Speaker
Description
In the island divertor of Wendelstein 7-X, modular divertor targets intersect a chain of resonant magnetic islands that act as the scrape-off layer (SOL). The island SOL is characterized by long parallel connection lengths of several 100m, resulting in a high efficiency of perpendicular (radial and bi-normal) transport compared to parallel transport. The two main perpendicular transport channels are believed to be turbulent transport and stationary ExB drift flows, where the former is dominant in the radial direction (normal to the last closed flux surface) due to the main pressure gradients. The ExB flows are mostly oriented in the bi-normal direction, which can deviate from the island flux surfaces.
Due to the 3D geometry and the toroidally discontinuous divertor targets, perpendicular pressure gradients are not uniform, resulting in a spatially varying impact of turbulent transport on the SOL plasma. Further, radial electric fields feature multiple shear layers (including sign flips) across the SOL. As a consequence, counter-propagating bi-normal ExB drift flows are observed [1]. The 3D drift and turbulent transport act back on the plasma parameter distribution, resulting in a self-consistent equilibrium.
We employ two key diagnostic systems installed in approximately toroidally symmetric positions of W7-X and give insight into both turbulence and drifts. Reciprocating electric probe arrays provide electron temperature and density as well as estimates of electric fields and turbulent radial particle transport via floating potential measurement. Gas Puff Imaging observes H-alpha emission fluctuations from a localized gas puff with high temporal resolution. From these measurements, turbulence properties and drift flows (via spatio-temporal analysis) are obtained [2].
We find that the velocity and therefore the impact of drift flows decreases for high plasma densities. Accordingly, the asymmetries between upper / lower divertors, which are presumably caused by drift flows, decrease for higher densities, but still persist up to (at least) $n_{av}=7\cdot 10^{19}m^{-3}$. Turbulent transport, however, increases in terms of absolute particle fluxes while moving to higher plasma densities. This is mostly a consequence of steeper SOL density gradients, while diffusion coefficients estimated from particle flux and density gradient measurements remain roughly in line with initial findings in the test divertor operation phase of W7-X [3].
[1] Killer NF 65 056026 (2025)
[2] Baek NME 43 101937 (2025)
[3] Killer NF 61 096038 (2021)