[en] Due to the high mass and charge of the heavy ions, centrifugal and electrostatic forces cause a significant variation in their poloidal density. The impact of these forces on the poloidal density profile of tungsten was investigated utilizing the detailed two-dimensional SXR emissivity profiles from the ASDEX Upgrade tokamak. The perturbation in the electrostatic potential generated by magnetic trapping of the non-thermal ions from neutral beam injection was found to be responsible for significant changes in the poloidal distribution of tungsten ions. An excellent match with the results from fast particle modeling was obtained, validating the model for the poloidal fast particle distribution. Additionally, an enhancement of the neoclassical transport due to an outboard side impurity localization was measured in the experiment when analyzing the tungsten flux between sawtooth crashes. A qualitative match with neoclassical modeling was found, demonstrating the possibility of minimizing neoclassical transport by an optimization of the poloidal asymmetry profile of the impurity. (paper)

[en] The recently proposed set of zeroth-order moment equations to model poloidal density asymmetries induced by temperature anisotropies in rotating tokamak plasmas (Bilato et al 2014 Nucl. Fusion 54 072003) is here extended to account for the effects of the localization of the ion-cyclotron (IC) resonance on the poloidal inhomogeneity of the density of the cyclotron-heated ion species. This additional effect has a significant impact on the poloidal density asymmetries due to radio-frequency heating, a key issue for high-Z impurity transport, leading, in particular, to a reduced ICRF heating impact on the in–out accumulation of high-Z impurities when the IC resonance is located on the HFS. (paper)

[en] Full text: The gradients in the ion and electron temperature profiles, T_i and T_e, are a key component for driving turbulent transport in plasmas. Since the 1980s, 'profile resilience' has been observed on many tokamaks, which describes the fact that T_i and T_e are limited by a critical normalized temperature gradient. Beyond this critical R/LT the heat diffusivities increase drastically. Hence, with stiff profiles the edge temperature is a key to attaining higher core temperatures and higher plasma confinement. Understanding the transport processes in the H-mode transport barrier, where turbulence is strongly reduced, is essential for a reliable scaling to next step fusion devices. In this contribution we analyze the ion heat and momentum transport at the plasma edge of ASDEX-Upgrade (AUG) H-mode plasmas. The experimentally determined ion heat diffusivities are compared to neoclassical theory and the impact of ELMs on the edge ion heat transport is studied in detail. During the inter-ELM phase the ion heat diffusivity in the pedestal region is close to the neoclassical level. High time-resolution CXRS measurements (100 μs) enables detailed studies of the ion heat transport during the entire ELM cycle. The measurements show that during the ELM crash the radial ion temperature gradient flattens and the temperature close to the separatrix increases as a result of the ELM heat transport. The pre-ELM level in the ion heat transport is established approximately 2-3 ms after the ELM crash. In order to study the edge momentum an H-mode edge rotation database was created at AUG. The data reveals a strong dependence of the impurity toroidal rotation on the ion collisionality. Below a certain collisionality threshold the impurity toroidal rotation switches sign from co- to countercurrent. The edge rotation is modelled using ASTRA. Here, the toroidal torque balance equation including diffusion, pinch and external momentum sources is solved. Comparison between the experimental profiles and the simulations shows good agreement within the experimental uncertainties, indicating that diffusion and external momentum sources are the dominant players. The sign change of the impurity toroidal rotation observed at low collisionality can be explained by a negative edge torque combined with a large differential toroidal rotation, while the main ion toroidal rotation is almost unaffected. (author)

