Coherent scattering in Nd-laser beat-wave experiments: modulational instability in multiphoton ionization plasmas

Abstract The generation of long, 1 cm, and homogeneous plasmas, n ∼ 10 17 cm −3 , Δn / n ⩽ 1%, by multiphoton ionization of low pressure gases, H 2 , D 2 and N 2 at pressures of a few torr, has been characterized by Thomson thermal scattering. After the initial full ionization, the hydrodynamic evolution due to the ponderomotive force of the laser itself is observed. In such plasmas, the modulational instability of high phase velocity plasma waves generated by Nd-laser beat-waves (Φ ∼ 10 14 W cm −2 ) has been observed by Thomson scattering of an auxiliary green beam. Both ion waves, electron waves and their harmonics are detected when the plasma frequency is equal to the difference frequency between the two pump lasers.