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We report the observation of a quasi-coherent density fluctuation (QCF) bythe Doppler backscattering system in the scrape-off layer (SOL) region of theDIII-D tokamak. This QCF is observed in high-power, high-performance hybridplasmas with near double-null divertor (DND) shape during the electroncyclotron heating period. This mode is correlated with a steepened SOL densityprofile and leads to significantly elevated particle and heat fluxes betweenELMs. The SOL QCF is a long-wavelength ion-scale fluctuation and propagates inthe ion diamagnetic direction in the plasma frame. Its radial expanse is about1.5-2 cm, well beyond the typical width of heat flux on DIII-D. Also, the SOLQCF does not show any clear dependence on the effective SOL collisionality andthus may raise issues on the control of plasma-material interactions in lowcollisionality plasmas in which the blob-induced transport is reduced. A linearsimulation using BOUT++ with a 5-field reduced model is performed and comparedwith experimental observations. In simulation results, an interchange-likedensity perturbation can be driven by the SOL density gradient, and its peaklocation and the radial width of the density perturbation are in agreement withthe experimental observations.

Observation of quasi-coherent density fluctuation in scrape-off layer enhancing boundary transport in high-betaN hybrid plasmas on DIII-D