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Description
The leading concept for power exhaust in stellarators is the island-divertor configuration. At Wendelstein 7-X (W7-X) the scrape-off layer (SOL) of the standard magnetic configuration is characterized by a 5 island-chain. The islands topology and their interaction with the divertor targets lead to a complex behavior of the impurity radiation structure. Moreover, the more dominant bi-normal transport compared to the SOL of tokamaks further complicates predictions. In this context, the 3D transport code EMC3-Eirene is often used to assess the SOL of W7-X. This contribution will focus on the validation of such simulations with experimental total radiated power data.
In [S. Togo et al (submitted) Plasma Phys. Control. Fusion], density scans have been simulated with EMC3-Eirene for 5 different magnetic configurations with planar-coil currents $I_{PC}=-1.5,-1,0,1,1.5$ kA, corresponding to an increasing outward radial shift of the islands. W7-X density scan-experiments with corresponding magnetic configurations are chosen and data from the 3 available total radiated power measurement diagnostics analyzed (imaging bolometer, triangular cross-section bolometry system and bean-shaped bolometry system). However, while the imaging bolometer offers poloidal and toroidal coverage of the upper divertor region, the latter two are characterized by a mainly poloidal coverage of the triangular and tear-shaped cross-sections respectively. In order to compare the simulated and experimental radiation data, synthetic diagnostic data generation tools and tomographic inversions are employed. This allows to compare the dynamics of the poloidal localization of the radiation features, for the different geometries. The simulations show a shift of the target-localized radiation towards the island O-points, when increasing the density at the separatrix. The features are gradually spread throughout the SOL and, in the $+1.5$ kA case, eventually start penetrating the separatrix for $f_{rad}\simeq1$. A similar effect has been observed in [V.R Winters et al (2024) Nucl. Fusion 64 126047] for the low-$\iota$ magnetic configuration of W7-X. Moreover, the observed broadening of the radiation features seen in the simulations is associated with a change in the density scaling of $f_{rad}$. Previous observations have already seen a discrepancy between simulations and experiments in the dependence of these trends on the radial position of the islands. For better characterization, experimental density profiles will be employed in addition to the bolometry data. Since the mentioned studies suggest a strong correlation between these dynamics and the ability to reach high $f_{rad}$, this systematic experimental validation of the trends predicted by EMC3-Eirene is expected to be of relevance to the topic.