On the nonlinear turbulent dynamics of shear-flow decorrelation and zonal flow generation

Sheared flows, thought to be generated by turbulence in magnetized fusion plasmas, are predicted to mediate the transport of mass, momentum, and heat across the shear flow region. In this paper we show that an examination of three-wave coupling processes using the bispectrum and bicoherence of turbulent fields provides an experimentally accessible test of the turbulence-generated shear flow hypothesis. Results from the Continuous Current Tokamak (CCT), Princeton Beta Experiment–Modified (PBX–M), and DIII–D tokamaks indicate that the relative strength of three-wave coupling increases during low-mode to high-mode $\mathrm{(L–H)}$ transitions and that this increase is localized to the region of strong flow and strong flow shear. These results appear to be qualitatively consistent with the turbulence-generated shear flow hypothesis.