Turbulent Ion Fluctuation Measurements in Negative Triangularity Plasmas

A new detector array on the UF-CHERS (Ultra Fast CHarge Exchange Recombination Spectroscopy) diagnostic at DIII-D has resulted in significantly improved signal to noise ratio and sensitivity to ion thermal fluctuations. UF-CHERS measures local, long-wavelength Carbon density, ion temperature, and toroidal velocity fluctuations at turbulence-relevant spatiotemporal scales (1 $\mu $s time resolution, \textasciitilde 1 cm spatial resolution which is approximately the turbulence correlation length) from emission of the CVI n$=$8$\to $7 transition. UF-CHERS and BES fluctuation measurements were obtained in equivalent positive and negative triangularity ($\delta )$ discharges with an L-mode edge to compare with theoretical models of turbulence-driven transport and elucidate the mechanisms for improved confinement with negative-$\delta $. Finite coherence is observed between UF-CHERS and co-located BES channels, demonstrating that critical multifield fluctuations such as \textless n*T$_{\mathrm{i}}$\textgreater and \textless n*v$_{\mathrm{tor}}$\textgreater can be measured. Initial analysis shows positive-$\delta $ has radially decreasing, low coherency between \textasciitilde 20-200 kHz for main ion density (BES) and Carbon density, ion temperature, and toroidal rotation (UF-CHERS) fluctuations.