A RADAR-like modification of enhanced microwave-scattering diagnostics is investigated both analytically and numerically. The method uses the fact that backscattering off the density fluctuation is localized to a narrow layer, whose position depends on the fluctuation wavenumber. Therefore, when a plasma is irradiated by a short microwave pulse, the time delay of the backscattered pulse contains the information on the shape of the fluctuation spectrum. This effect is strongly pronounced for scattering near the upper hybrid resonance where both incoming and outgoing waves are very slow. In laboratory and small tokamak plasmas, this technique permits observation of small-scale density fluctuations with a high-spatial and wavenumber resolution.
Fluctuation spectra and velocity profile from Doppler backscattering on Tore Supra