Study of short-pulsed laser-induced plasma confined in a microhole

Summary form only given. The evolution of the plasma induced by nanosecond laser ablation of a metal target and confined in a microhole has been studied using a physics-based model. This kind of research work has been rarely reported in literature. In the model, 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 and the target condensed phases are related through the Knudsen layer relation. The plasma evolution in microholes with different diameters has been studied. It has been found that the plasma will start “feeling” the spatial confinement effect of the hole sidewall through the compressed air in between well before its size reaches the hole diameter. The reduction of microhole diameter will increase the plasma front expansion velocity. When the hole diameter is sufficiently small, a shock wave with a flat front will be generated in the ambient air ahead of the plasma front. Also, a high-density region will be induced in the plasma near its expanding front, which will last for a relatively long time. Other effects of the microhole on the plasma evolution will also be discussed. The study provides useful information for the fundamental study of laser-induced plasma, and for the practical applications of lasers in microhole drilling, laser-induced breakdown spectroscopy and other areas, where laser-induced plasma may play an important role.