G. Birkenmeier

An experimental investigation of the high density transition of the scrape-off layer transport in ASDEX Upgrade

A multidiagnostic approach, utilizing Langmuir probes in the midplane, X-point and divertor walls, along with lithium beam and infrared measurements is employed to evaluate the evolution of the scrape-off layer (SOL) of ASDEX Upgrade across the L-mode density transition leading to the formation of a density shoulder. The flattening of the SOL density profiles is linked to a regime change of filaments, which become faster and larger, and to a similar flattening of the q∥ profile. This transition is related to the beginning of outer divertor detachment and leads to the onset of a velocity shear layer in the SOL. Experimental measurements are in good agreement with several filament models which describe the process as a transition from conduction to convection-dominated SOL perpendicular transport caused by an increase of parallel collisionality. These results could be of great relevance since both ITER and DEMO will feature detached divertors and densities largely over the transition values, and might therefore exhibit convective transport levels different to those observed typically in present-day devices.

Experimental Validation of a Filament Transport Model in Turbulent Magnetized Plasmas

In a wide variety of natural and laboratory magnetized plasmas, filaments appear as a result of interchange instability. These convective structures substantially enhance transport in the direction perpendicular to the magnetic field. According to filament models, their propagation may follow different regimes depending on the parallel closure of charge conservation. This is of paramount importance in magnetic fusion plasmas, as high collisionality in the scrape-off layer may trigger a regime transition leading to strongly enhanced perpendicular particle fluxes. This work reports for the first time on an experimental verification of this process, linking enhanced transport with a regime transition as predicted by models. Based on these results, a novel scaling for global perpendicular particle transport in reactor relevant tokamaks such as ASDEX-Upgrade and JET is found, leading to important implications for next generation fusion devices.

Filament velocity scaling laws for warm ions

The dynamics of filaments or blobs in the scrape-off layer of magnetic fusion devices are studied by magnitude estimates of a comprehensive drift-interchange-Alfven fluid model. The standard blob models are reproduced in the cold ion case. Even though usually neglected, in the scrape-off layer, the ion temperature can exceed the electron temperature by an order of magnitude. The ion pressure affects the dynamics of filaments amongst others by adding up to the interchange drive and the polarisation current. It is shown how both effects modify the scaling laws for filament velocity in dependence of its size. Simplifications for experimentally relevant limit regimes are given. These are the sheath dissipation, collisional, and electromagnetic regime.

Characterization of the Li-BES at ASDEX Upgrade

The lithium beam emission spectroscopy (Li-BES) is a powerful diagnostic to resolve the plasma edge density with high temporal and spatial resolution. The recent upgrades of the Li-BES at ASDEX Upgrade and the resulting gain in photon flux allow the plasma edge density to be determined with an advanced level of accuracy. Furthermore, electron density fluctuations are measured using Li-BES. The Li-BES capabilities and limitations to measure electron density profiles as well as density fluctuations are presented. It is well suited to characterize electron density turbulence in the scrape off layer (SOL) with decreasing sensitivity towards the plasma core. This is demonstrated by simulations as well as by comparisons with other diagnostics. The Li-BES is an appropriate tool to study transport phenomena in the SOL over a wide range of plasma parameters due to its robustness and routine usage.

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.

Magnetic field dependence of the blob dynamics in the edge of ASDEX upgrade L-mode plasmas

The magnetic field dependence of intermittently expelled density filaments (blobs) is investigated in the scrape-off layer of ASDEX Upgrade low confinement (L-mode) plasmas. It is demonstrated that lithium beam emission spectroscopy can be used to determine the frequency, radial size and velocity of the blobs. The measured radial blob sizes depend only weakly on magnetic field B. Normalizing the blob sizes to the drift parameter ρs ∝ B−1 results in a large variation beneficial for a quantitative comparison with theoretical blob scaling laws. The blob velocity scales inversely proportional to the square of the blob size in agreement with analytic models for blobs in the sheath-connected regime. The measurements point towards an influence of finite ion temperature on radial blob transport.

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.

Velocimetry analysis of type-I edge localized mode precursors in ASDEX Upgrade

When the electron transport barrier remains in its final shape before a type-I edge localized mode (ELM) crash in ASDEX Upgrade, ELM precursors appear as electron temperature fluctuations. In order to relate these precursors to an instability, spatial scales, parity and the cross-phase between electron temperature and radial velocity fluctuations are evaluated by means of velocimetry of measured 2D electron temperature fluctuations. A comprehensive comparison with properties of different instabilities points to microtearing modes. Bispectral analysis indicates a nonlinear coupling of these precursors to a ballooning-type mode prior to the ELM onset.

Filament transport, warm ions and erosion in ASDEX Upgrade L-modes

The dynamics of blob filaments are investigated in the scrape-off layer of ASDEX Upgrade by means of lithium beam emission spectroscopy. A comparison of the measurements in L-mode with a recently developed analytical blob model based on a drift-interchange-Alfven fluid model indicates an influence of a finite ion temperature on the blob dynamics which has typically been neglected in other blob models. The blob dynamics agree well with the sheath-connected regime at lower plasma densities, and inertial effects play only a minor role. At higher densities, a transition into another regime with large blob amplitudes and increased transport is found. This points to a prominent role of blob transport at higher Greenwald fractions. On the basis of the measured blob properties, the erosion on plasma facing components is estimated. For pure deuterium plasmas, the high ion temperatures of blobs lead to a dominant erosion induced by blobs. However, if an impurity concentration of 1% is taken into account, the blob-induced erosion plays a minor role and background plasma parameters determine the total gross erosion.