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A linear response, local model for the DBS amplitude applied to gyrokineticsimulations shows that radial correlation Doppler reflectometry measurements(RCDR, Schirmer et al., Plasma Phys. Control. Fusion 49 1019 (2007)) are notsensitive to the average turbulence radial correlation length, but to acorrelation length that depends on the binormal wavenumber $k_\perp$ selectedby the Doppler backscattering (DBS) signal. Nonlinear gyrokinetic simulationsshow that the turbulence naturally exhibits a non-separable power law spectrumin wavenumber space, leading to a power law dependence of the radialcorrelation length with binormal wavenumber $l_r \sim C k_\perp^{-\alpha}(\alpha \approx 1)$ which agrees with the inverse proportionality relationshipbetween the measured $l_r$ and $k_\perp $ in experiments (Fernandez-Marina etal., Nucl. Fusion 54 072001 (2014)). This offers the possibility ofcharacterizing the eddy aspect ratio in the perpendicular plane to the magneticfield and motivates future use of a non-separable turbulent spectrum toquantitatively interpret RCDR and potentially other turbulence diagnostics. Theradial correlation length is only measurable when the radial resolution at thecutoff location $W_n$ satisfies $W_n \ll l_r$, while the measurement becomesdominated by $W_n$ for $W_n \gg l_r$. This suggests that $l_r$ is likelyinaccessible for electron-scale DBS measurements ($k_\perp\rho_s > 1$). Theeffect of $W_n$ on ion-scale radial correlation lengths could benon-negligible.

Interpreting Radial Correlation Doppler Reflectometry using Gyrokinetic Simulations