Plasma Torch Development and Crater Growth Dynamics in the Course of Laser Processing of Materials and its Diangostics

Abstract

UDC 535.391:621.376 and V. I. Nedolugov aThe evolution of the plasma torch arising under the action of laser radiation on a metal specimen has been investigated by several methods. The dependence of the time form and the spectrum of acoustic vibrations on the parameters of the irradiated material and the law of increase in the crater depth has been determined. It has been found that under the action of a lser pulse of length ~20 μs on the surface of a copper specimen the growth time of the destruction zone is about 40-50 μs, which is in good agreement with the lifetime of plasma formation near the surface of the target subjected to laser-plasma processing (~50 μs). It has been shown that the use of the model of a loaded zone with moving boundaries emitting acoustic waves into an elastic medium permits solving an important practical problem -determining the law of time growth of the zone of irreversible deformations on the surface of the specimen subjected to pulsed laser plasma processing.Introduction. The action of laser radiation (LR) on a substance is accompanied by a whole number of phenomena. One of such phenomena is acoustic emission of the zone of laser destruction of the material. The essence of this phenomenon is that under the action of LR on the surface of a solid, the laser destruction zone under the action of high pressure in the vapor-plasma formation formed from the evaporated material and air (laser torch) begins to vibrate and emit sound waves into the irradiated medium [1]. Experiments and calculations show that between the parameters of emitted acoustic waves and the parameters of the destruction zone there is a unique relation. Thus, the destruction zone parameters can be determined by the acoustic emission parameters, which can have a practical application [2].In the above works, in considering the emission, a model of a loaded region emitting acoustic waves into an elastic medium is used [1, 2]. The time dependences and the spectra of pressure pulses in the elastic wave calculated with the use of the given method agree well with experimental data but only beginning from the second quarter of the period of pressure oscillations in the experimentally registered acoustic wave. The discrepancy is explained by the generation of acoustic waves in the process of growth of the formed zone of inelastic deformations [3].The aim of the present work is to investigate simultaneously the development of the laser torch and the acoustic emission of the destruction zone formed under the action of pulsed laser radiation on the surface of a metal and study the relationship between the parameters of time dependences of the pressure in the generated elastic wave and the change in the sizes (in particular, the depth) of the irreversibly deformed zone on the surface of the irradiated specimen.Experimental Facility and Results of the Experiment. The experimental facility is schematically represented in Fig. 1. The radiation from a laser based on an ethanol solution of rhodamine 6G with a coaxial pump lamp 1 [4] pas...