| An experimental study of the stimulated Brillouin scattering (SBS) instability has investigated the effects of velocity gradients and kinetic effects on the saturation of ion-acoustic waves in a plasma. For intensities less than I < 1.5 × 1015 W cm−2, the SBS instability is moderated primarily by velocity gradients, and for intensities above this threshold, non-linear trapping is invoked to saturate the instability. We report direct evidence of detuning of SBS by a velocity gradient which was achieved by directly measuring the frequency shift of the SBS driven acoustic wave relative to the local resonant acoustic frequency. Furthermore, a novel use of Thomson scattering has allowed us to gather direct evidence of kinetic effects associated with the SBS process. Specifically, a measured two-fold increase of the ion temperature has been linked with laser beam excitation of ion-acoustic waves to large amplitudes by the SBS instability. Ion-acoustic waves were excited to large amplitude with a 2ω 1.2-ns long interaction beam with intensities up to 5 × 1015 W cm−2. The local frequency, amplitude, and spatial range of these waves were measured with a 3ω 200ps Thomson-scattering probe beam. These detailed and accurate measurements in well-characterized plasma conditions allow for the first time a direct test of non-linear models of the saturation of SBS. The measured two-fold increase of the ion temperature and its correlation with SBS reflectivity measurements is the first quantitative evidence of hot ions created by ion trapping in laser plasmas. |
