[en] Tungsten (W) is the main candidate for the first wall of a reactor due to its robustness against physical sputtering by the plasma ions, however, when W reaches concentrations of 10^-4 in the plasma, it causes unduly large plasma cooling by radiation. This implies restrictive impurity control for W, which needs reliable diagnostic by plasma spectroscopy. The most intense spectral lines of highly ionized W are emitted in the VUV and soft X-ray range. To perform calculations on atomic data the code packages incorporated in the ADAS project are used. The electronic structure of nearly all W-ions is calculated by the Cowan-code (Hartree-Fock algorithm). In a second step, the cross sections for electron impact excitation are evaluated via the Cowan-code using the plane wave Born-approximation. A detailed collisional-radiative model is employed to calculate the model-spectra for each ion in equilibrium. Finally, ionization and recombination rates of W are evaluated by semi-empirical formulae, which make use of the electronic structure calculations of the Cowan-code. All atomic data are confronted with experimental measurements from the Garching tokamak ASDEX Upgrade and the Berlin electron-beam ion trap (EBIT). The experimental investigations extend up to 5 keV electron temperatures, which is the maximum of the routine operation at ASDEX Upgrade. 'Impurity accumulation', which is characterized by a strong peaking of the impurity density profile, enables unique investigations on the fractional abundance of Ag-like W²⁷⁺ up to Co-like W⁴⁷⁺. The recombination rates for few states are corrected empirically satisfying boundary conditions which arise from experimental evidence. Both spectral features have been studied also for isoelectronic sequences by injecting the impurities hafnium, tantalum, rhenium, gold, lead and bismuth. Additionally, xenon is targeted by the same code packages, as xenon might be injected in future experiments or a reactor for intentional plasma cooling. Predictions on radiative plasma cooling (cooling factor CF) have been based up to now on the rough 'Average Ion Model' (AIM) and a further result of the work is the analysis of plasma cooling with the outlined, superior model. All data, which are benchmarked by experimental spectra, are used to calculate the CF of the high-Z elements. (Orig.)

[en] High-resolution measurements of K-shell emission from O, F, and Ne have been performed at the ASDEX Upgrade tokamak in Garching, Germany. Independently measured temperature and density profiles of the plasma provide a unique test bed for model validation. We present comparisons of measured spectra with calculations based on transport and collisional-radiative models and discuss the reliability of commonly used diagnostic line ratios

[en] A new edge CXRS diagnostic, which utilizes one of the neutral heating beams, has been installed in the ASDEX Upgrade tokamak. The system provides highly resolved radial profiles (∝5 mm) of impurity ion temperature, density and poloidal rotation, which are determined directly from the observed spectra, i.e. from the Doppler width, the Doppler shift and the line intensity. From these profiles, in conjunction with the data from a second edge CXRS diagnostic, which provides toroidal plasma rotation, the edge radial electric field (E_r) can be determined. E_r can be calculated using the radial force balance which relates E_r with ∇p/n as well as with both poloidal and toroidal rotation. It is widely accepted that E x B velocity shear is fundamental for suppressing edge turbulence thus, aiding the formation of the edge transport barriers and enabling the L-H transition. However, the origin and development of E_r is still an open issue. The E_r profile is determined in type-I ELMy H-mode discharges using CX measurements of several different fully ionized impurity ions. This provides a consistency check and validates the new CX system as all analyses must arrive at the same E_r profile regardless of the impurity species used. In addition, the effect of the recently installed resonant magnetic perturbation coils on the E_r profile is investigated.

[en] Today carbon is the most common first wall material in fusion experiments, whereas the first wall of the next step device will consist of a mixture of elements. Especially tungsten has been shown to be an alternative to low-Z materials. However, even with 40% of tungsten coated plasma facing components, carbon is still the dominant impurity at ASDEX Upgrade. A consistent picture of the carbon migration in ASDEX Upgrade has been achieved. Primary carbon sources are the protection limiters at the low field side of the main chamber. Eroded carbon is distributed all over the main chamber. So, the initially tungsten coated central column acts as the main carbon source during discharges, even though a considerable amount of tungsten surfaces persists. Carbon coverage of the central column can significantly change on a shot to shot basis. The divertor target plates act as a strong carbon sink. Deposits are found at the inner and outer divertor, which may be re-eroded forming precursors for layer production at remote areas. In ASDEX Upgrade, deposits on the subdivertor structure are formed by hydrocarbons with a high effective sticking coefficient. A parasitic plasma at these locations may enhance the surface loss probability by surface activation. At more remote areas, such as the pump ducts, a very small deposition is found. Non sticking hydro-carbons are effectively pumped by the cryopump and turbo molecular pumps

