Investigation of plasma formation during pulseperiodical laser treating of metals in air
In the introduction an object of the investigation is pointed - the metal samples under laser radiation with temperate (technological) intensity action both in mono-pulse and two-pulse regimes. The reasons of actuality of these investigations are mentioned. The purpose of research was investigation of plasma during mono-pulse and two-pulse action of laser radiation with wavelength λ = 0,69 λm upon the metal samples in air on atmospheric pressure and its influence on laser destructed zone form. In the main part of the work, the method of experimental investigation of interaction between laser radiation and metal samples both in mono- pulse and two-pulse regimes of action of radiation on the matter is described. Specifically the application of fast holographic filming for visualization of development of laser torch forming near the surface of irradiated is described in detail. The method of reconstruction of interference pictures from the holograms and calculation of electron density fields at different temporal moments from the beginning of irradiation of the target is described. It is shown that the density of plasma formation on the border of irradiated spot and in the centre are different. This effect is connected with different conditions of energy dissipation from the plasma connected with lateral unloading. The difference of concentration maximums on different regimes of laser treating is also observed. For instance, during mono-pulse action maximum is disposed in the centre of irradiated spot, during two-pulse action two maximums symmetrically removed from the centre are observed. This effect can be connected with the refraction of laser radiation in the already existing plasma. Fast photo-scanning shows that each laser pulse brings separate contribution in laser plasma formation on the surface of irradiated sample. For plasma plumes forming by the second and by next laser pulses it is typical presence of several plasma fronts supported by laser radiation and spreading along the laser beam in the already existing plasma. It is pointed in the conclusions that the discovered peculiarities of pulse-periodical laser treating of metal samples expends the possibilities of application of pulse laser radiation for treating of matters. The results of this work can be applied at the industrial enterprises using laser treating of materials.