Investigation of phase modulation and propagation-route effect from unmatched large-scale structures for Doppler reflectometry measurement through 2D full-wave modeling

To interpret the common symmetric peaks caused by the large-scale structure in the complex S(f) spectrum from the heterodyne Doppler reflectometry (DR) measurement in EAST, a 2D circular-shaped O-mode full-wave model based on the finite-difference time-domain method is built. The scattering characteristics and the influences on the DR signal from various scales are investigated. When the structure is located around the cutoff layer, a moving radial or poloidal large-scale structure k_θ ≲ k_{θ, match} (k_{θ, match} is the theoretic wavenumber of Bragg scattering) could both generate an oscillation phase term called ‘phase modulation’, and symmetrical peaks in the complex S(f) spectrum. It was found that the image-rejection ratio A₋₁/A₊₁ (A_±1 represents the amplitudes of ±1 order modulation peaks) could be a feasible indicator for experiment comparison. In the case when the structure is near the cutoff layer with the same arrangement as the experiment for the edge DR channel, the curve of A₋₁/A₊₁ versus k_θ can be divided into three regions, weak asymmetrical range with k_θ/k₀ ≲ 0.15 (k₀ is the vacuum wavenumber), harmonics range with 0.15 ≲ k_θ/k₀ ≲ 0.4, and Bragg scattering range of 0.4 ≲ k_θ/k₀ ≲ 0.7. In the case when the structure is located away from the cutoff layer, the final complex S(f) spectrum is the simple superimposing of modulation and Bragg scattering, and the modulation peaks have an amplitude response nearly proportional to the local density fluctuation, called the ‘propagation-route effect’. Under the H-mode experiment arrangement for the core DR, a critical fluctuation amplitude A m p\left(n_\mathrme, \text Mod.@route \right) / A m p\left(n_\mathrme, \text Tur.@MSA \sim 1.3-4.1\left(\operatornameAmp\left(n_\mathrme, \text Mod.@route \right)\right.\right. refers to the pedestal large-scale structure amplitude and Amp(n_{e, Tur.@MSA}) refers to turbulence amplitude at the main scattering area) is needed for the structure in the pedestal to be observed by the core DR measurement. The simulations are well consistent with the experimental results. These effects need to be carefully considered during the DR signal analyses as the injecting beam passes through the plasma region with large-scale structures.