Reflectometry and Backscattering for Broad-$k_r$ Microturbulence Measurements in NSTX

On NSTX, the unique combination of reflectometry hardware (FM-CW, fixed-frequency, and correlation reflectometers) is well-suited to turbulence measurements. Recently, the FM-CW reflectometers have been used as radial backscattering diagnostics for probing microturbulence over a broad range of radial wavenumbers ($k_r$$\sim$0$-$20 cm$^{-1}$). This new method utilizes the reflection from the cutoff layer to determine a detailed reconstruction of the density profile. Time-of-flight information is then used to map the backscattered signal to radial locations away from the cutoff layer, allowing visualization of the turbulence intensity in $k_r$ vs $R$ (major radius) space with excellent space and time resolution. Further details of the method are demonstrated using modeled turbulence and the GPU-accelerated UCLA 1-D and 2-D FDTD full-wave codes. Initial measurements during the L-H transition show a steep drop in the turbulence intensity over a broad range of $k_r$ and localized to a narrow spatial region around the edge transport barrier location.