[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] The transport of argon as trace impurity has been investigated in electron cyclotron resonance heated L-mode discharges at ASDEX Upgrade to test recent theories predicting the rise of an outward impurity convection. The profiles of the argon transport coefficients for r/a < 0.65 have been determined by analysing the linear flux-gradient dependence of the total argon ion density evolution after the puff. A new methodology to experimentally obtain the total impurity ion density from the integrated use of two spectroscopic diagnostic and the 1D impurity transport code STRAHL has been developed. Results confirm the enhancement in diffusivity and the rise of the positive convection observed in previous studies, but show for the first time how these effects are strongly localized around the electron cyclotron resonance heating deposition radius. These experimental results are found to be in promising qualitative agreement with a set of quasi-linear gyrokinetic simulations with the code GS2.

[en] During Ion Cyclotron Range of Frequencies (ICRF) operation with metallic plasma facing components, plasma wall interaction processes are modified, which needs to be understood in order to reduce the ICRF-related rise in impurity concentration and maximize ICRF coupling. This contribution is focused on gathering and analyzing a database on W-behavior during ICRF spanning a multi-parameter space. The gas puff dependence is investigated in selected pulses that feature the injection of deuterium (D_2) main gas, with impurity gas nitrogen (N_2), argon (Ar) and krypton (Kr). C_W was found to decrease with the total gas injection rate, which was also correlated with the changes of electron density in the location of the C_W measurements. Although the addition of N_2, Ar and Kr impacted on the increase in sputtering, to achieve a similar C_W level with mixed puffing, one needs to inject considerably more gas. Moreover, C_W is shown to change after implementation of boron coatings on the ICRF antenna limiters

[en] A new method for the reconstruction of two-dimensional (2D) electron temperature profiles in the presence of saturated magneto-hydro-dynamic (MHD) modes from the one-dimensional (1D) electron cyclotron emission (ECE) diagnostic is presented. The analysis relies on harmonic decomposition of the electron temperature oscillations through short time Fourier transforms and requires rigid poloidal mode rotation as the only assumption. The method is applicable to any magnetic perturbation as long as the poloidal and toroidal mode numbers m and n are known. Its application to the case of a (m, n) = (1, 1) internal kink mode on ASDEX Upgrade is presented and a new way to estimate the mode displacement is explained. For such modes, it is shown that the higher order harmonics usually visible in the ECE spectrogram arise also for the pure m = n = 1 mode and that they cannot be directly associated with m = n > 1 magnetic perturbations. This method opens up new possibilities for electron heat transport studies in the presence of saturated MHD modes and a way to disentangle the impurity density contributions from electron temperature effects in the analysis of the soft x-ray data. (paper)

[en] Radiative cooling is an important tool to lower the power loads to plasma facing components. Amongst the different species used, nitrogen has been shown to provide the best cooling effect in ASDEX Upgrade discharges, while maintaining or even improving the confinement. Depending on the background plasma density and the divertor temperature a reduction or increase of the W sputtering yield compared to the unseeded phases was observed. In discharges with very high density, divertor plasma temperatures below 5 eV can be achieved leading to a complete suppression of the W sputtering. The W influx is a necessary prerequisite for the central W concentration (c_W), nevertheless it has been shown that c_W is mainly governed by transport. Although the energy confinement increases often in N₂ seeded discharges no strong increase in c_W is observed and there are no signs of central density peaking and W accumulation. Moreover the sustained regular ELM activity prevents a strong inward transport of W across the pedestal region.

[en] Future fusion reactors will operate under more demanding conditions compared to present devices. They will require high divertor and core radiation by impurity seeding to reduce heat loads on divertor target plates. In addition, high core densities are required to reach adequate fusion performance. These scenarios are addressed at the ASDEX upgrade tokamak. Here we present the transport analysis of such scenarios. 'Plasmas with high radiation by impurity seeding': a non coronal radiation model was developed and compared to the bolometric measurements in order to provide a reliable radiation profile for transport calculations. Power balance analyses taking into account the radiation distribution show no change in the core transport during impurity seeding. 'High density plasmas with pellets': very good agreement between experimental values and transport calculations is found. Both reveal that τ_E remains constant despite the density increase. Hence the density dependence of ITER98 (τ_E ∝ n_e^0.41) is not adequate to describe this regime. The kinetic profiles reveal a transient phase at the pellet start due to a slower density build up compared to the temperature decrease. The low particle diffusion can explain the confinement behaviour.

