Ion Acoustic and Entropy Fluctuations in Collisional Plasmas Measured by Laser Light Scattering

The ion acoustic and entropy fluctuation resonance peaks in the electron density fluctuation spectrum S( vec k,omega) of high density, low temperature collisional plasmas (Ar, T_{e} = 2 eV, N_{e} = 1 times 10^{17 } cm^{-3}; He, T_{e} = 2.3 eV, N _{e} = 1 times 10^{17} cm ^{-3}) are measured by means of CO_2 laser scattering. The spectrum consists of three peaks, two due to ion acoustic waves, and a central entropy fluctuation peak. The ion acoustic peaks (at omega ~ kC_{s}) were measured in the previous work (Mostovych, 1984). The entropy peak (at omega = 0) is observed in a plasma for the first time. Two theoretical models are introduced and their predictions compared with the observations. A two-species BGK kinetic model includes the collisional interactions between ions and electrons, but fails to correctly model the relative amplitudes of the spectral peaks. Alternatively, Braginskii's two-fluid transport equations may be used with the Fluctuation-Dissipation theorem to derive an analytic expression for the fluctuation spectrum, which gives relation of the thermal transport coefficients to the spectrum. The prediction of Braginskii's equations is used to estimate the transport coefficients experimentally by least-squares fitting the theory to the experiment. The effects on the spectrum of the second-stage ionization species and impurity in the plasmas are estimated with a three-fluid model.