well and diagnosing fast-timescale changes with Phase Contrast Imaging on DIII-D*

Measurements with the Phase Contrast Imaging diagnostic on DIII-D have identified density turbulence situated near the center of the E_r well in H-mode plasmas. These fluctuations are seen prominently in ELM-free H-mode and wide-pedestal QH-mode plasmas. The turbulent mode forms on sub-ms times scales after L-H transitions and ELMs, consistent with the time scale for changes in the E_r well. The lab-frame phase velocity is approximately equal to the ExB velocity at the center of the E_r well and varies with the E_r well depth. Observations of this turbulence during limit-cycle oscillations at the L-H transition show that the E_r well collapses and reforms on ms timescales, progressively deeper with each cycle. Initial linear simulations with CGYRO have found a high phase velocity, ion scale instability near the center of the E_r well in wide-pedestal QH-mode plasmas in general agreement with these experimental observations. These observations provide information on the E_r well evolution on a time scale of 10s of μs, much faster than is available by direct observation.