Formation of the condensed phase of metals exposed to submicrosecond laser pulses

Abstract

Using the method of laser probing, the laws governing the formation of the liquid-droplet phase of a number of metals (Ni, Zn, Pb, Ag, and Cu) on exposure of metal targets to intense submicrosecond pulses have been determined. It has been established that condensation from the vapor of the erosive laser jet is the main mechanism of formation of the liquid-droplet phase of a metal under given conditions. Introduction. With the advent of powerful lasers, the range of practical problems whose solution has become possible due to the introduction of rapidly developing laser technologies has extended considerably. The intense use of lasers was begun in both scientific investigations and in industry, where lasers are applied in various technologies of metal working (hardening, welding, cutting, drilling). First of all, these are solid-state lasers based on glasses and crys- tals activated with neodymium. The energy and time characteristics of radiation pulses generated by such lasers differ considerably. While for rather long (10 μsec-10 msec) pulses of moderate power density the processes of interaction of laser radiation with metals have been well studied (1-4), reliable experimental information on these processes for short pulses of high power density is obviously scarce. The present work is devoted to investigation of the principal aspects of the formation of the liquid-droplet phase of metals in erosive jets in the case of submicrosecond (200 nsec) intense laser action. Experimental Techniques. The parameters of the condensed phase of metals were controlled with the aid of the technique of laser probing based on the dependence of the parameters of absorption and scattering of the particles of the condensed phase of metals on their size. This technique allows one, in real time, to follow changes in the av- erage size of particles and in their concentration in the assigned near-surface region of a target (for more details see (5)). From the dependence of the increase in the scattered component on time one can also determine the approximate velocity of the condensed phase motion. For this investigation we selected the metals Ni, Zn, Pb, Ag, and Cu, which differ greatly in such parameters as the melting and boiling temperatures and in the heat of evaporation. This allowed us to understand, at the qualita- tive level, the basic laws governing the processes of formation of the liquid-droplet phase of metals under the given conditions of laser action. As the actuating laser we used a GOS-1001 standard laser setup operating in the Q-switching mode. To implement such a regime, we used a rotating prism of complete internal reflection (speed of rotation 7000 rev ⁄ min). The setup allowed us to obtain laser radiation single pulses with the parameters λ = 1064 nm, τ 200 nsec, and E = 2-5 J and with an outgoing beam diameter of 30 mm. The laser pulse form is shown in Fig. 1. On focusing such radiation to spots of diameters 1 and 3 mm, a power density of 10 9 and 10 8 W ⁄ cm 2 , respectively, was attained on the target surface. The probing of the erosive jet was made by the radiation emitted by a ruby laser operating in the free-gen- eration regime. This regime was realized with the aid of a cavity made from plane-parallel mirrors (Fabry-Perot cav- ity) with the reflection coefficients 0.98 and 0.70. The time structure of the ruby laser radiation operating in the free-generation regime is a set of approximately 100 spikes of length of about 1 μsec each (total duration of the gen- eration pulse 900-1000 μsec), thus ensuring the time resolution of about 5-10 μsec realizable by means of registration