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(theta) less than or similar to k(theta) ,match (k(theta),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. can be divided into three regions, weak asymmetrical range with k(theta)/k(theta). 0.15 (k(0) is the vacuum wavenumber), harmonics range with 0.15 less than or similar to k(theta)/k(0) <= 0.4, and Bragg scattering range of 0.4 less than or similar to k(theta)/k(0) <= 0.7. In the case when the structure is located away from the cutoff layer, the final complex S( f) spectrum is