B. Nold

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.

Influence of temperature fluctuations on plasma turbulence investigations with Langmuir probes

The reliability of Langmuir probe measurements for plasma-turbulence investigations is studied on GEMR gyro-fluid simulations and compared with the results from conditionally sampled I?V characteristics as well as electron-emitting probe measurements close to the last closed flux surface of the tokamak ASDEX Upgrade. In this region, simulation and experiment consistently show coherent in-phase fluctuations in density, plasma potential and also electron temperature. Ion-saturation current measurements turn out to reproduce density fluctuations quite well. Fluctuations in the floating potential, however, are strongly influenced by temperature fluctuations and, hence, are strongly distorted compared to the actual plasma potential. These results suggest that interpreting floating as plasma-potential fluctuations while disregarding temperature effects is not justified near the separatrix of hot fusion plasmas. Here, floating potential measurements led to corrupted results on the E???B dynamics of turbulent structures in the context of, e.g., turbulent particle and momentum transport or turbulence characterization on the basis of density?potential phase relations.

Estimation of edge electron temperature profiles via forward modelling of the electron cyclotron radiation transport at ASDEX Upgrade

We present a method to obtain reliable edge profiles of the electron temperature by forward modelling of the electron cyclotron radiation transport. While for the core of ASDEX Upgrade plasmas, straightforward analysis of electron cyclotron intensity measurements based on the optically thick plasma approximation is usually justified, reasonable analysis of the steep and optically thin plasma edge needs to consider broadened emission and absorption profiles and radiation transport processes. This is carried out in the framework of integrated data analysis which applies Bayesian probability theory for joint analysis of the electron density and temperature with data of different interdependent and complementary diagnostics. By this means, electron cyclotron radiation intensity delivers highly spatially resolved electron temperature data for the plasma edge. In H-mode, the edge gradient of the electron temperature can be several times higher than the one of the radiation temperature. Furthermore, we are able to reproduce the ‘shine-through’ peak—the observation of increased radiation temperatures at frequencies resonant in the optically thin scrape-off layer. This phenomenon is caused by strongly down-shifted radiation of Maxwellian tail electrons located in the H-mode edge region and, therefore, contains valuable information about the electron temperature edge gradient.

Blob properties in L- and H-mode from gas-puff imaging in ASDEX Upgrade

Blob properties are studied in the scrape-off layer of the tokamak ASDEX Upgrade with a fast camera. The gas-puff imaging technique is used to investigate the detection rate as well as the blob size and velocity scaling. The experiments were performed in L- and H-mode phases of the same discharges to study the change in blob properties after the L-H transition. In both regimes the detection rate is of the order of a few thousand blobs per second, which is compatible with the picture of blob generation by edge micro instabilities. The blob size increases in H-mode, while the radial velocity decreases slightly. The changes are, however, not indicating a drastic change in the blob dynamics in both phases. The experimentally found blob properties were compared to predictions from a novel blob model including effects due to a finite ion temperature, which should be more appropriate for the conditions in the SOL of fusion plasmas.

The influence of plasma edge dynamics on blob properties in the stellarator TJ-K

Blob properties are studied in the scrape-off layer of the stellarator TJ-K. Langmuir probes and a fast camera are used to investigate the generation rate as well as the blob size and velocity scaling. Discharges with different ion species and magnetic field strengths provide access to a large range of plasma parameters. It was found that almost every large amplitude drift wave in the edge of the confined plasma triggers blob generation in the scrape-off layer, which implies that the birth rate of blobs is determined by the turbulence in the edge. Furthermore, the cross-field size of the blobs seems to correlate with the size of the generating drift waves. Since the observed radial propagation velocity of the blobs is well described by a size dependent blob velocity model, the size coupling between the drift waves and the blobs also has an impact on the blob velocities. Thus, the presented results imply that the dynamics in the edge have a large influence on the blob properties in the scrape-off layer.

Spatial structure of drift-wave turbulence and transport in a stellarator

The three-dimensional structure of drift-wave turbulence and turbulent transport is investigated in plasmas of the stellarator experiment TJ-K. By means of two poloidal Langmuir probe arrays placed at different toroidal positions, density and potential fluctuations are recorded simultaneously at 128 positions on a single flux surface. From these data, the spatial drift-wave turbulence pattern including perpendicular and parallel structure sizes are obtained using a cross-correlation technique. A comparison with the magnetic field structure indicates an initially perfect alignment of turbulent structures with magnetic field lines. Passing over regions with different field-line pitches according to the local variation of the rotational transform, however, results in a measured displacement of turbulent structures with respect to the field lines during their radial propagation. A reduction in the perpendicular correlation lengths in regions of high absolute values of local magnetic shear is found. Prominent and poloidally narrow turbulent transport maxima are measured at different toroidal positions. They are connected by the magnetic field lines and located in regions of negative normal curvature. The poloidal propagation pattern of turbulent structures and the exact position of the transport maximum depend on the magnetic field direction.