[en] Combining high plasma core performance with a suitable power exhaust solution is one of the major challenges in magnetic confinement fusion research. One of the most important power exhaust parameters is the power fall-off length in the scrape-off layer. Two infrared thermography based power fall-off length data sets from JET operated with carbon and ITER-like wall are revisited and compared to recently published scaling laws as well as to confinement and pedestal top parameters. It is shown that the power fall-off length is correlated to confinement, with the highest correlation among the tested parameters being the pedestal top density. The power fall-off length decreases with increasing pedestal top density in variance to the multi-machine scaling law. A similar trend is observed for the pedestal top pressure. This is in agreement with findings at C-Mod showing a scaling of the power fall-off length in various confinement regimes with the volume averaged pressure. Further, it is shown that a variation of the safety factor at constant pedestal top density is not changing the power fall-off length significantly in the two JET data sets. (paper)

[en] Highlights: • The power spreading in the divertor region is described by the competition of perpendicular heat diffusion and parallel electron conduction. • The in/out asymmetry of the power fall-off length λ_q is studied for a closed divertor configuration in hydrogen, deuterium and helium. • A strong density dependence of λ_q,i on the inner divertor target with density is observed. • An interpretation of the observed dependencies of λ_q using a drift based model is given. - Abstract: Understanding the heat transport in the scrape-off layer (SOL) and the divertor region is essential for the design of large fusion devices such as ITER and DEMO. Current scalings for the power fall-off length λ_q in H-Mode [1] are available only for the outer divertor target at low densities with low recycling divertor conditions. For the divertor power spreading S only an empirical scaling for ASDEX Upgrade L-Mode is available based on global plasma parameters [2]. Modelling using SOLPS shows a dependence of S on the divertor electron temperature [3]. A more detailed analysis of the heat transport forming λ_q and S is presented for ASDEX Upgrade L-Mode discharges in hydrogen (H), deuterium (D) and helium (He). For low densities the power fall-off length λ_q,o on the outer divertor target in H and D is described by the same parametric dependencies as the H-Mode scaling [1] but with a larger absolute size of the power fall-off length in L-Mode. The divertor power spreading S is studied using the local divertor measurements of the target electron temperature T_e,tar and density n_e,tar. It is found that the competition of the diffusive transport parallel and perpendicular to the magnetic field forming $S\propto \sqrt{{\chi }_{\perp }/{\chi }_{\parallel }}$ is dominated by the temperature dependence of parallel electron conduction. For high divertor temperatures the ion gyro radius has a significant contribution to S, resulting in a minimum of S at ∼30 eV. A recent study [4] with an open divertor configuration found an asymmetry of the power fall-off length between inner and outer target with a smaller power fall-off length λ_q,i on the inner divertor target. Measurements with a closed divertor configuration find a similar asymmetry for low recycling divertor conditions. It is found, in the experiment, that the in/out asymmetry λ_q,i/λ_q,o is strongly increasing with increasing density. Most notably the heat flux density at the inner divertor target is reducing with increasing λ_q,i whilst the total power onto each divertor target stays constant. It is found that λ_q,o exhibits no significant density dependence for hydrogen and deuterium but increases with about the square root of the electron density for helium. The difference between H,D and He could be due to the different recycling behaviour in the divertor. These findings may help current modelling attempts to parametrize the density dependence of the widening of the power channel and thus allow for detailed comparison to both divertor effects like recycling or increased upstream SOL cross field transport.

[en] Power exhaust is one of the major challenges for a future fusion device. Applying a non-axisymmetric external magnetic perturbation is one technique that is studied in order to mitigate or suppress large edge localized modes which accompany the high confinement regime in tokamaks. The external magnetic perturbation induces breaking in the axisymmetry of a tokamak and leads to a 2D heat flux pattern on the divertor target. The 2D heat flux pattern at the outer divertor target is studied on ASDEX Upgrade in stationary L-mode discharges. The amplitude of the 2D characteristic of the heat flux depends on the alignment between the field lines at the edge and the vacuum response of the applied magnetic perturbation spectrum. The 2D characteristic reduces with increasing density. The increasing divertor broadening, S, with increasing density is proposed as the main actuator. This is supported by a generic model using field line tracing and the vacuum field approach that is in quantitative agreement with the measured heat flux. The perturbed heat flux, averaged over a full toroidal rotation of the magnetic perturbation, is identical to the non-perturbed heat flux without magnetic perturbation. The transport qualifiers, power fall-off length ${\mathrm{\lambda <!--\; ??\; -->}}_q$ and divertor broadening, S, are the same within the uncertainty compared to the unperturbed reference. No additional cross field transport is observed. (paper)

