Reply to criticism of the scintillation technique to measure plasma density fluctuations by Weisen (1986)

H. Weisen in a publication 'Imaging methods for the observation of plasma density fluctuations' (ibid., vol.28, p.1147, 1986) refers to a paper by Sharp (Plasma Physics vol.25, p.781 1983) and assesses that the technique of phase scintillations (phase fluctuations impressed on a diffracted electromagnetic wave) described in that paper has serious limitations in its ability to measure those plasma density fluctuations for which it was designed. This was said to be in part due to its inability to measure absolute phase variations, while, in order to obtain sufficient sensitivity, the necessary distance from the plasma at which observations must be made violated the requirement that they made in the near field. These conclusions are based on the totally incorrect assertion that Sharp proposed that the measurements be performed without the use of a reference beam or use of an interferometer. Sharp points out that his paper clearly describes measurements made with an interferometer operating at 2 mm wavelength to measure the phase fluctuations and that the measurement technique did indeed have a reference 'beam', did in fact measure absolute phase, and consequently has the sensitivity to measure phase fluctuations in the near field.