[en] The toroidal rotation of H-mode plasmas in ASDEX Upgrade is studied in the outermost 5 cm of the confined plasma. The projection of the rotation velocity along the line of sight (approximately toroidal) is measured using charge exchange recombination spectroscopy, with a radial resolution of up to 3 mm and a temporal resolution of 1.9 ms. At about 1 cm inside the separatrix the rotation exhibits a local minimum. From there, the rotation in codirection increases towards the plasma center and towards the separatrix. The latter increase is the focus of this work. It is situated in the region of the edge transport barrier and amounts to 10-20 km/s. It is observed for D⁺, He²⁺, B⁵⁺, and C⁶⁺. The described rotation feature at the edge is not visible during an ELM crash and is probably connected to the occurrence of steep gradients in this plasma region

[en] This paper presents an overview of results from the Imaging Motional Stark Effect (IMSE) diagnostic obtained during its first measurement campaign at ASDEX Upgrade since installation as a permanent diagnostic. A brief overview of the IMSE technique is given, followed by measurements of a standard H-mode discharge, which are compared to equilibrium reconstructions showing good agreement where expected. The development of special discharges for the calibration of pitch angle is reported and safety factor profile changes during sawteeth crashes are shown, which can be resolved to a few percent due to the high sensitivity at good time resolution of the new IMSE system.

[en] Impurity transport in sawtoothing plasmas in the presence of long lived inter-crash $(m,n)=(1,1)$ MHD activity is analysed in an ASDEX Upgrade discharge. In order to describe the time-evolution of the soft x-ray (SXR) time-traces after argon trace impurity injection, two sets of transport coefficients, switching at the onset of the $(1,1)$ mode, are necessary. The non-linear time evolution of the background SXR emissivity leads on the other hand to systematic errors that cannot be eliminated from the transport analysis. Typical experimental methods for the determination of the transport coefficients are demonstrated to be inapplicable and a way to determine the intrinsic $W$ density form a combination of SXR and vaccum-ultraviolet measurements is explained. Ideas for new ways to probe impurity transport in two dimensions in the presence of long-lived MHD activity are given. (paper)

[en] In the fully tungsten clad ASDEX Upgrade, the sputtering rates of tungsten have been determined at all relevant plasma facing components using fast spectroscopic measurements with temporal resolution down to 0.5 ms. The sputtering strongly increases during an edge-localized mode (ELM) and the ELMs are often the dominant cause of tungsten sputtering. A modelling approach was employed to calculate the tungsten source at the limiters and the resulting tungsten density at the pedestal top inside the H-mode edge transport barrier (ETB). In the ETB, it is assumed that tungsten transport is collisional, i.e. behaves like other impurities. The collisional transport leads to strong inward drifts and steep density gradients in the ETB, which are flattened during an ELM causing an efflux of tungsten. The collisional transport in the ETB is also calculated for typical ITER conditions and the resulting tungsten density profiles as well as the transport of the helium ash through the ETB are evaluated.

[en] The parallel flows in the H-mode edge of ASDEX Upgrade are investigated. Beam-based charge-exchange recombination spectroscopy (CXRS) provides the toroidal and poloidal impurity flow velocities at the outboard midplane, while a deuterium-puff based CXRS measurement provides the toroidal impurity flow velocities at the inboard midplane. In order to more easily compare these measurements to fundamental boundary conditions, a basic overview of flows on a flux surface is presented. The boundary conditions are given by the continuity equation and mean that the flow velocities on a flux surface must have a specific structure in order to provide zero divergence. At first, poloidal impurity density asymmetries and radial transport are neglected. Inside of the pedestal-top of the electron density profile the measurements agree with the postulated flow structure, while they do not agree at the pedestal itself. Here, an extension of the theoretical scheme, which allows for a poloidal impurity density asymmetry, suggests that the measured flow velocities could be explained by an excess impurity density at the inboard midplane. In detail, the inboard impurity density is postulated to be at the separatrix up to a factor of 6.5 higher than impurity density at the outboard midplane. Near the pedestal-top of the electron density, this asymmetry disappears. Radial transport is considered as an explanation for that asymmetry. A conclusive disentanglement of the driving mechanisms requires further investigation. (paper)

[en] The influence of passive edge emission on the charge exchange (CX) spectra of carbon (C⁵⁺, n = 8 → 7 at 529 nm) measured in fusion plasmas at the ASDEX Upgrade tokamak is investigated. The spectra are obtained viewing the plasma edge tangentially with eight lines of sight while the plasma is swept to enhance the spatial density of the measurements. A forward model to deconvolute the measured line-integrals is employed. The local emissions are then compared with the simulated radiation obtained with the 1D impurity transport code STRAHL using transport coefficients which are determined independently. Depending on the background neutral deuterium densities the simulation predicts the absolute line intensities and the relative contributions of electron impact excitation and thermal CX to the measured signals. Therefore, a background neutral deuterium density profile has been determined. For the passive emission line of C⁵⁺, the comparison between forward model and simulations yields that electron impact excitation and thermal CX are both important for understanding the passive line. Indeed, thermal CX proves to be affecting the passive emission line considerably via two mechanisms, i.e. change in the ionization equilibrium through CX recombination and radiation due to local CX reactions.