Small-angle scattering and spatial resolution of fluctuation reflectometry: comparison of 2D analytical theory with numerical calculations

Three methods to study the effect of density fluctuations on the reflected signal, based on a 2D full-wave analysis and Born approximation numerical and analytical calculations, are developed. The explicit analytical expression for the signal produced by an island-like perturbation running in poloidal direction is obtained and shown to be in nice agreement with results of numerical techniques for small level density perturbations. It is shown, both numerically and analytically, that due to small-angle scattering effects, fluctuations far from the cut-off layer may produce a significant and, in some realistic cases, dominant response. The conditions under which the fluctuation reflectometry signal is generated only in the cut-off vicinity are given.