Detection of Ion Plasma Waves by Collective Thomson Scattering

Abstract

The first unambiguous experimental detection of ion plasma waves is reported. These short-wavelength ( $k{\ensuremath{\lambda}}_{\mathrm{De}}>1$), purely electrostatic ion waves were first predicted by Tonks and Langmuir in 1929. The waves are observed in a long-scalelength ( $L>{10}^{4}/k$), multiply ionized ( $Z\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}22$), laser-produced plasma. The frequency $(\ensuremath{\omega})$, 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 $({\ensuremath{\omega}}_{\mathrm{pi}})$ when $k{\ensuremath{\lambda}}_{\mathrm{De}}$ exceeds 1, as predicted by the theoretical dispersion relation.