Photon-Counting as a Technique for Determining Correlations in a Plasma

Abstract

It has been suggested 1 that counting photons scattered from a single-mode laser beam passing through a plasma, may be used for determining correlations in the plasma. Higher-order moments of the distribution of photon counts are related to an ensemble average of higher-order products of the electric field. Hence, this method yields also information about higher-order correlations of the electron density fluctuations, in contrast with the measurement of the scattering cross section which depends only on second-order correlations. In this paper, the connection between the statistics of photon counts and the properties of electron density fluctuations is discussed in detail, concentrating on the first and second moment of the photon count distribution. The incident wave is assumed to be plane and ideally monochromatic. For stable plasmas in a steady state, the results are written in terms of the distribution functions of the electron and ion velocities. Weakly turbulent unstable plasmas in the phase of exponential growth are also considered, using as a characteristic parameter the spectrum of the electric micro-field.