[en] In this article we present experimental investigations of divertor detachment in ASDEX Upgrade. For this purpose we performed a series of ohmic and L-Mode density ramp discharges. The electron density in the divertor volume is determined with a spectroscopic measurement of the Stark broadened D_ϵ line shape. These measurements are combined with several other diagnostics, yielding a consistent picture of detachment. It will be shown that detachment in ASDEX Upgrade is not a continuously evolving process but can be divided into three different states. Within these states, the conditions in the inner and outer divertor are strongly coupled. Radiative fluctuations, measured with fast diode bolometers, combined with a region of high electron density appear at the X-point during one of these states. We show how N₂ seeding during this state alters the divertor conditions. Finally an unstable situation was found, where the divertor plasma oscillates between two detachment states

[en] In this paper, a new experimental classification of divertor detachment in ASDEX Upgrade is presented. For this purpose, a series of ohmic and L-mode density ramp discharges at different heating powers, magnetic field directions and plasma species were carried out. For the first time at ASDEX Upgrade the electron density in the divertor volume and the occurrence of volume recombination were measured by means of spectroscopy. It is shown that detachment is not a continuously evolving process but rather undergoes three distinct states while the characteristics of the inner and outer divertor are strongly coupled. Before the complete detachment of the inner and outer divertor, radiative fluctuations occur in the inner divertor close to the X-point, observed for the first time via new fast diode bolometers. Finally, the effect of an externally applied magnetic perturbation field on the detachment process is investigated. (paper)

[en] Improvements to the Thomson scattering diagnostic have enabled the study of near scrape-off layer (SOL) decay lengths in the 2014 ASDEX Upgrade experimental campaign. A database of H-mode discharges has been studied using a two-line fit method for the core and log-linear fit for the near SOL region under both attached and detached divertor conditions. SOL electron temperature $T_e$ profiles have been found to have a radial exponential decay distribution which does not vary poloidally, consistent with the two-point model. In attached H-mode regimes, a log-linear regression shows that the SOL upstream dataset has the same main parametric dependencies as the scaling inferred from downstream Infrared camera measurements. A simple collisional relation from two-point model is found to best relate the upstream decay lengths and downstream divertor power widths. The SOL $T_e$ gradient length appears to be independent of $T_e$ pedestal parameters, but may correlate with the pedestal electron pressure parameters. Both the pedestal and SOL density and temperature scale lengths are linearly correlated with an almost constant gradient ratio, ${\mathrm{\eta <!--\; ??\; -->}}_e$. The smaller gradient ratio ${\mathrm{\eta <!--\; ??\; -->}}_e$ and the fact that the Spitzer–Härm model is more valid, agrees with the studied plasma lying in the collisional regime. A transition to flat SOL ne profiles, previously reported for L-mode plasmas in many machines, has been observed in AUG detatched H-mode regimes. When the flattening of density profile happens in H-mode detached plasmas, the broadening of near SOL $T_e$ decay length ${\mathrm{\lambda <!--\; ??\; -->}}_{T_{e,u}}$ also appears which may be good news for future machines. (paper)

