We report on comprehensive experimental studies of turbulence spreading in edge plasmas. These studies demonstrate the relation of turbulence spreading and entrainment to intermittent convective density fluctuation events or bursts (i.e. blobs and holes). The non-diffusive character of turbulence spreading is thus elucidated. The turbulence spreading velocity (or mean jet velocity) manifests a linear correlation with the skewness of density fluctuations, and increases with the auto-correlation time of density fluctuations. Turbulence spreading by positive density fluctuations is outward, while spreading by negative density fluctuations is inward. The degree of symmetry breaking between outward propagating blobs and inward propagating holes increases with the amplitude of density fluctuations. Thus, blob-hole asymmetry emerges as crucial to turbulence spreading. These results highlight the important role of intermittent convective events in conveying the spreading of turbulence, and constitute a fundamental challenge to existing diffusive models of spreading.
This paper reports on experiments that reveal the non-diffusive nature of turbulence spreading in edge plasmas. It shows that intermittent convective events, such as blobs and holes, are crucial in conveying the spreading of turbulence, challenging existing diffusive models. The turbulence spreading velocity is found to correlate with the skewness and auto-correlation time of density fluctuations, with outward and inward spreading driven by positive and negative density fluctuations, respectively.