ORDINARY-MODE REFLECTOMETRY - MODIFICATION OF THE SCATTERING AND CUTOFF RESPONSES DUE TO THE SHAPE OF LOCALIZED DENSITY-FLUCTUATIONS

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 th e perturbation. It is shown that it strongly depends upon the perturba tion shape and size. For a small perturbation wavenumber, the response is due to the oscillation of the cut-off layer. For larger wavenumber s, two regimes of resonant Bragg scattering are found: for a broad per turbation, the phase response is an image of the perturbation itself; for a narrow perturbation, it is an image of the Fourier transform. Th ese features are enhanced for a broadband perturbation (modulated squa re 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 cu t-off due to the sharp boundary effects of this perturbation. Because of this peculiarity, the phase response obtained for a damped square p erturbation reproduces the results of an earlier experiment (Rhodes er al, Rev. Sci. Instrum. 1992).