Turbulence-driven Transport and Spreading at the Edge of DIII-D L- and H-mode Plasmas

Turbulent radial particle flux, Γ_r , heat flux, Q_r, electron intensity flux, 〈V_rn²〉, and kinetic and potential energy flux, 〈 V_r(ε_k+ε_p)〉 are measured at the edge of DIII-D in: 1) L-mode, 2) during the transition from L- to H-mode, and 3) H-mode plasma. The radial turbulent particle and energy fluxes are directed outward both inside and outside the last closed flux surface (LCFS), but the turbulent energy flux is inward inside the LCFS and outward in the SOL. All turbulent fluxes are reduced during the transition to, and within H-mode. This work challenges recent theories of turbulence spreading that may result in a mechanism to control the scrape-off layer (SOL) width and the heat flux footprint in tokamak plasmas, which defines a critical limitation in future reactors.

The root-mean-aquare levels of density and electric field fluctuations are found to decay monotonically with radius towards the wall, and although a reduction (8X) of the fluctuation magnitude occurs in H-mode, the general profile of the turbulent energy flux remains similar.

The direction of the energy flux relates to regions of hole-blob pairs inside the LCFS and in the SOL. Indicating that energy flux is influenced by the holes, consistent with holes propagating radially inwards carrying colder plasma at lower density. Hole-blob dynamics has been understood as the result of interchange instabilities in the edge.