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(k,{ω}) of high-density, low-temperature collisional plasmas (Ar, Tₑ=2 eV, Nₑ=1×{}10¹⁷ cm^-3; He, Tₑ=2.3 eV, Nₑ=1×{}10¹⁷ cm^-3) are measured by means of CO₂ laser scattering. The spectrum consists of three peaks, two due to ion-acoustic waves, and a central entropy-fluctuation peak. The entropy peak (at {ω}=0) is observed in a plasma. 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, which predicts the fluctuation spectrum better than the BGK model, not including the ion-electron collisional interactions. 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 the 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 in the plasmas are estimated wtih a three-fluid model.