Criteria for excitation of a shock wave and its intensity during an explosion in a rarefied gas

Abstract

The effect of the initial pressure of the surrounding gas on the intensity of the shock wave (SW) formed during the dispersion of material vaporized by a powerful laser pulse is examined. The initial stage of expansion of the plasma generated through the focusing of powerful laser radiation on the surface of a solid material in air was studied experimentally in [1, 2]. The times of formation and the initial radii of the SW were recorded on the photo-scans of the SW front radiation presented in these reports. It is found that at an air pressure below ∼ 0.1 mm Hg the recordings of the intrinsic radiation of a flare do not differ from the corresponding recordings in a vacuum. For instance, in [2] a bright shock front was observed at a pressure of 0.18 mm Hg, while at a pressure of 0.1 mm Hg SW radiation was not detected. In [2] the hypothesis was made that at an air pressure below ∼ 0.1 mm Hg a SW is not formed and the interaction of the vaporized material with the surrounding gas has a diffusion nature. However, in [1] SW were detected by the Schlieren method at a considerably lower pressure, about 2 · 10−2 mm Hg. It will be shown below that the sharp decrease observed in the brightness of the radiation of SW fronts generated during laser heating of a solid material in a rarefied gas is explained by the rapid decrease in the maximum SW velocity at a pressure below ∼ 0.1 mm Hg. The expansion of the vaporized material at a pressure of the surrounding gas much less than 0.1 mm Hg is also examined.