Scrape-off Layer Of ASDEX Upgrade Research Articles

Full-f electromagnetic gyrokinetic turbulence simulations of the edge and scrape-off layer of ASDEX Upgrade with GENE-X

Understanding and predicting turbulence in the edge and scrape-off layer (SOL) is critical for the optimization of magnetic confinement fusion devices. While there has been progress along these lines, especially with the help of fluid codes, the development of full-f electromagnetic gyrokinetic codes for the edge and SOL, in general, diverted geometries, remains crucial. In this work, we present simulations of the edge and SOL of the ASDEX Upgrade tokamak with the novel grid-based gyrokinetic (continuum) code GENE-X. The presented simulations are performed at both reduced and realistic electron-to-ion mass ratios on millisecond time scales, studying profile evolution. We compare the resulting plasma profiles to experimental measurements and to previous simulations with the Braginskii fluid code GRILLIX. Furthermore, we measure and validate the SOL power falloff length λq according to the Eich fit function. Based on the results, we discuss the influence of the ion-to-electron mass ratio and collisional effects on gyrokinetic SOL turbulence.

Numerical modelling of an enhanced perpendicular transport regime in the scrape-off layer of ASDEX Upgrade

A desirable scenario for future fusion devices is one in which dissipative processes in the scrape-off layer (SOL) are maximized, aiming to detach the divertor plasma. The access to such a regime in current devices is thought to be correlated to the increase of the perpendicular particle transport in the SOL. In this work we investigated numerically how increasing perpendicular transport globally affects the SOL plasma through the SOLPS-ITER code package. For this we modelled one L-mode discharge, performed at the ASDEX Upgrade tokamak, trying to obtain the most accurate fit to the experimental data at the outer midplane. Studying the plasma solutions and analyzing the resulting momentum and power balances in the SOL allowed to characterize how enhancing perpendicular SOL transport leads to the experimentally observed phenomena, i.e. the formation of a density shoulder at the midplane and the partial detachment of the divertor plasma. The results suggest that strong momentum losses caused by the increase of transport are able to explain the qualitatively observed detachment in the modelled discharge. The concurrent enhanced ionization of neutrals resulting from divertor recycling, triggered by an increase of radial energy transport in the SOL, can be invoked as a cause for the shoulder formation.

Measurement of N+ flows in the high-field side scrape-off layer of ASDEX upgrade with different degrees of inner divertor detachment

Toroidal and poloidal flows of injected N+ ions were measured in the high-field side (HFS) scrape-off layer (SOL) of ASDEX Upgrade by Doppler spectroscopy with different degrees of HFS divertor detachment. In high-recycling conditions, the results suggest reversed parallel N+ flow away from the inner divertor in the near SOL close to the separatrix, while the flow is towards the inner divertor throughout the SOL in detached conditions. The measured poloidal N+ flows were directed away from the HFS divertor in the near SOL for all density cases. Divertor plasma oscillations, characterized by momentary peaking of the HFS target ion flux and decrease of the HFS SOL density, were observed slightly before the roll-over of the ion saturation current to the HFS target and lead to an increase in the N+ flow towards the HFS divertor. SOLPS and ERO simulations of the experiment predict entrainment below 50% between the velocities of N+ and D+ ions, suggesting that N+ ions are quantitatively a limited proxy for measuring D+ flows. ERO simulations show significantly higher entrainment for higher ionization states, e.g., N2+ and N3+.

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-Alfvén 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.

Intermittent transport across the scrape-off layer: latest results from ASDEX Upgrade

We report the latest results of turbulence and transport studies in the ASDEX Upgrade scrape-off layer (SOL). Dissimilarity between the plasma and the floating potential fluctuations is studied experimentally and by gyrofluid simulations. Measurements by a retarding field analyser reveal that both, edge-localized mode (ELM) and turbulent filaments, convey hot ions over large radial distances in the SOL. The measured far SOL ELM ion temperature increases with the ELM energy, consistent with earlier observations that large ELMs deposit a large fraction of their energy outside the divertor. In the SOL, the ELM suppression by magnetic perturbations (MPs) results in lower ELM ion energy in the far SOL. At the same time, large filaments of ion saturation current are replaced by more continuous bursts. Splitting of the divertor strike zones observed by the infrared imaging in H-mode with MPs agree with predictions from the EMC3-Eirene simulations. This suggests that the ‘lobe’ structures due to perturbation fields observed near the X-point are not significantly affected by plasma screening, and can be described by a vacuum approach, as in the EMC3-Eirene. Finally, some effects of the MPs on the L-mode SOL are addressed.

