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Microwave reflectometry is one of the few non-intrusive diagnostics that can be applied to measure density profiles and fluctuations in fusion plasmas. Since the temporal resolution is sufficiently high the reflectometry data are an important input for the modelling and understanding of plasma turbulence. It is, however, still under discussion how the absolute value of the density fluctuation at the cut-off can be deduced from measured phase fluctuations. There are additional contributions of small angle forward scattering and of resonant Bragg backscattering, both located outside the cut-off. We address this topic by performing 1D full wave simulations in a turbulent model plasma, comparing the model density fluctuations with those of the model reflectometry phase. In order to check whether the 1D results are valid on a fusion machine we study in 2D the propagation of a Gaussian beam in a snapshot of the model turbulence. Density profile and beam width parameters correspond to the set-up on Tore Supra. A linear 1D relation can be established between the reflectometry phase fluctuation level σrms and the density fluctuation level given the Born approximation is valid. For O-mode launched into a linear density profile with the gradient length L, and a superimposed homogeneous turbulence with a Gaussian correlation function, the relation is displayed in (1).

On the Absolute Value of the Density Fluctuation Measured by Reflectometry