Characteristics of Plasma and Detonation Waves Induced on an Aluminum Target by Nanosecond Laser Ablation

摘要

Investigation of plasma and detonation wave characteristics induced by laser ablating target is very important to recognizing the mechanism of laser driven flyer and laser destruction. In this study a laser ablating aluminium target experiment has been carried out, employing Schlieren photography technique for visualizing detonation wave temporal evolution and to analyse detonation wave attenuation law. A numerical model of detonation wave evolution induced by laser ablation has been established: phase transformation volume fraction is employed to describe target vaporization; plasma state equation is obtained by calculating vapour ionization degree; interaction of laser and plasma is considered. Numerical simulation of laser ablating aluminium target has been conducted, analysing the unsteady ablation process of target. Comparison of shockwave propagation speed and compressed air thickness between calculated results and experimental data validates the calculation method. Results show that shockwave evolution has a complicated structure with multi-density discontinuities. During detonation wave evolution, wave front transforms from half elliptical in early stage into half spherical and peak density location changes from wave front to interior. The fluctuating range of target ablation rate under laser action (672 mJ, duration is 24 ns) is 0.08 to 0.32 mms. Plasma density distribution is related to laser energy variation. After laser pulse, ionization degree and electron number density of plasma first decreases slowly and then dropped dramatically.