front have been measured. Moreover, the turbulence scales involved in the energy transfer of the predator-prey process have been identified.

Multi-device studies of pedestal physics and confinement in the I-mode regime

This paper describes joint ITPA studies of the I-mode regime, which features an edge thermal barrier together with L-mode-like particle and impurity transport and no edge localized modes (ELMs). The regime has been demonstrated on the Alcator C-Mod, ASDEX Upgrade and DIII-D tokamaks, over a wide range of device parameters and pedestal conditions. Dimensionless parameters at the pedestal show overlap across devices and extend to low collisionality. When they are matched, pedestal temperature profiles are also similar. Pedestals are stable to peeling–ballooning modes, consistent with lack of ELMs. Access to I-mode is independent of heating method (neutral beam injection, ion cyclotron and/or electron cyclotron resonance heating). Normalized energy confinement H 98,y2 ≥ 1 has been achieved for a range of 3 ≤ q 95 ≤ 4.9 and scales favourably with power. Changes in turbulence in the pedestal region accompany the transition from L-mode to I-mode. The L–I threshold increases with plasma density and current, and with device size, but has a weak dependence on toroidal magnetic field B T. The upper limit of power for I-modes, which is set by I–H transitions, increases with B T and the power range is largest on Alcator C-Mod at B > 5 T. Issues for extrapolation to ITER and other future fusion devices are discussed.

Geodesic oscillations and the weakly coherent mode in the I-mode of ASDEX Upgrade

Density fluctuations in I-mode discharges in ASDEX Upgrade are studied. The I-mode specific weakly coherent mode (WCM) appears at the transition from the L to I-mode. The WCM but also the turbulence in general are strongly modulated by a low frequency mode which can be related to the geodesic acoustic mode (GAM). The GAM induces an energy transfer away from the central WCM frequency, indicating an underlying instability responsible for the WCM. During the I-mode magnetic fluctuations close to the WCM frequency are intensified, which can be assigned to the geodesic Alfvenic oscillation. The geodesic Alfvenic oscillation is already present in the L-mode, and does not follow changes of frequency of the WCM, therefore it is not responsible for the WCM.