Measurements of Short Wavelength Plasma Fluctuations Using the DIII-D Phase Contrast Imaging Diagnostic

The DIII-D Phase Contrast Imaging (PCI) diagnostic has been upgraded and used to measure turbulence in the outer plasma region ($0.7 < r/a < 1$) covering an operational range of 10~kHz through 10~MHz and 2-30~cm$^{-1}$. A novel rotating mask has been used to measure turbulence as a function of propagation angle about the PCI chord. This technique provides localized measurements along the PCI chord for turbulence with $k_\parallel \sim 0$, and an estimate of the turbulence $k_\parallel$ value otherwise. Long wavelength ($\vert k\vert \buildrel<\over\sim 12~$cm$^{-1}$) turbulence is localized to within the instrumental width of the last closed flux surface (LCFS) ($r/a\buildrel>\over\sim 0.9$). Modes with finite (and theoretically unexpected) {\it parallel~wavenumber} have been seen to propagate at angles as large as $k_\parallel/k \sim 0.1-0.4$. Due to the finite $k_\parallel$, these modes cannot be localized with the present techniques. A theoretical explanation for these modes is lacking at the present time.