Advanced Ion Temperature and Velocity Fluctuation Diagnostic Design and Preliminary Measurements

A high-efficiency, high-throughput custom spectroscopic system is being designed and implemented at DIII-D to measure localized, long-wavelength ion thermal fluctuations associated with drift wave turbulence. A high-transmission grism (prism-coupled transmission grating) and high-throughput collection optics are employed to observe Doppler-shifted emission from the $n=\,$8-7 transition of C VI centered near $\lambda =\,$529~nm. The diagnostic achieves 0.25~nm resolution for 2.0~nm spectral band using eight discrete spectral channels. A turbulence-relevant time resolution of 1~$\mu$s will be achieved using high-speed photodiodes and low-noise preamplifiers. The system sensitivity should allow for measurements of normalized ion temperature fluctuations on the order of $\tilde{T}_I/T_I \leq 1$\%. These measurements will be combined with 2D BES measurements to determine the local multi-field turbulence dynamics. The predicted signal-to-noise for turbulence measurements, signal analysis techniques, and preliminary data will be presented.