Reconstruction of wavenumber spectra of plasma turbulence in microwave reflectometry

A method of measuring the fluctuation spectrum by means of conventional microwave reflectometry is proposed and discussed. The technique is based on the analytical solution of the full-wave equation of microwave propagation and scattering inside the plasma. The reflectometer signal simulated using the reconstructed fluctuations is consistent with the amplitude spectrum of the experimental reflectometer signal. The accuracy of the method is tested on a number of specially modelled data sets, which correspond to isotropic and anisotropic fluctuations, multiple scattering by strong turbulence, and random motion of turbulent modes. We conclude that for the case of small fluctuations our method allows us to reconstruct the profile of the poloidal wavenumber spectrum with a high accuracy, while the turbulence amplitude is estimated within a 40-60% error bar. Multiple scattering deteriorates both the wavenumber spectrum profile and turbulence amplitude estimations. As an illustration, we apply our technique to estimate the fluctuation level and poloidal wavenumber spectrum shape in the ELM-free H mode plasma of the JT-60U tokamak