Observation of laser-induced anisotropic changes in the frequency and damping of ion acoustic waves in a laser plasma

Temporally and spectrally resolved large-angle Thomson scattering was used to study the frequency and damping of short-wavelength (kλDe{~}1) ion acoustic fluctuations in a plasma in the presence of a high-frequency (ω₀>ωₚₑ) laser beam. At high laser intensities (v_osc/vₑ>1), the frequency of the ion acoustic waves which propagated parallel to the electric field of the laser (i.e., parallel to the electron quiver velocity) was observed to be larger than the frequency of the waves which propagated in the perpendicular direction. An increase in the fluctuation amplitude for those waves that propagated parallel to the electric field was observed to be coincident with the shift in the ion acoustic frequency. Both the frequency shift and the increase in the fluctuation amplitude can be explained in terms of the off-resonance ion acoustic decay and the off-resonance oscillating two-stream instabilities. The experimental results are in good agreement with the predictions of theory.