Ordinary wave reflectometry in a plasma containing a localized density perturbation is studied with a one-dimensional (1D) model. The phase response is studied as a function of the wavenumber and position of the perturbation. It is shown that it strongly depends upon the perturbation shape and size. For a small perturbation wavenumber, the response is due to the oscillation of the cut-off layer. For larger wavenumbers, two regimes of resonant Bragg scattering are found: for a broad perturbation, the phase response is an image of the perturbation itself; for a narrow perturbation, it is an image of the Fourier transform. These features are enhanced for a broadband perturbation (modulated square wave) and scattering can occur over the whole region up to the cut-off. Furthermore, in that case there is a specific behaviour at the cut-off due to the sharp boundary effects of this perturbation. Because of this peculiarity, the phase response obtained for a damped square perturbation reproduces the results of an earlier experiment (Rhodes et al, Rev. Sci. Instrum. 1992).

Full-wave 1D simulations of fluctuation reflectometry are compared with the nonlinear theory recently published (Gusakov et al 2002 Plasma Phys. Control. Fusion 44 2327), for the case of time-independent turbulence, as well as for the case of space-time turbulence. Homogeneous and inhomogeneous turbulence models are considered. The simulations confirm quantitatively predictions of both linear and nonlinear theory for the correlation time and correlation length of the signal. The localization properties of the signal in the nonlinear regime are also found to be in good agreement with theory. Whereas the correlation time of the signal depends on the turbulence all over the plasma, its correlation length is essentially driven by the turbulence in the close vicinity of the cut-off. It turns out that theory gives a value of the correlation length within a factor of 2 even for turbulence levels as high as 20%, well beyond its validity domain.

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.

The phase shift due to the resonant Bragg scattering of an ordinary wave in a fluctuating plasma is numerically computed for large amplitude density fluctuations, i.e. well beyond the Born approximation. The phase response (the phase shift against the perturbation position in the density gradient) is strongly distorted with respect to the small amplitude case, and 2π phase jumps occur. For localized quasi-monochromatic fluctuations, the results are explained by an analytical model where the Helmholtz equation is approximated by a Mathieu equation. The maximum phase shift and the phase jumps are well predicted by the model. It turns out that the theoretical predictions also apply to moderate amplitudes. For an actual phase measurement where the number of different frequencies (i.e. the number of perturbation positions) is finite, most phase jumps are missed, due to the steepness of the phase jumps, and the measured phase shift increases continuously.