Time evolution of laser-ablation plumes and induced shock waves in low-pressure gas

Abstract

We investigated correlations between the temporal evolutions of shock waves and plasma plumes generated by pulsed laser ablation of an aluminum target under various background gas pressures. Using a probe-beam deflection technique with a high-gain amplifier, we succeeded in detecting relatively weak shock waves in a thin gas with a pressure down to 200 Pa, which is considered to be a suitable condition for cluster formation. The behavior of the expanding plume was also observed using a high-speed framing camera and compared with that of the shock wave. The result shows that the shock front forms just ahead of the plume in the early expansion stages. The plume expansion rapidly attenuates with time and finally ceases, whereas the shock wave continues to propagate and gradually converts into a sound wave. The point-explosion blast wave model is able to estimate the transition of the temperature behind the shock front at low background pressures, giving valuable information for investigating the growth of clusters in the boundary region between the plume and background gas.