[en] Power exhaust is one of the major challenges for the development of a fusion power plant. Predictions based upon a multimachine database give a scrape-off layer power fall-off length ${\mathrm{\lambda <!--\; ??\; -->}}_q⩽<!--\; ???\; -->1$ mm for large fusion devices such as ITER. The power deposition profile on the target is broadened in the divertor by heat transport perpendicular to the magnetic field lines. This profile broadening is described by the power spreading S. Hence both ${\mathrm{\lambda <!--\; ??\; -->}}_q$ and S need to be understood in order to estimate the expected divertor heat load for future fusion devices. For the investigation of S and ${\mathrm{\lambda <!--\; ??\; -->}}_q$ L-Mode discharges with stable divertor conditions in hydrogen and deuterium were conducted in ASDEX Upgrade. A strong dependence of S on the divertor electron temperature and density is found which is the result of the competition between parallel electron heat conductivity and perpendicular diffusion in the divertor region. For high divertor temperatures it is found that the ion gyro radius at the divertor target needs to be considered. The dependence of the in/out asymmetry of the divertor power load on the electron density is investigated. The influence of the main ion species on the asymmetric behaviour is shown for hydrogen, deuterium and helium. A possible explanation for the observed asymmetry behaviour based on vertical drifts is proposed. (paper)

[en] Highlights: • The 2D heat flux pattern in presence of MP is studied in ASDEX Upgrade. • Transport qualifiers in the SOL do not change in presence of MP. • The heat flux variation due to the MP decreases with increasing density. - Abstract: Power exhaust is one of the major challenges for a future fusion device like ITER. Future devices are foreseen to operate in a regime that exhibits periodic outbursts of particles and energy, so called edge localised modes (ELMs). These ELMs might limit the life time of the divertor target. One of the techniques that is studied in order to control ELMs is the application of a non-axisymmetric magnetic perturbation (MP). This technique leads to a 2D heat flux pattern at the divertor target. The 2D heat flux pattern is characterised for a perturbation with a toroidal mode number n = 2. The lobe structure is visible with a resonant configuration in a low density L-Mode. Varying the differential phase reduces the extent of the heat flux modulation compared to the axisymmetric reference. The heat flux distribution shows no significant modulation with the differential phase opposite to the resonant configuration. The toroidally averaged profile is described by an 1D diffusive model for all differential phases. No significant deviation between the axisymmetric reference and the averaged profiles is observed for both transport qualifiers, power fall-off length λ_q and divertor broadening S. It was reported from previous studies in ASDEX Upgrade, that in L-Mode both λ_q and S increase with increasing density. This dependence is confirmed for the axisymmetric reference phase. The toroidally averaged heat flux profiles with MP show the same increase with density as the reference without MP. The modulation of the heat flux decreases with increasing density. This is attributed to the increasing divertor broadening S which distributes the heat flux more uniformly.

[en] The change of the scrape-off layer power width in dependence of the toroidal magnetic field direction is investigated in L-mode discharges in the upper single null (USN) configuration in ASDEX Upgrade. The heat flux onto the outer and inner divertor plates is measured using a fast 2D infrared camera. The heat flux distribution is described by an exponential power fall-off length ${\mathrm{\lambda <!--\; ??\; -->}}_q$ and a diffusive broadening in the divertor region $S$. In this paper the parameters, $S$ and ${\mathrm{\lambda <!--\; ??\; -->}}_q$, for the inner and outer divertor target are compared for both toroidal magnetic field directions. For the divertor broadening $S$ no dependence on the toroidal magnetic field direction is observed. The comparison between the near scrape-off layer electron temperature fall-off length ${\mathrm{\lambda <!--\; ??\; -->}}_{T_e}$ and the power fall-off length ${\mathrm{\lambda <!--\; ??\; -->}}_q$ are in agreement with the 2-point model. It is concluded that electron conduction is the main contribution for the scrape-off layer parallel transport in these discharges. The ratio between inner, ${\mathrm{\lambda <!--\; ??\; -->}}_q^{\text{inner}}$, and outer, ${\mathrm{\lambda <!--\; ??\; -->}}_q^{\text{outer}}$, power fall-off length is dependent on the toroidal magnetic field direction. The numerical values are ${\mathrm{\lambda <!--\; ??\; -->}}_q^{\text{inner}}/{\mathrm{\lambda <!--\; ??\; -->}}_q^{\text{outer}}=0.44$ for favourable $\mathbf{B}\times <!--\; ??\; -->\mathrm{\nabla <!--\; ???\; -->}B$ ion drift direction and ${\mathrm{\lambda <!--\; ??\; -->}}_q^{\text{inner}}$/${\mathrm{\lambda <!--\; ??\; -->}}_q^{\text{outer}}=0.85$ for non-favourable drift direction. The different ratios are explained by vertical drifts, which are dependent on the toroidal magnetic field direction. (paper)

