The influence of edge magnetic islands on scrape-off layer plasma and energy confinement of W7-X

Compared to tokamak magnetic field configuration, the scrape-off layer (SOL) of Wendelstein 7-X stellarator is much more complicated because of the presence of large natural magnetic islands. These islands are intended to separate the confined plasma from the divertor targets by providing well defined exhaust channels for heat and particles to the divertor. However, in this configuration, the magnetic islands are observed to introduce complex patterns of sheared plasma ExB drift flows in the SOL, with typical velocities of a few km/s. These flows compete with parallel transport channels and lead to unexpected (up-down asymmetric) density distributions in the divertor. Such effects are raising concerns about accuracy of EMC3-Eirene predictions for the design of future stellarator reactor, if drift flows are still not included. Magnetic configurations, where islands are pushed inside the last closed flux surface, surprisingly lead to confinement improvement up to 30%. Under such conditions, intermittent crashes in plasma stored energy occur, superficially resembling the H-mode. These crashes, however, are shown to be a large scale m=1 modes, which originate from the outer-core plasma, that temporarily suppress turbulent fluctuations as density profile gradient is temporarily increased. The island-induced sheared flows inside the confinement region are shown to play a key role in such confinement-improved scenarios.