Doppler Reflectrometry

Doppler reflectometry is micro-wave scattering diagnostic which is based on Bragg-back-scattering. The incident microwave is reflected at a cut-off position that depends on the local plasma density and magnetic field (m=0). Doppler reflectometry utilizes the m=-1 scattered component for which the Bragg-condition reads k_y=2k₀sin(θ), where k_y is the fluctuation wavenumber and θ the incidence angle to the flux surface normal. From the measured Doppler shift ω=k_yu_y the perpendicular velocity can be estimated which allows to calculate the radial electric field if the fluctuation phase velocity is small. For the W7-X systems the line-of-sight geometry is fixed. Radial resolution is obtained by changing the microwave frequency. Since θ changes according to the probed radius the Bragg condition selects a different ky at each radius. The different reflectometers allow to characterize zonal perturbations (k_|| = 0) as well as the radial correlation of fluctuations.