[en] Full text: Large burning plasma fusion devices such as ITER and DEMO require divertor detachment to not exceed the tolerable power load densities (< 5 MW/m²) for long pulse operation. The understanding of the processes leading to divertor detachment is currently incomplete and a reliable prediction for future large scale devices out of reach. In ASDEX Upgrade divertor detachment has been studied for Ohmic and L-mode density ramp discharges with deuterium and hydrogen as a fuelling species and both toroidal field directions. The effect of seeding nitrogen has been tested. Prior to the detachment of the outer divertor a fluctuating detachment state appears in the SOL of the inner divertor, characterized by strong radiative fluctuations close to the X-point. Simultaneously a region with high ne appears in the inner far SOL and X-point regions. Once these radiative fluctuations disappear detachment occurs along the entire inner target plate and the outer divertor reaches a state of complete detachment. The integrated ion flux at the inner target reaches its peak value at an only ∼ 10 - 20% lower line averaged density than for the outer divertor target. However, the maximum of the integrated ion flux to the inner target remains well below what is observed at the outer target. Numerical transport code packages, such as SOLPS 5.0, contain an as complete as possible model of our current understanding of the basic processes present in the Scrape Off Layer. Under common assumptions for the model it is found that the roll over of the ion flux at the inner and outer targets occurs at a similar n^sep. Contrary to experimental findings the simulated peak values remain comparable for both target plates and no strong reduction of the ion flux density is seen for the inner divertor. The experimental observation of the high n_e in the far SOL during the fluctuating phase indicates that plasma is transported outward into the far SOL and/or strong radiation spontaneously sets in over a large volume in the far SOL. Based on these observations various levels of perpendicular transport in the X-point region of the inner divertor are assumed and recycling impurities such as oxygen and nitrogen are included. This leads to a shift of the simulated peak ion flux into the far inner SOL and a reduction of its peak value at the roll over by more than a factor of 3. (author)

[en] Tungsten rods of 1×1×3 mm³ were exposed in single H-mode discharges at the outer divertor target plate of ASDEX Upgrade using the divertor manipulator system. Melting of the W rod at a pre-defined time was induced by moving the initially far away outer strike point close to the W-rod position. Visible light emissions of both the W pin and consecutively ejected W droplets were recorded by two fast cameras with crossed viewing cones. The time evolution of the local W source at the pin location was measured by spectroscopic observation of the WI line emission at 400.9 nm and compared to the subsequent increase of tungsten concentration in the confined plasma derived from tungsten vacuum UV line emission. Combining these measurements with the total amount of released tungsten due to the pin melt events and ejected droplets allowed us to derive an estimate of the screening factor for this type of tungsten source. The resulting values of the tungsten divertor retention in the range 10-20 agree with those found in previous studies using a W source of sublimated W(CO)₆ vapour at the same exposure location. Ejected droplets were found to be always accelerated in the general direction of the plasma flow, attributed to friction forces and to rocket forces. Furthermore, the vertically inclined target plates cause the droplets, which are repelled by the target plate surface potential due to their electric charge, to move upwards against gravity due to the centrifugal force component parallel to the target plate.

[en] The first stable completely detached H-mode plasma in the full tungsten ASDEX Upgrade has been achieved. Complete detachment of both targets is induced by nitrogen seeding into the divertor. Two new phases are added to the detachment classification described in Potzel et al (2014 Nucl. Fusion 54 013001): first, the line integrated density increases by about 15% with partial detachment of the outer divertor. Second, complete detachment of both targets is correlated to the appearance of intense, strongly localized, stable radiation at the X-point. Radiated power fractions, f_rad, increase from about 50% to 85% with nitrogen seeding. X-point radiation is accompanied by a loss of pedestal top plasma pressure of about 60%. However, the core pressure at ρ_pol < 0.7 changes only by about 10%. H₉₈ = 0.8–1.0 is observed during detached operation. With nitrogen seeding the edge-localized mode (ELM) frequency increases from the 100 Hz range to a broadband distribution at 1–2 kHz with a large reduction in ELM size. (paper)

[en] The L–H transition and the H-mode behaviour in the presence of non-axisymmetric n = 2 magnetic perturbations have been investigated. At low density no effect on the L–H transition is observed. Within a rather narrow density window around 50% of the Greenwald density limit, a transition to H-mode with small ELMs only and good confinement can be achieved. However, a strong density dependence of the L–H threshold power in the presence of magnetic perturbations forces the plasmas to remain in L-mode when the density is above 60% of the Greenwald value. The H-mode confinement time is not affected by the presence of the magnetic perturbations. All of these H-modes, with and without ELM mitigation, exhibit a common confinement degradation with increasing recycling. (paper)

