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

Abstract

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-shape 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 cut-off layer, a moving radial or poloidal large scale structure $k_\theta\lesssim k_{\theta,match}$ ($k_{\theta,match}$ 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 $A_{-1}/A_{+1}$ ($A_{\pm1}$ represent the amplitudes of $\pm1$ order modulation peaks) could be a feasible indicator for experiment comparison. In the case when the structure is near the cut-off layer same as experiment arrangement for the edge DR channel, the curve of $A_{-1}/A_{+1}$ versus to $k_\theta$ could be divided into three regions, weak asymmetrical range with $k_\theta/k_0\lesssim0.15$ ($k_0$ is the vacuum wavenumber), harmonics range with $0.15\lesssim k_\theta/k_0\lesssim0.4$, and Bragg scattering range $0.4\lesssim k_{\theta}/k_0\lesssim0.7$. In the case when the structure is located away from the cut-off 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 $Amp(n_{e,Mod.@route})/Amp(n_{e,Tur.@MSA})\sim1.3-4.1$ ($Amp(n_{e,Mod.@route})$ 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 passing through the plasma region with large scale structures.

Interplay among turbulence, flow and impurities for sustaining magnetic island

“…From figure 9, as the radiation increasing, the turbulence intensity and the absolute values of T e gradient inside MI all increased, while only the T e gradient at X-point remained almost unchanged. It should be noted that although the ray tracing in figure 5(a) suggested that the turbulence measurement location is at the inner side of MI (positive R 0 /L Te ), the possibility that turbulence may trigged by the inverted R 0 /L Te could not be ruled out, because that turbulence spreading [29,30] and DR measurement principle [31][32][33] should both be considered. So, the relationships between turbulence amplitude and positive R 0 /L Te and inverted R 0 /L Te are both plotted in panel (d).…”