[en] A comparison of the impact of additional central electron cyclotron resonance heating (ECRH) and ion cyclotron resonance heating (ICRH) on the behaviour of the tungsten (W) density in the core of H-mode plasmas heated with neutral beam injection (NBI) is performed in ASDEX Upgrade. Both localized and broad profiles of the power density of the ECRH have been obtained, where broad profiles reproduce the profile shape of the ICRH power density, which is applied with a hydrogen minority heating scheme. In contrast to ECRH, which produces direct electron heating only, ICRH eventually heats both electrons and ions in almost equal fractions. It is found that both additional RF heating systems reduce the peaking of the W density profile with increasing central RF heating power. Approximately the same values of W density peaking are obtained when the same values of electron heating are produced by the two RF heating systems, which implies that less total heating power is required with ECRH than with ICRH to reduce the W density peaking. A related modelling activity shows that an important ingredient to explain the experimentally observed trend is the variation of the turbulent W diffusion as a function of the electron to ion heat flux ratio. Additional effects are connected with the more favorable W neoclassical transport convection in the presence of ICRH, produced by the combination of stronger central ion temperature gradients and the impact of the H minority on the W poloidal density asymmetry. (paper)

[en] In tokamaks, the role of turbulent transport of heavy impurities, relative to that of neoclassical transport, increases with increasing size of the plasma, as clarified by means of general scalings, which use the ITER standard scenario parameters as reference, and by actual results from a selection of discharges from ASDEX Upgrade and JET. This motivates the theoretical investigation of the properties of the turbulent convection of heavy impurities by nonlinear gyrokinetic simulations in the experimentally relevant conditions of comparable ion and electron heat fluxes. These conditions also correspond to an intermediate regime between dominant ion temperature gradient turbulence and trapped electron mode turbulence. At moderate plasma toroidal rotation, the turbulent convection of heavy impurities, computed with nonlinear gyrokinetic simulations, is found to be directed outward, in contrast to that obtained by quasi-linear calculations based on the most unstable linear mode, which is directed inward. In this mixed turbulence regime, with comparable electron and ion heat fluxes, the nonlinear results of the impurity transport can be explained by the coexistence of both ion temperature gradient and trapped electron modes in the turbulent state, both contributing to the turbulent convection and diffusion of the impurity. The impact of toroidal rotation on the turbulent convection is also clarified. (paper)

[en] We present a fully Bayesian approach for the inference of radial profiles of impurity transport coefficients and compare its results to neoclassical, gyrofluid and gyrokinetic modeling. Using nested sampling, the Bayesian impurity transport inference (BITE) framework can handle complex parameter spaces with multiple possible solutions, offering great advantages in interpretative power and reliability with respect to previously demonstrated methods. BITE employs a forward model based on the pySTRAHL package, built on the success of the well-known STRAHL code (Dux 2003 Fusion Sci Technol. 44 708–15), to simulate impurity transport in magnetically-confined plasmas. In this paper, we focus on calcium (Ca, Z = 20) laser blow-off injections into Alcator C-Mod plasmas. Multiple Ca atomic lines are diagnosed via high-resolution x-ray imaging crystal spectroscopy and vacuum ultra-violet measurements. We analyze a sawtoothing I-mode discharge for which neoclassical and turbulent (quasilinear and non-linear) predictions are also obtained. We find good agreement in diffusion across the entire radial extent, while turbulent convection and density profile peaking are estimated to be larger in experiment than suggested by theory. Efforts and challenges associated with the inference of experimental pedestal impurity transport are discussed. (paper)

[en] This contribution reviews an important example of current developments in diagnostic systems and data analysis tools aimed at improved understanding and control of transport processes in magnetically confined high temperature plasmas. The choice of tungsten for the plasma facing components of ITER and probably also DEMO means that impurity control in fusion plasmas is now a crucial challenge. Soft X-ray (SXR) diagnostic systems serve as a key sensor for experimental studies of plasma impurity transport with a clear prospective of its control via actuators based mainly on plasma heating systems. The SXR diagnostic systems typically feature high temporal resolution but limited spatial resolution due to access restrictions. In order to reconstruct the spatial distribution of the SXR radiation from line integrated measurements, appropriate tomographic methods have been developed and validated, while novel numerical methods relevant for real-time control have been proposed. Furthermore, in order to identify the main contributors to the SXR plasma radiation, at least partial control over the spectral sensitivity range of the detectors would be beneficial, which motivates for developments of novel SXR diagnostic methods. Last, but not least, semiconductor photosensitive elements cannot survive in harsh conditions of future fusion reactors due to radiation damage, which calls for development of radiation hard SXR detectors. Present research in this field is exemplified on recent results from tokamaks COMPASS, TORE SUPRA and the Joint European Torus JET. Further planning is outlined. (paper)

