Pedestal Turbulence Dynamics in ELMing and ELM-free H-mode Plasmas

Measurements of long wavelength density fluctuations in the pedestal region of DIII-D plasmas have revealed several important features of edge instabilities during type I edge localized mode (ELMing) and ELMfree discharges. These features, measured with a 2D array of beam emission spectroscopy channels, include: toroidal field and spatial dependence of two distinct frequency bands (50–150 kHz and 200–400 kHz) of density fluctuations modulated with the ELM cycle; the low frequency band (50–150 kHz) has dynamics correlated with that of the pedestal electron pressure. High frequency coherent modes observed during quiescent H-mode plasmas exhibit long poloidal scale length, short decorrelation times of a few μs and mode frequency close to ion diamagnetic frequency, qualitatively similar to characteristics expected for kinetic ballooning modes. Core turbulence increases dramatically in plasmas for which ELMs are stabilized via application of an n=3 resonant magnetic perturbation. These observations provide key insights into the underlying turbulence and instability properties that limit pedestal height and width, and will help develop a predictive model for pedestal.