Increased nonlinear coupling between turbulence and low-frequency fluctuations at the L-H transition - art. no. 135001

The nonlinear coupling between small scale high-frequency turbulence and la rger scale lower-frequency fluctuations increases transiently in transition s to improved confinement in the DIII-D tokamak. This increase starts befor e 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 t he transition, the coupling returns to L-mode levels. These results are con sistent with expectations for spontaneous transitions to improved confineme nt triggered by a turbulence-driven sheared flow.