Radial transport in the far scrape-off layer of ASDEX Upgrade during L-mode and ELMy H-mode

The radial turbulent particle flux and the Reynolds stress in the scrape-off layer (SOL) of ASDEX Upgrade were investigated for two limited L-mode (low confinement) and one ELMy H-mode (high confinement) discharge. A fast reciprocating probe was used with a probe head containing five Langmuir probes. One of the probe pins was protruding radially above the other pins. The radial electric field component was derived from the difference of the floating potentials of the protruding pin and another pin nearby. The poloidal electric field component was derived from the difference of the floating potentials of two probes on the same poloidal meridian. In both cases, equal electron temperatures on all probe pins had to be assumed. Of the other pins one was biased to ion saturation (−70 V), whereas one was swept to record the current–voltage characteristic. A detailed statistical analysis of the plasma conditions in L- and H-modes and, more specifically, also during and in between type-I edge-localized mode (ELM) events is presented. We found evidence for similarities in the statistics of transport in L-mode, during ELM events and in between ELMs, suggesting that in all three cases the transport is undertaken by blobs and filamentary structures, respectively. During the intervals in between ELMs the blob structures appear to be relatively smaller as compared with the L-mode.

Measurements of the parallel ion velocity distribution at the plasma–sheath interface

The ion velocity distribution parallel to magnetic field lines, f is extracted from retarding field analyzer (RFA) measurements in the ASDEX Upgrade scrape-off layer. The RFA ion current–voltage characteristic is transformed into a system of linear equations, from which f is unfolded using a standard regularization method. The algorithm is validated on numerically generated data. The experimentally measured f compares favorably with the ion velocity distributions calculated from a quasi-neutral kinetic simulation of the plasma pre-sheath. The technique can be used to measure f by RFAs in plasma processing, high-energy particle accelerators or by satellite-born RFAs.

Ion temperature fluctuations in the ASDEX Upgrade scrape-off layer

The ion temperature in turbulent plasma filaments Ti fil is measured by a retarding field analyser during L-mode discharges in the ASDEX Upgrade tokamak. At 2 cm outside the separatrix Ti fil ≈ 80–110 eV, which is 3–4 times that of the background ions and 50–70% of the ion temperature at the separatrix. Ti fil is reproduced by a fluid model of the parallel filament transport assuming the radial filament propagation speed of 400–1000 m s−1 in the near scrape-off layer, consistent with earlier experimental estimates. The conditional sampling used in experiment to measure Ti fil is tested on artificial time series obtained from a gyrofluid turbulence simulation.

Latest investigations on fluctuations, ELM filaments and turbulent transport in the SOL of ASDEX Upgrade

This paper presents turbulence investigations in the scrape-off layer (SOL) of ASDEX Upgrade in ohmic, L-mode and H-mode discharges using electrostatic and electromagnetic probes. Detailed studies are performed on small scale turbulence and on ELM filaments. Simultaneous measurements of floating and plasma potential fluctuations revealed significant differences between these quantities. Large errors can occur when the electric field is extracted from floating potential measurements, even in ohmic discharges. Turbulence studies in ohmic plasmas show the existence of density holes inside the separatrix and blobs outside. Close to the separatrix a reversal of the poloidal blob propagation velocity occurs. Investigations on the Reynolds stress in the scrape-off layer (SOL) show its importance for the momentum transport in L-mode while its impact for momentum transport during ELMs in H-mode is rather small. In the far SOL the electron density and temperature were measured during type-I ELMy H-mode at ASDEX Upgrade resolving ELM filaments. Strong density peaks and temperatures of several 10 eV were detected during the ELM events. Additional investigations on the ions in ELM filaments by a retarding field analyser indicate ion temperatures of 50–80 eV. ELMs also expel current concentrated in filaments into the SOL. Furthermore, discharges with small ELMs were studied. In N2 seeded discharges the type-I ELM frequency rises and the ELM duration decreases. For discharges with small type-II ELMs the mean turbulent radial particle flux is increased over the mean particle flux in type-I ELM discharges at otherwise similar plasma parameters.