[en] Magnetic perturbation (MP) fields are currently studied in ASDEX Upgrade and many other tokamaks in terms of edge localized mode control and the implication onto steady state divertor power load and access to detachment. Previous studies at ASDEX Upgrade in low density, attached L-mode (Faitsch et al 2017 Plasma Phys. Control. Fusion 59 095006) are combined with high density L- and H-mode studies (Brida et al 2017 Nucl. Fusion 57 116006). A consistent interpretation is presented for the steady state power load variation due to the 3D MP. The deviation from an axisymmetric power load is reducing with increasing density, being an effect of the increasing broadening in the divertor region. No deleterious effect on the access to a detached divertor regime is found. Experimental results from ASDEX Upgrade reveal similar power load profiles for plasmas without and a toroidal averaged profile in presence of an external MP. EMC3-Eirene modeling is showing agreement only if screening currents in the plasma as a response to the external magnetic field are added. In plasma scenarios with field penetration and locking of internal modes a fundamentally different power load pattern is observed. The pattern is in agreement with edge ergodization and the extent can be used to determine the magnitude of current needed to describe the internal mode. (paper)

[en] Infrared (IR) thermography is widely used in fusion research to study power exhaust and incident heat load onto the plasma facing components. Due to the short pulse duration of today’s fusion experiments, IR systems have mostly been designed for off-line data analysis. For future long pulse devices (e.g., Wendelstein 7-X, ITER), a real time evaluation of the target temperature and heat flux is mandatory. This paper shows the development of a real time capable IR system for ASDEX Upgrade. A compact IR camera has been designed incorporating the necessary magnetic and electric shielding for the detector, cooler assembly. The camera communication is based on the Camera Link industry standard. The data acquisition hardware is based on National Instruments hardware, consisting of a PXIe chassis inside and a fibre optical connected industry computer outside the torus hall. Image processing and data evaluation are performed using real time LabVIEW

[en] In divertor tokamaks electric currents flowing in the Scrape-Off Layer (SOL) are a frequently observed and well known feature. However, the convective heat flux carried by these currents is often assumed to be small compared to the total parallel heat flux. In this paper it is shown that in ASDEX Upgrade the electric current, which was measured by Langmuir probes at the outer target, can be several times larger than the ion saturation current in low density L- and H-mode discharges. Therefore they have to be taken into account when calculating the target heat flux from the Langmuir probe data. Moreover, a detailed analysis reveals that for low density L- and H-mode discharges the heat flux profile is mainly determined by the current profile. By applying a 1D heat flux transport model it is demonstrated that the electric current can carry a large fraction (>  50 %) of the electron heat flux along the SOL convectively. Thus a Spitzer-Härm approach, where it is assumed that heat is transported only conductively, might not be valid in the investigated regime. (paper)

[en] In this work we carry out quantitative measurements of particle and heat transport associated to SOL filaments in a tokamak, and relate density shoulder formation to the advection of energy in the far SOL. For the first time, this attempt includes direct measurements of ion and electron temperatures for background and filaments. With this aim, we combine data from a number of equivalent L-mode discharges from the ASDEX Upgrade tokamak in which different probe heads were installed on the midplane manipulator. This approach is validated by a comparison with independent diagnostics. Results indicate an increase of heat transport associated to filaments after the shoulder formation. Several centimeters into the SOL, filaments are still found to carry a substantial fraction (up to one fifth) of the power ejected at the separatrix. (paper)