Characterisation of small-scale laser breakdown-induced blast waves with a PVDF gauge

Abstract

The study of explosions at the laboratory scale is difficult and limited due to the pyrotechnic safety and regulation. The laser-induced breakdown is a good alternative to overcome these constraints, but is inherently limited to very small scales (wave propagation distance of the order of a few centimetres). One of the key issues at such small scales is pressure measurement, which is one of the main experimental outputs at larger scales. Polyvinylidene fluoride (PVDF) pressure sensors offer suitable characteristics to address this problem. In this project, we performed laser breakdown experiments in air, using a pulsed, 1064-nm, Nd:YAG laser to induce a blast wave propagation. The laser delivers a maximum energy of 2.3 J over 7.5 ns. The beam was focused on to a millimetric spot to ensure a favourable power density leading to breakdown. The pressure field was measured with a face-on PVDF pressure gauge at various distances from the source. The signal was well fitted by a Friedlander-like profile. High-speed shadowgraph was used to image the shock wave propagation. By matching the laser breakdown energy to a TNT equivalent for chemical explosions, we evidenced a quantitative agreement with free-field TNT explosion results and showed that the classical laws for such explosions are relevant to describe the evolution of all the characteristics of the pressure wave (maximum overpressure, time of arrival, positive impulse, and positive phase duration). Those features confirm that face-on PVDF pressure gauges are well suited to measure blast effects at the laboratory scale.