The Role of Zonal Flows and Predator-Prey Oscillations in Triggering the \hbox{L-H} Transition and in Internal Transport Barriers

Low frequency Zonal Flows (ZFs) have been observed to trigger the \hbox{L-H} transition near the power threshold, by either an extended predator-prey limit cycle oscillation (LCO [1]) or a short ($\sim 0.5-1.5$~ms) ZF burst executing only part of one limit cycle. Localized turbulence suppression ($k_\theta \rho_s\sim 0.5$) is initiated as the ZF shearing rate approaches the turbulence decorrelation rate. Turbulence-flow correlations (via Doppler Backscattering) show that the ZF amplitude and shear initially lag the rms fluctuation level by 90$^\circ$ during LCO, transitioning to 180$^\circ$ as the increasing ion pressure gradient and resulting equilibrium {\bf E}x{\bf B} shear secure the final transition to ELM-free \hbox{H-mode}. In a separate experiment, localized suppression of electron-scale fluctuations ($k_\theta \rho_s\sim 3$) by ZF shear is also observed in an internal thermal electron transport barrier. However, in contrast to the \hbox{L-H} transition, here the density fluctuation level is always anti-correlated (180$^\circ$ out of phase) with the ZF shearing rate. \vskip4pt\noindent [1] L. Schmitz et al., Phys. Rev. Lett. {\bf 108}, 155002 (2012).