Doppler Reflectometry Measurements of Medium Wavenumber Density Fluctuations and Zonal Flows in DIII-D

Doppler reflectometry is a versatile diagnostic for poloidal plasma flow measurements and local density fluctuation spectra. Depending on the launch angle and frequency of the probing beam, the signal back-scattered from the plasma cut-off layer is sensitive to density fluctuations at a specific poloidal wavenumber $k_{\theta }$ (3 cm$^{-1}$~$<~k_{\_}$~$<$ 9 cm$^{-1}$, calculated using GENRAY ray tracing code). The plasma flow velocity $v_{\theta }$ is obtained with high time resolution from the Doppler shift \textit{$\omega $}$_{D}$ of the back-scattered signal ($v_{\theta }$\textit{ = $\omega $}$_{D}/k_{\theta }).$ Doppler reflectometry is well-suited for the detection of zonal flows, characterized by poloidal flow fluctuations ($v_{\theta }~=$~$k_{r}\Phi $/$B_{\phi })$. Zonal flows are thought to regulate the local turbulence level and radial correlation. We present first reflectometry measurements of geodesic acoustic modes (GAMs) and low frequency zonal flows in DIII-D L{\-}mode plasmas (0.6~$<$~$r/a$~$<$~0.9). The interaction of these time-dependent plasma flows with medium wavenumber density fluctuations is investigated in order to study turbulence self-organization.