Laser scattering measurements of thermal ion-acoustic fluctuations in collisionally dominated plasmas

The thermal ion-acoustic fluctuation spectrum of a high-density, low-temperature (${10}^{17}$ ${\mathrm{cm}}^{\mathrm{-}3}$, 2 eV) collisional plasma was measured using ${\mathrm{CO}}_2$-laser scattering. The measured spectra differed substantially from spectra observed in collisionless plasmas. Fluctuations at the ion-acoustic frequency were strongly enhanced and the width of the resonance was significantly narrowed in comparison to the collisionless case. Comparison to the predictions of various collisional theories shows the Bhatnagar-Gross-Krook theory to be the most successful in describing the data.