Intermediate-Wavenumber Pedestal Fluctuation Measurements with Charge eXchange Imaging on DIII-D

Localized 2D measurements of low-to-intermediate wavenumber density fluctuations in the H-mode pedestal reveal a range of broadband turbulent modes during the inter-ELM cycle. These measurements are obtained with a new higher radial resolution multichannel Charge eXchange Imaging (CXI) diagnostic. CXI observes charge exchange emission between neutral beam atoms and intrinsic carbon impurity ions by detecting the n=8-7 hydrogenic emission line. Fluctuations are measured in a 12 (radial) x 2 (poloidal) array at 2 MHz with a resolution of dR ~ 0.4 cm and dZ ~ 1.2 cm. Optimal CXI signal to noise ratios are predicted for experiments that maximize the NBI injection voltage and minimize the background neutral density content. Initial measurements include observations of broadband carbon density fluctuations in ELM’ing H-mode pedestal regions near rho=0.95-1.0. First measurements demonstrate excellent spatial resolution and signal to noise ratios at all frequencies, and strongly correlated fluctuation measurements remain coherent up to 400 kHz. Two distinct and counter-propagating modes are occasionally observed near the base of the pedestal, and EHO observations in QH-Mode are observed alongside coherent turbulence.