Impact of drift flows and turbulence on island divertor operation in Wendelstein 7-X

The island divertor in Wendelstein 7-X employs a chain of stationary magnetic islands to separate the confined plasma from the divertor targets. In this configuration, the scrape-off layer (SOL) exhibits complex patterns of counter-propagating bi-normal E × B plasma flows with typical velocities of a few km s−1. Due to the long parallel connection length in the island divertor, such flows can well compete or dominate parallel transport on the open field lines to the divertor targets, which could significantly alter heat and particle flux patterns. Reversed field experiments clearly indicate an important role of drift flows for density distributions in the SOL and at the divertor. These effects cannot be reproduced by state-of-the-art models such as EMC3-EIRENE, where drift flow physics is not yet included. The mismatch between experiment and simulation poses a critical challenge in predicting heat loads for optimizing W7-X divertor operation at higher heating powers, and for the design of future stellarator reactors. In a separatrix plasma density scan in attached plasmas, a complex picture emerges: drift flow velocities decrease slightly towards higher densities, and divertor heat loads become more up-down symmetric. However, the density distributions in the SOL and at the divertor still diverge from expectations. The role of turbulent transport as another cross-field transport mechanism in this context is explored.