[en] In this article we present the development of a new diagnostic capable of determining the electron density in the divertor volume of ASDEX Upgrade (AUG). It is based on the spectroscopic measurement of the Stark broadening of the Balmer lines. In this work two approaches of calculating the Stark broadening, i.e. the unified theory and the model microfield method, are compared. It will be shown that both approaches yield similar results in the case of Balmer lines with high upper principal quantum numbers n. In addition, for typical AUG parameters the influence of the Zeeman splitting on the high n Balmer lines is found to be negligible. Moreover, an assumption for the Doppler broadening of T_n = 5 eV, which is the maximum Frank–Condon dissociation energy of recycled neutrals, is sufficient. The initial electron density measurements performed using this method are found to be consistent with both Langmuir probe and pressure gauge data. (paper)

[en] A database with attached, partially detached and completely detached divertors has been constructed in ASDEX Upgrade discharges in both H-mode and L-mode plasmas with Thomson Scattering data suitable for the analysis of the upstream SOL electron profiles. By comparing upstream temperature decay width, ${\mathrm{\lambda <!--\; ??\; -->}}_{T_{e,u}},$ with the scaling of the SOL power decay width, ${\mathrm{\lambda <!--\; ??\; -->}}_{q_{\parallel <!--\; ???\; -->e}},$ based on the downstream IR measurements, it is found that a simple relation based on classical electron conduction can relate ${\mathrm{\lambda <!--\; ??\; -->}}_{T_{e,u}}$ and ${\mathrm{\lambda <!--\; ??\; -->}}_{q_{\parallel <!--\; ???\; -->e}}$ well. The combined dataset can be described by both a single scaling and a separate scaling for H-modes and L-modes. For the single scaling, a strong inverse dependence of, ${\mathrm{\lambda <!--\; ??\; -->}}_{T_{e,u}}$ on the separatrix temperature, $T_{e,u},$ is found, suggesting the classical parallel Spitzer–Harm conductivity as dominant mechanism controlling the SOL width in both L-mode and H-mode over a large set of plasma parameters. This dependence on $T_{e,u}$ explains why, for the same global plasma parameters, ${\mathrm{\lambda <!--\; ??\; -->}}_{q_{\parallel <!--\; ???\; -->e}}$ in L-mode is approximately twice that in H-mode and under detached conditions, the SOL upstream electron profile broadens when the density reaches a critical value. Comparing the derived scaling from experimental data with power balance, gives the cross-field thermal diffusivity as ${\mathrm{\chi <!--\; ??\; -->}}_{\perp <!--\; ???\; -->}\propto <!--\; ???\; -->T_e^{1/2}/n_e,$ consistent with earlier studies on Compass-D, JET and Alcator C-Mod. However, the possibility of the separate scalings for different regimes cannot be excluded, which gives results similar to those previously reported for the H-mode, but here the wider SOL width for L-mode plasmas is explained simply by the larger premultiplying coefficient. The relative merits of the two scalings in representing the data and their theoretical implications are discussed. (paper)

[en] A novel fast-ion D-alpha (FIDA) diagnostic that is based on charge exchange spectroscopy has been installed at ASDEX Upgrade. The diagnostic uses a newly developed high-photon-throughput spectrometer together with a low-noise EM-CCD camera that allow measurements with 2 ms exposure time. Absolute intensities are obtained by calibrating the system with an integrating sphere and the wavelength dependence is determined to high accuracy using a neon lamp. Additional perturbative contributions to the spectra, such as D₂-molecular lines, the Stark broadened edge D-alpha emission, and passive FIDA radiation have been identified and can be subtracted or avoided experimentally. The FIDA radiation from fast deuterium ions after charge exchange reactions can therefore be analyzed continuously without superimposed line emissions at large Doppler shifts. Radial information on the fast ions is obtained from radially distributed lines of sight. The investigation of the fast-ion velocity distribution is possible due to three different viewing geometries. The independent viewing geometries access distinct parts of the fast-ion velocity space and make tomographic reconstructions possible