DYNAMICS OF LASER-PLASMA INTERACTION AT 10(18)-W CM(2)/

The interaction of 2 ps FWHM laser pulses with solid targets at intens ities 10(16)-10(18) W/cm2 is studied for two essentially different cas es of laser front steepness resulting in preplasma and preplasma-free interaction. Red and blue frequency shifts of a backscattered fundamen tal wave and its second harmonic depending on the incident laser pulse shape are observed. They are associated with Doppler shifts correspon ding to inward or outward movement of the critical density surface. A model including ponderomotive force was developed to explain the exper imental results.