[en] Highlights: • Solving the exhaust problem is crucial for DEMO. • Here, we discuss the new impurity radiation model in the systems code PROCESS. • Furthermore, we assess its effect on DEMO design. • More appropriate scalings will significantly enhance predictions for DEMO. • The controllability of highly radiative scenarios remains to be shown. - Abstract: For fusion reactors with ITER divertor technology, it will be imperative to significantly reduce the heat flux into the divertor e.g. by seeded impurity radiation. This has to be done without affecting the accessibility of a high performance scenario. To assess the implications of seeded plasma impurities on DEMO design, we have developed an impurity radiation model for radiation inside the separatrix. Evaluating the validity of our model, we find the assumption of a local ionisation equilibrium to be appropriate for our purposes and the assumption of flat impurity profiles – even though not satisfactory – to represent the best currently possible. Benchmarking our model against other codes highlights the need to use up to date atomic loss function data. From the impurity radiation perspective, the main uncertainties in current DEMO design stem from the lack of confinement and L-H-threshold scalings that can be robustly extrapolated to highly radiative DEMO scenarios as well as the lack of appropriate models for the power flow from the separatrix into the divertor that include radiation in the scrape off layer. Despite these uncertainties in the model we can exclude that significant fuel dilution through seeded impurities (with Z ≥ Z_A_r) will be an issue for DEMO, but the controllability of highly radiative scenarios still needs to be coherently shown.

[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] Fishbones are ubiquitous in high-performance JET plasmas and are typically considered to be unimportant for scenario design. However, during recent high-performance hybrid scenario experiments, sporadic and explosive fishbone oscillations with sawtooth reconnection were observed coinciding with reduced performance and a main chamber hotspot. Fast ion loss diagnostics showed fusion products ejected from the plasma by the fishbones. We present calculations of the perturbed motion of non-resonant fusion products in the presence of fishbones assuming a fixed linear mode structure and frequency. Using careful reconstruction of the equilibrium and measurements of the perturbation, we show that the measured fishbone spatial structure in these experiments can be well modelled as a linear MHD internal kink mode. Both drift and full-orbit calculations predict losses of fusion products at the same location of the observed hotspot, however the calculated energy content of those losses is negligible and cannot be contributing significantly. The fast ions responsible for the hotspot and the reason for their loss both remain unexplained. (paper)

[en] Full text: The understanding of impurity transport from the wall to the center of the plasma and the identification of reliable methods to control central impurity accumulation are essential elements toward the achievement of practical fusion energy. A combination of theoretical and experimental research is required to identify the physical mechanisms from the theoretical standpoint, and to validate their impact on the measured impurity density profiles from the experimental side. In this contribution, advances in the theoretical description of turbulent impurity transport, particularly related to the inclusion of rotational effects, are presented. An analytical fluid model, which still captures the main elements of the physics, and linear and nonlinear numerical calculations with the gyrokinetic codes GKW and GS2 are presented and compared. In particular, GKW has the unique feature for a gyrokinetic code of including also centrifugal effects on turbulent transport. The impurity transport mechanisms produced by a radial gradient of the toroidal velocity and by centrifugal effects are singled out in the analytical calculations, and identified in the numerical results. These advances allow, in particular, the consistent prediction of the two dimensional impurity density distribution over the poloidal cross section. This more comprehensive theoretical description is also applied to the modelling of ASDEX Upgrade measurements of impurity density profiles. In neutral beam injection heated H-mode plasmas, central electron cyclotron heating is observed to increase the peaking of both the electron density and the boron density profiles. An inverse correlation is observed between the peaking of the boron density profile and the plasma toroidal rotation, as well as the boron logarithmic temperature gradient. The theoretical explanation of this phenomenology relates the boron response to the reduction of plasma rotation in the presence of central electron heating. (author)