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Microwave coherent backscattering from acoustic or electronic waves in a magnetized plasma

A microwave coherent backscattering experiment has been carried out on Mirabelle, a weakly ionized plasma device, with the objective of measuring the electron-density fluctuation level. The experiment is a preliminary step in order to prepare the detection system for a microwave stimulated-backscattering experiment. The incident electromagnetic wave is focused in front of a plane grid, which excites ion acoustic or electron Bernstein waves and induces fluctuations in the plasma. The backscattering signal is collected by the launching circuit and detected by homodyne mixing. The typical ratio of the scattered power to the incident power is about 10−12 and the relative density fluctuations is of the order of δne/ne ≈10−3 against a background electron density ne=(1–5)×109 cm−3. The backscattering measurement is also compared with Langmuir-probe measurements, and gives good agreement with the relative density fluctuations. The spectral width of the backscattered signal has also been studied, by taking into account effects due to the incident-wave focusing and plasma-wave damping.