[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] Full text: Heat and momentum transport play a key role in achieving high confinement in fusion plasmas. Recent advances in the diagnostic capabilities at AUG now allow us to measure the edge profiles on a sub-ms to ms time-scale and with a spatial resolution of less than 5 mm, making it ideal to study the profile recovery after an ELM crash. Here, we present the dynamic behaviour of the energy and momentum transport during edge localized mode cycles at the plasma edge of AUG by combining a comprehensive set of pedestal measurements with interpretive and predictive modelling. The main ion temperature and toroidal rotation profiles were measured in helium plasmas with unprecedented temporal resolution of 250 μs. A local increase of T_i close to the separatrix is observed at the ELM onset, thus reducing the gradient in the pedestal, similar to the behaviour in D plasmas. Shortly after the initial separatrix increase, the whole profile drops and then the pedestal starts to build up again. The pre-ELM profile is fully recovered 3-4 ms after the ELM crash. Transport analysis of the ion energy reveals that the ion heat transport is at the neoclassical level before the ELM crash in the region where the edge ion temperature gradient is maximal. Further inwards, the ion heat transport is about a factor of 4-5 above the neoclassical level. The dynamics of the edge ion heat transport during the pedestal build-up after the crash is also consistent with neoclassical theory. Helium plasmas provide the unique opportunity to measure both main ion and impurity flows simultaneously. Compared to the impurity (here nitrogen) toroidal rotation, which exhibits a local minimum at the plasma edge during the inter-ELM phase, the edge main ion toroidal rotation has a much less pronounced dip and is rather flat. During the ELM the main ion toroidal rotation in the pedestal drops by about 5-10 km/s. This is in contrast to the behaviour of the impurity toroidal rotation, which shows a flattening of the toroidal dip feature. TRANSP simulations and predictive modelling with ASTRA solving the toroidal momentum balance including diffusion, pinch and external sources are used to quantify how much momentum is transported during the ELM and will be presented. (author)

[en] Edge localized modes (ELMs) have a detrimental effect on the plasma facing components and pose one of the most serious obstacles for steady-state operation in a future fusion device. For future fusion machines, the control or even full suppression of ELMs is mandatory. In the past years, extensive effort has been directed to the development of operational regimes that maintain the high confinement and good performance of the H-mode, while at the same time ELMs are suppressed or mitigated. Several natural ELM-free and small-ELM regimes, such as the quiescent H-mode, the improved energy confinement mode, the type-II and the grassy ELM-regime, have been obtained in various tokamaks. The state-of-the-art and recent advances of these ELM-free and small-ELM scenarios are reviewed, and the access and sustainment as well as their applicability to ITER are discussed. (special topic)

[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.

[en] Full text: Recently, ASDEX Upgrade has made significant contributions to momentum transport studies thanks to the upgrade of the core charge exchange recombination spectroscopy system, which now produces much higher quality ion temperature and toroidal rotation profiles. This upgrade enabled the development of an intrinsic rotation database that contains over 200 observations. The edge rotation on AUG is always co-current, while the core rotation can be either co- or counter-current directed. The latter results in a null point in the profile at finite rotation gradient, which is clear evidence of a localized residual stress momentum flux. Moreover, the Mach number in the center of the plasma appears to be determined largely by the normalized gradient of the toroidal rotation at mid-radius, u¹. This correlation holds for all of the observations regardless of plasma confinement regime or type of auxiliary heating. Further examination of the database reveals that u¹ exhibits the strongest correlation with the local logarithmic electron density gradient, R/L_ne and : hollow rotation profiles coincide with peaked n and profiles, while co-current rotation corresponds to low R/L_ne. The known relationship between density peaking and plasma turbulence suggests a connection between the turbulence and the intrinsic rotation behavior as well. A study based on local linear gyro-kinetic calculations found good quantitative agreement between the predicted and measured values of u¹ through the imposition of a finite tilting angle of -0.3 radians on the turbulent mode structure. The mechanism expected to produce such a tilting is a combination of E x B and profile shearing residual stress. These database results are also consistent with observations of residual stress in non-intrinsic rotation scenarios. Flat to hollow rotation profiles are observed concomitant with peaked electron density profiles when sufficient ECRH power is added to NBI heated H-modes causing the turbulent regime to transition from ITG to TEM. Momentum transport analyzes of these plasmas show that the observations can only be explained by the presence of a core localized, counter-current directed, residual stress induced torque of the same order of magnitude as the applied NBI. These results have important implications for torque modulation experiments, which often assume that the residual stress is negligibly small. (author)

