Pedestal instabilities during the L-H transition in ASDEX-Upgrade plasmas

Abstract The high confinement regime, or H-mode, is the result of a transport barrier at the plasma edge, where the pressure rises sharply to form a pedestal. This paper reports on a collection of experimental observations of instabilities and turbulence occurring in this decisive and narrow edge region. Using the high sampling rate enabled by an ultra-fast frequency sweeping reflectometer, a high spatial and temporal resolution analysis of density, gradients, and turbulence allows new and thorough insights on plasma instabilities arising from the microturbulence and flow competing system during the I-phase to the edge localized modes and blob-filaments during the established H-mode. The measurements provide the observation on how different instabilities can arise in the pedestal during the L-H transition and address their specific turbulent natures.