M. Ramisch

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.

ρs scaling of characteristic turbulent structures in the torsatron TJ-K

The scalings of correlation lengths and times with the drift-scale parameter ρs and other dimensionless parameters are investigated. Using a novel 8×8 Langmuir probe matrix, the two-dimensional structure of drift-wave turbulence has been measured inside the confinement region of the toroidal low-temperature plasma in the TJ-K torsatron [N. Krause et al., Rev. Sci. Instrum. 73, 3474 (2002)]. Using five different gases from hydrogen to argon, ρs could be varied by a factor of 10. For small ion masses, the scalings of the microscopic parameters turn out to be close to the predictions from drift-wave turbulence, which lead to a gyro-Bohm scaling of the diffusivity. Including heavier ions turns the scaling to more Bohm-like. It is shown that ρs scaling studies carried out on the diffusivity can be misleading if residual dependencies on other parameters are present and the cross phase between poloidal electric field and density fluctuations is not constant as in the present case. The measured turbulent diffusiv...

On the interaction of turbulence and flows in toroidal plasmas

In toroidally confined plasmas, background E × B flows, microturbulence and zonal flows constitute a tightly coupled dynamic system and the description of confinement transitions needs a self-consistent treatment of these players. The background radial electric field, linked to neoclassical ambipolar transport, has an impact on the interaction between zonal flows and turbulence by tilting and anisotropization of turbulent eddies. Zonal-flow drive is shown to be non-local in wavenumber space and is described as a straining-out process instead as a local inverse cascade. The straining-out process is also discussed as an option to explain turbulence suppression in sheared flows and could be the cause of predator–prey oscillations in the turbulence zonal-flow system.

Gyrokinetic turbulent transport simulation of a high ion temperature plasma in large helical device experiment

Ion temperature gradient turbulent transport in the large helical device (LHD) is investigated by means of gyrokinetic simulations in comparison with the experimental density fluctuation measurements of ion-scale turbulence. The local gyrokinetic Vlasov simulations are carried out incorporating full geometrical effects of the LHD configuration, and reproduce the turbulent transport levels comparable to the experimental results. Reasonable agreements are also found in the poloidal wavenumber spectra of the density fluctuations obtained from the simulation and the experiment. Numerical analysis of the spectra of the turbulent potential fluctuations on the two-dimensional wavenumber space perpendicular to the magnetic field clarifies the spectral transfer into a high radial wavenumber region which correlates with the regulation of the turbulent transport due to the zonal flows. The resultant transport levels at different flux surfaces are expressed in terms of a simple linear relation between the transport c...

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.

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.

Poloidal mode structure of long-distance correlation of fluctuations under strong E×B shear in the torsatron TJ-K

Long-distance correlations of density and potential fluctuations are investigated on the poloidal circumference of flux surface in the TJ-K plasmas. Shear flow as induced by biasing causes a strong increase in the long-distance correlation of potential and density fluctuations. In the potential fluctuations, zonal-flow-like modes are excited. The density long-distance correlation is dominated by an m=3 mode. As a new feature a coherency analysis reveals also the existence of m=0 density fluctuations. As an explanation, background profile fluctuations or a turbulent zonal density drive are proposed.

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.

Experimental estimation of the dual cascade in two-dimensional drift-wave turbulence

The dual turbulent cascade is studied experimentally in two-dimensional wavenumber space. A two-field model is used for the analysis and tested on simulated data first. It is found that the energy of the turbulent fluctuations follows an inverse cascade to larger scales while enstrophy is transported in a direct cascade to smaller scales. This provides experimental evidence for the turbulent dual cascade in magnetized plasmas. The analysis also reveals the importance of non-local transfer to the cascading process.