Increased Nonlinear Coupling between Turbulence and Low-Frequency Fluctuations at theL−HTransition

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 $E\times B$ 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.