Effect of E × B driven transport on the deposition of carbon in the outer divertor of ASDEX Upgrade

Reversal of the toroidal magnetic field and plasma current is observed to considerably change the re-deposition of 13C injected into the outer divertor scrape-off layer of ASDEX Upgrade. In forward field low-density L-mode plasmas, 24–32% of injected carbon is found locally re-deposited. The deposition tails are aligned toroidally both upstream and downstream of the exit holes and indicate transport towards the strike point. In reversed field with similar main plasma parameters, the re-deposition efficiency is a factor of 2 smaller. The deposition is more localized and shows transport towards the outer scrape-off layer. Numerical modelling with the SOLPS5.0 and ERO codes shows that these differences in 13C deposition can be attributed to the combination of the E×B drift reversal directly influencing the transport of carbon and changes in local plasma conditions due to the drift reversal.

Deuterium plasma flow in the scrape-off layer of ASDEX Upgrade

Measurements of the plasma flow parallel to the magnetic field in the scrape-off layer (SOL) of ASDEX Upgrade were performed using Mach probes. The parallel flow was studied at various densities in L- and H-mode discharges with lower and upper single null configuration applying normal and reversed toroidal field. Two reciprocating probes allow for flow measurements in the outer midplane and in front of the lower divertor, slightly below the X-point, covering the inner and outer divertor leg. Strong parallel flows were observed with Mach numbers of up to 0.6 in the outer midplane and even larger than 1.0 above the divertor. Such a flow can connect the particle loss of the confined plasma predominantly happening on the torus outside to the strong recycling observed at the torus inside. The plasma flow varies with plasma density and ion drift direction.

Modeling of tungsten transport in the SOL for sources at the central column of ASDEX Upgrade using DIVIMP

Tungsten erosion at the central column and tungsten transport in the scrape-off layer of ASDEX Upgrade have been modeled with the 2D Monte-Carlo impurity transport code DIVIMP. The main objectives were the reproduction of the erosion and migration patterns that are observed experimentally. This was accomplished by implementing a new erosion routine which allows to calculate the tungsten source term at the central column and subsequently the poloidal distribution of the W density. First results on W transport in the boundary plasma as well as erosion and migration patterns of W are discussed in relation to different models of the plasma in front of the central column. A comparison between measured and modeled deposition of W in the divertor already shows satisfactory qualitative agreement.

Computational Modelling of a Purely Neoclassical Scrape-Off Layer

We study the Scrape-off Layer of ASDEX Upgrade in a scenario where the neoclassical transport phenomena are dominant with respect to the anomalous contributions. To represent such a neoclassical scenario, we implemented the expressions for the energy and momentum radial transport, toroidicity effects being introduced automatically by the tokamak geometry. Correspondingly, we reduced the anomalous transport coefficients, and accounted partially for drift effects. An extremely large difference between the two divertor plates was found, especially regarding the electron energy flux and temperature profiles.

2D modelling of the ASDEX-Upgrade scrape-off layer and divertor plasma

Due to the open field lines, the scrape-off layer and divertor region of tokamak plasmas is a complex, two-dimensional system, involving transport parallel and perpendicular to the magnetic field, as well as interaction of the plasma with surfaces and with the neutral gas. Therefore sophisticated two-dimensional codes are required to model the divertor and edge physics. In this paper, the B2-EIRENE code package is used to simulate the ASDEX-Upgrade scrape-off layer plasma and the neutral gas dynamics in a fully self-consistent way. Specific ASDEX-Upgrade discharges are modelled using the actual magnetic configuration and in-vessel components. Single fluid as well as multifluid calculations including self-consistent target and wall erosion of carbon are described. At given input power and bulk plasma radiation, typical divertor plasma profiles from Langmuir probes are fitted by varying the separatrix density and the transport coefficients. On the basis of such multifluid fits, spectroscopic divertor diagnostics are numerically modelled and compared with measured profiles, and reasonable agreement is found.

B2-EIRENE simulation of ASDEX and ASDEX-Upgrade scrape-off layer plasmas

The 2D multifluid edge code B2 coupled with the 3D neutral gas Monte Carlo code EIRENE is being used for edge interpretation and model validation on the axisymmetric poloidal divertor experiments ASDEX and ASDEX-Upgrade. For this purpose B2 was significantly improved especially by a fully implicit treatment of the topological cuts appearing in X-point configurations, and a reasonably accurate handling of inclined target plates. A fast, automatic grid generator has been developed, which allows direct implementation of experimental MHD equilibria into B2-EIRENE. Typical ASDEX and ASDEX-Upgrade simulations are presented and discussed.