Plasma evolution induced by long nanosecond laser pulse ablation: Time-resolved measurement and physics-based modeling

Abstract

Extensive research has been performed in literature for the plasma induced by nanosecond (ns) laser ablation of metal targets. However, most of the previous investigations employ relatively short ns laser pulses (duration less than ∼50 ns). The study employing relatively long ns laser pulses on the order of ∼100 ns is much less. This kind of plasma has been studied in this work through ns time-resolved observation using an intensified charge-coupled device (ICCD) camera, which is coupled with a microscope tube for a high spatial resolution. The study shows that the plasma radiation intensity is not spatially uniform. Instead, high-radiation-intensity (HRI) regions have been observed right above the target surface and behind the plasma expanding front. The HRI region right above the target surface disappears after the completion of the laser pulse. This kind of experimental observation has been rarely reported in literature. To fundamentally understand the observed plasma evolution, a physics-based model has been developed, where the heat transfer equation is solved for the target condensed phase, while the two-dimensional axisymmetric gas dynamic equations are solved for the plasma (ionized target vapor) and ambient air region. The governing equations for the gaseous phase and the target condensed phase are related through the Knudsen layer relation. The simulation results are well consistent with the experimental observations. The simulations show that the region right above the target surface and the region behind the plasma expanding front both have relatively high temperatures and densities. However, for the former region, the temperature and the density drop very quickly after the end of the laser pulse. The study provides very useful information for a good understanding of the dynamic temporal evolution and the spatial distribution of long ns laser-induced plasma properties from metal targets.