COHERENT SCATTERING IN ND-LASER BEAT-WAVE EXPERIMENTS - MODULATIONAL INSTABILITY IN MULTIPHOTON IONIZATION PLASMAS

The generation of long, 1 cm, and homogeneous plasmas, n - 10(17) cm(- 3), Delta n/n less than or equal to 1%, by multi-photon ionization of low pressure gases, H-2, D-2 and N-2 at pressures of a few torr, has b een characterized by Thomson thermal scattering. After the initial ful l ionization, the hydrodynamic evolution due to the ponderomotive forc e of the laser itself is observed. In such plasmas, the modulational i nstability of high phase velocity plasma waves generated by Nd-laser b eat-waves (Phi - 10(14) W cm(-2)) has been observed by Thomson scatter ing of an auxiliary green beam. Both ion waves, electron waves and the ir harmonics are detected when the plasma frequency is equal to the di fference frequency between the two pump lasers. (C) 1996 Elsevier Scie nce Ltd.