Spatial and spectral resolution of the plasma Doppler reflectometry

An approach to numerical simulations of microwave scattering in Doppler reflectometry is developed that is based on calculations of the two-dimensional spatial weighting function in the Born approximation. The knowledge of the spatial weighting (or instrumental) function, which contains complete information on both forward and backward scattering processes, allows one to calculate the output signal of a quadrature diagnostic detector for any given distribution of the electron plasma density fluctuations. The weighting functions were computed for axisymmetric distributions of the background plasma density and for arbitrarily specified fields in the mouths of the emitting and receiving antennas. Simulations carried out for different model representations of the scattering fluctuations yielded quantitative estimates of both the spatial and wavenumber resolutions of Doppler reflectometry. The simulations showed that the resolution of the method proposed worsens with increasing curvature of the cut-off surface. Conditions are determined under which the resolution of the diagnostics in small- and medium-size tokamaks can be substantially improved by using converging microwave beams. The possibility of utilizing focused probing microwave beams in Doppler reflectometry on ITER-scale devices is discussed.