DYNAMICS OF HIGH-TEMPERATURE PLASMA FORMATION DURING LASER IRRADIATION OF 3-DIMENSIONALLY STRUCTURED, LOW-DENSITY MATTER

Results are presented from an experimental investigation of the proper ties of the plasma produced by the action of a radiation pulse at the second harmonic of a Nd laser, with average intensity similar to 5 . 1 0(14) W/cm(2) in the focal spot, on flat targets consisting of porous polypropylene (CH)(x) with an average density of 0.02 g/cm(3) (close t o the critical plasma density) and with similar to 50 mu m pores. The properties of the laser plasma obtained with porous and continuous tar gets are substantially different. The main differences are volume abso rption of the laser radiation in the porous material and much larger s patial scales of energy transfer. The experimentally measured longitud inal ablation velocity in the porous material was equal to (1.5-3). 10 (7) cm/s, which corresponds to a mass velocity of (3-6). 10(5) g/cm(2) . s, and the transverse (with respect to the direction of the laser b eam) propagation velocity of the thermal wave was equal to similar to( 1-2). 10(7) cm/s. The spatial dimensions of the plasma plume were simi lar to 20-30 mu m. The plasma was localized in a 200-400 mu m region i nside the target. (C) 1996 American Institute of Physics.