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Evidence for Reynolds-stress driven shear flows using bispectral analysis: Theory and experiment


Spontaneous shear flow generation in magnetized fusion plasmas is thought to occur by an interaction of the turbulent Reynolds stress with the shear flow. This interaction can be viewed as a transfer of turbulent energy to the shear flow scales via a three-wave coupling process, which suggests that investigations of bispectral quantities may be of interest. In this paper, we discuss the theory of mean flow generation when described via the bispectrum. Simple theoretical analysis is used to demonstrate the connection between the bispectrum and the nonlinear amplification mechanism for zonal shear flows. We also discuss results from analysis of edge fluctuations from DIII-D during an L-H transition. These results indicate a transient rise and fall of three-wave coupling during the transition, which is spatially localized to inside the separatrix. Future efforts to determine the direction and rate of turbulent energy transfer during shear-flow formation will also be discussed.

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