Ray Tracing for Doppler Backscattering System in the Experimental Advanced Superconducting Tokamak

The Doppler backscattering system has been widely used for turbulence measure?ments, and the microwave beam will be backscattered near the cut-o? layer when the Brag con?dition is fulfilled. In tokamak, the ray-tracing code is used to obtain the radial position and perpendicular wave number of the scattering layer for turbulence velocity measurement and the WKB (Wentzel-Kramers-Brillouin) approximation should be satisfied for optical propagation. To calculate the backscattering location and wave number at the cut-o? layer only, a single ray tracing in the cross section is enough, while for spatial and wave number resolution calculation, multiple rays reflecting the microwave beam size should be used. Considering the angle between the wave vector and the magnetic field, a three-dimension quasi-optical Gaussian ray tracing is sometimes needed.