[en] We present design, analysis, and performance evaluation of a new, low cost and high speed visible-light camera diagnostic system for tokamak experiments. The system is based on the camera Casio EX-F1, with the overall price of approximately a thousand USD. The achieved temporal resolution is up to 40 kHz. This new diagnostic was successfully implemented and tested at the university tokamak GOLEM (R = 0.4 m, a = 0.085 m, B_T < 0.5 T, I_p < 4 kA). One possible application of this new diagnostic at GOLEM is discussed in detail. This application is tomographic reconstruction for estimation of plasma position and emissivity.

[en] Highlights: • Spline basis current functions with second-order linear regularisation. • Perturbations of magnetic probe measurements due to ferromagnetic tiles on the inner wall and from oscillations in the fast position coil current are corrected. • A constraint of the safety factor on the magnetic axis is introduced. Soft X-ray tomography is used to assess the quality of the real-time magnetic equilibrium reconstruction. • External loop voltage measurements and magnetic probe pairs inside and outside the vessel wall were used to measure the vacuum vessel wall resistivity. - Abstract: Real-time magnetic equilibria are needed for NTM stabilization and disruption avoidance experiments on ASDEX Upgrade. Five improvements to real-time magnetic equilibrium reconstruction on ASDEX Upgrade have been investigated. The aim is to include as many features of the offline magnetic equilibrium reconstruction code in the real-time equilibrium reconstruction code. Firstly, spline current density basis functions with regularization are used in the offline equilibrium reconstruction code, CLISTE [1]. It is now possible to have the same number of spline basis functions in the real-time code. Secondly, in the presence of edge localized modes, (ELM's), it is found to be necessary to include the low pass filter effect of the vacuum vessel on the fast position control coil currents to correctly compensate the magnetic probes for current oscillations in these coils. Thirdly, the introduction of ferromagnetic tiles in ASDEX Upgrade means that a real-time algorithm for including the perturbations of the magnetic equilibrium generated by these tiles is required. A methodology based on tile surface currents is described. Fourthly, during current ramps it was seen that the difference between fitted and measured magnetic measurements in the equilibrium reconstruction were larger than in the constant current phase. External loop voltage measurements and magnetic probe pairs inside and outside the vessel wall were used to measure the vacuum vessel wall resistivity. This is the first step towards including vacuum vessel currents during the plasma current ramp in the real-time equilibrium reconstruction. Fifthly, the introduction of a constraint of the safety factor on the magnetic axis is found to be a helpful method to improve the prediction of the location of rational surfaces for NTM stabilization and disruption avoidance experiments. Soft X-ray tomography is used to assess the quality of the real-time magnetic equilibrium reconstruction using this internal constraint.

[en] The confinement fast ions, generated by neutral beam injection (NBI), has been investigated at the ASDEX Upgrade tokamak. In plasmas that exhibit strong sawtooth crashes, a significant sawtooth-induced internal redistribution of mainly passing fast ions is observed, which is in very good agreement with the theoretical predictions based on the Kadomtsev model. Between the sawtooth crashes, the fishbone modes are excited which, however, do not cause measurable changes in the global fast-ion population. During experiments with on- and off-axis NBI and without strong magnetohydrodynamic (MHD) modes, the fast-ion measurements agree very well with the neo-classical predictions. This shows that the MHD-induced (large-scale), as well as a possible turbulence-induced (small-scale) fast-ion transport is negligible under these conditions. However, in discharges performed to study the off-axis NBI current drive efficiency with up to 10 MW of heating power, the fast-ion measurements agree best with the theoretical predictions that assume a weak level anomalous fast-ion transport. This is also in agreement with measurements of the internal inductance, a Motional Stark Effect diagnostic and a novel polarimetry diagnostic: the fast-ion driven current profile is clearly modified when changing the NBI injection geometry and the measurements agree best with the predictions that assume weak anomalous fast-ion diffusion. (paper)