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

Sheared flows, thought to be generated by turbulence in magnetized fusion p lasmas, 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 turbu lent fields provides an experimentally accessible test of the turbulence-ge nerated shear flow hypothesis. Results from the Continuous Current Tokamak (CCT), Princeton Beta Experiment-Modified (PBX-M), and DIII-D tokamaks indi cate that the relative strength of three-wave coupling increases during low -mode to high-mode (L-H) transitions and that this increase is localized to the region of strong flow and strong flow shear. These results appear to b e qualitatively consistent with the turbulence-generated shear flow hypothe sis. (C) 2001 American Institute of Physics.