30 / 2021-04-09 18:56:23

Turbulence in the SOL and island divertor of W7-X

stellarator,SOL,Turbulence

In magnetic fusion devices, the heat flux width of the Scrape-Off Layer (SOL) is a fundamental control parameter for safe divertor operation. The SOL width is determined by the competition between parallel transport to the targets and (anomalous) perpendicular transport, where the latter is widely assumed to be dominated by turbulent transport. While one might naively assume that the magnitude of turbulent transport depends on the gradients in the SOL, turbulence spreading of high turbulence levels into shallow gradient regions of the SOL is commonly observed in tokamaks.



In the optimized stellarator Wendelstein 7-X, the plasma exhaust is controlled by an island divertor configuration, where the Scrape-Off Layer (SOL) is formed by a chain of resonant magnetic islands directly adjacent to the last closed flux surface. The islands effectively widen the SOL, as they offer a quasi-radial transport channel on the radial scale of the island width simply by parallel transport along field lines on island flux surfaces. As a consequence, the gradients in the SOL of the island divertor can have a (non-uniform) 3D distribution and are often shallow compared to a tokamak SOL.



In this presentation, the relation of SOL gradients to SOL turbulence in the island divertor of W7-X is investigated. We show that often (especially in the private flux region and shadowed regions of the islands) a linear relation between perpendicular pressure gradients ∇rp and perpendicular turbulent transport Γr is observed, implying that the turbulence originates locally in the SOL, and allowing to describe Γr as a diffusive process for modeling purposes. Furthermore, blob-filaments that radially traverse the entire SOL (and significantly contribute to perpendicular transport in tokamaks) are not observed in W7-X due to the smaller magnetic curvature of W7-X and therefore smaller drive of the interchange instability.



In certain regions of the magnetic islands, however, the relation Γr~∇rp is broken as even for very flat profiles considerable fluctuation amplitudes and perpendicular particle flux is seen, indicating additional transport processes that appear to be associated to the magnetic islands and therefore hinting at turbulence spreading. To uncover these processes, the propagation of fluctuations from correlation analyses and the possible role of modes is discussed.