Turbulent Ion Temperature Fluctuation Measurements on DIII-D

A novel dual-channel, high throughput, high efficiency, charge exchange spectrometer (UF-CHERS) has been developed to measure impurity ion temperature and toroidal velocity fluctuations with 1 $\mu$s time resolution. These measurements are primarily needed for identifying underlying instabilities and validation of transport simulation codes. Based on the measured photon flux levels for the entire spectral line, a photon noise floor $\sim$1\% is expected. Statistical averaging over long data records should provide turbulence measurements to 1/5 to 1/10 of the estimated photon noise floor. Correlation measurements in DIII-D plasmas demonstrate broadband ion-temperature fluctuations from 0-150 kHz, while cross-correlation with BES measurements of density fluctuations exhibits cross-power between density and ion temperature fluctuations to 250 kHz in ECH-heated low-collisionality L-mode discharges. The fluctuation level is $\tilde{T}_i/T_i \sim 2$\%, with $\langle \tilde{n}\tilde{T}_i\rangle$ correlation lengths in the range of 3-5 cm. A new detector system will provide improved signal-to-noise level and higher frequency measurements.