Investigation of the processes of initiation of laser heating of a dispersed medium from a transparent dielectric

Experimental studies have been conducted on the formation of a laser torch when exposed to radiation from a fiber ytterbium laser with a wavelength of 1.065 microns on pressed micro-powders made of transparent dielectric materials (MgF₂, CaF₂, SiO₂, BaF₂, MgAl₂O₄, Al₂O₃, Nd :Y₂O₃, YSZ и TiO₂). The propagation of ytterbium laser radiation in the above-mentioned targets has been theoretically investigated. The refractive indices of the studied materials are in the range of 1.38 – 2.48. As a result of calculations, it was found that the combination of interference and scattering of radiation in a medium of micron particles leads to an increase in intensity in local areas by one to two orders of magnitude compared with the intensity of incident radiation. It is shown that as the refractive index of the particle material increases, this gain increases. It is assumed that in such local maxima, both nonlinear mechanisms of laser radiation absorption are triggered, which contribute to the throwing of electrons into the conduction band, and the processes of shock ionization, which lead to an avalanche-like increase in the concentration of electrons in this zone. As a result, the material begins to heat up until ablation. The characteristic times of the specified process for different materials are calculated numerically. The calculation results are in good agreement with the experimental data.