[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] Bounce-transit and drift resonance is one of the resonances in tokamaks with broken toroidal symmetry. It plays an important role in the wave-particle interactions. The nonlinear consequence of the resonance is the nonlinear particle trapping in the magnetic well, created by the radial drift motion resulting from the perturbed magnetic fields. These nonlinearly trapped particles form superbananas. When the effective collision frequency is less than the bounce frequency of the superbananas, the resonance is resolved by the nonlinear orbits. The transport theory for the superbananas is developed by solving the drift kinetic equation using the Eulerian approach. The neoclassical toroidal plasma viscosity, and the non-axisymmetric transport coefficients have a scaling , which can be significant even for weakly perturbed tokamaks. Here, is the collision frequency, r is the minor radius, is the typical magnitude of the perturbed magnetic field strength, B is the equilibrium magnetic field strength, and U is a function of the magnetic shear parameter, mode numbers, and with being the poloidal gyro-radius. The magnitude of the energy flux can be comparable to that of the axisymmetric tokamaks for energetic alpha particles when ~ and ~ 0.1. Thus, the theory sets a maximum magnitude of the tolerable perturbed magnetic field strength in fusion reactors, when nonlinear trapping is significant. (paper)

[en] Experimental investigations carried out in the ASDEX Upgrade tokamak under various conditions demonstrate that the ion heat flux at the plasma edge plays a key role in the L–H transition physics, while the electron heat flux does not seem to play any role. This is due to the fact that the ion heat flux governs the radial electric field well induced by the main ions which is responsible for the turbulence stabilization causing the L–H transition. The experiments have been carried out in the low density branch of the power threshold where the electron and ion heat channels can be well separated. In plasmas heated by electron heating, the edge ion heat flux has been increased to reach the L–H transition by using separately three actuators: heating power, density and plasma current. In addition, the key role of the edge ion heating has been confirmed in experiments taking advantage of the direct ion heating provided by neutral beam injection. The role of the ion heat flux explains the non-monotonic density dependence of the L–H threshold power. Based on these results, a formula for the density of the threshold minimum has been developed, which also describes well the values found in tokamaks of various size. For ITER it predicts a value which is close to the density presently foreseen to enter the H-mode and indicates that operation at half field and current would benefit from a very significantly lower density minimum and correspondingly low threshold power. (paper)

[en] The ASDEX Upgrade intrinsic rotation database has been expanded to include a large number of measurements from ohmic L-mode discharges covering a wide range of plasma densities, currents and magnetic fields. This database was then used to study the rotation behaviour across the transition from linear to saturated ohmic confinement (LOC/SOC). At low collisionality the plasma is in the LOC regime and the toroidal rotation profile is in the co-current direction. As the collisionality is increased and the plasma transitions from LOC to SOC, the core rotation decreases resulting in a hollow, counter-current profile. Even deeper in the SOC regime, however, a second reversal back towards the co-current direction occurs. Linear gyrokinetic calculations indicate that these reversals cannot be explained by a transition from a trapped electron mode (TEM) to an ion temperature gradient (ITG) dominated regime. Rather, the analysis indicates that the intrinsic normalized rotation gradient, u′, depends strongly on local plasma parameters, in particular on R/L_ne. Taken together with turbulent particle transport theory, these results suggest that the co- to counter-current directed rotation reversal occurs in the TEM regime due to profile changes and not due to transition from TEM to ITG. The second reversal back to the co-current direction is explained by the reduction of R/L_ne observed in ITG dominated plasmas. Lastly, a simple linear model for the residual stress assuming a fixed poloidal tilt angle of the turbulent eddies was applied to the database. This model is able to capture the main parameter dependencies observed in the data and demonstrates that at least two processes contributing to the poloidal tilt of the turbulent eddies are needed to reproduce the experimentally observed u′ values: one proportional to the sign of the turbulence propagation and the second independent of ω_r. (paper)