Observation of laser-induced anisotropy in ion acoustic waves in a plasma

Temporally and spectrally resolved, large-angle Thomson scattering was used to measure the frequency of thermal ion acoustic waves in a plasma in the presence of a high-frequency (ω₀>ωₚₑ) laser beam. At high laser intensities (v_osc/vₑ4), the frequency of the short-wavelength ion acoustic waves (kλ_De=0.3-1.2) which propagated parallel to the electric field of the laser (i.e., parallel to the oscillation velocity of the quivering electrons) was observed to be larger than the frequency of those waves which propagated perpendicular to the electric field (anisotropic sound). The measured frequency ratios are found to be in good agreement with the predictions of theory.