Detection of Ion Plasma Waves by Collective Thomson Scattering

The first unambiguous experimental detection of ion plasma waves is reported. These short-wavelength ( kλDe>1), purely electrostatic ion waves were first predicted by Tonks and Langmuir in 1929. The waves are observed in a long-scalelength ( L>10⁴/k), multiply ionized ( Z0ex0ex=0ex0ex22), laser-produced plasma. The frequency (ω), wave number $(k)$, and amplitude evolution of the ion waves are measured by time-resolved spectroscopy of the scattering from the ion waves of a probe laser beam. The wave frequency is approximately the ion plasma frequency (ωₚᵢ) when kλDe exceeds 1, as predicted by the theoretical dispersion relation.