Increased Nonlinear Coupling between Turbulence and Low-Frequency Fluctuations at the<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>L</mml:mi><mml:mi>−</mml:mi><mml:mi>H</mml:mi></mml:math>Transition

The nonlinear coupling between small scale high-frequency turbulence and larger scale lower-frequency fluctuations increases transiently in transitions to improved confinement in the DIII-D tokamak. This increase starts before the rapid turbulence suppression and E x B shear-flow development in the region that becomes the H-mode transport barrier/shear flow region. After the transition, the coupling returns to L-mode levels. These results are consistent with expectations for spontaneous transitions to improved confinement triggered by a turbulence-driven sheared flow.