The measurement of large-scale density fluctuations in toroidal plasmas from the 'phase scintillation' of a probing electromagnetic wave

The plasma electron density fluctuation spectrum can be obtained from the measurement of the phase fluctuation spectrum of a probing electromagnetic wave measured some distance from the plasma. If the mean square fluctuation of the probing beam is much less than unity and the observer-plasma distance is well within the Fresnel distance of the irregularities a direct relationship exists between the density and phase spectra. As an example of this procedure, an experiment is described which uses a probing beam wavelength of 2 mm to measure the density irregularities. It is then proposed that a suitable wavelength for the present large toroidal plasma devices is in the 10 μm region. (author)