Direct evidence of zonal flow (ZF) predator-prey oscillations and the synergistic roles of ZF- and equilibrium $\mathit{E}\times \mathit{B}$ flow shear in triggering the low- to high-confinement (L- to H-mode) transition in the DIII-D tokamak is presented. Periodic turbulence suppression is first observed in a narrow layer at and just inside the separatrix when the shearing rate transiently exceeds the turbulence decorrelation rate. The final transition to H mode with sustained turbulence and transport reduction is controlled by equilibrium $\mathit{E}\times \mathit{B}$ shear due to the increasing ion pressure gradient.

• Received 23 August 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.155002