The ion feature in a laboratory plasma: Theory and experiment using CO2 laser light scattering

The electron density fluctuation spectrum in the range of frequencies below the ion plasma frequency (the ‘‘ion feature’’) is investigated in plasmas where the ion acoustic waves are weakly damped. The situation typical in laboratory plasmas where the electron temperature is larger than the ion temperature is considered in detail. Analytical expressions are given in the limit of weak damping and the variations of the spectrum with the relevant parameters are readily obtained, including the dependence on frequency, wavenumber, temperature ratio, drift velocity, and the concentrations of light impurity ions. Velocity distributions with non‐Maxwellian tails are also considered. An experiment is described in which the scattering of CO2 laser radiation from a discharge plasma is studied. The observed fluctuation spectrum corresponds to the ion feature, in that the dependence of the observed frequencies on wavenumber corresponds to the dispersion relation of ion acoustic waves. They are only observed, however, in the edge plasma where drifting ions enhance the fluctuation level, and not in the plasma center, where light impurities in the argon plasma damp out the ion resonance.