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

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