Density Fluctuation Measurement by Far Infrared Light Scattering

Density fluctuation observation can be obtained by scattering of electromagnetic radiation. New developments and applications are presented. By heterodyne detection the scattered signal is shown to be the time variation of a space Fourier transform of the density. The antenna beam, that especially defines the sensitivity, is experimentally investigated. The time variation is used in two occurrences, to check quantum efficiency uniformity on detectors area, and in an air turbulence where the interspectrum is obtained between two scattering devices looking at different regions. Calibration formulas are given for quantitative measurement of density fluctuation spectrum. Observations are also reported on a tokamak plasma, on the drift wave spectrum, and on a new second order group of oscillations that were observed during ion cyclotron radio-frequency heating. The analysis shows this new group is a forced oscillation by beating of the drift waves with the high-frequency heating wave, and its amplitude provides a measurement of the HF wave electric field.