Confinement effect of primary plasma on glass sample induced by irradiation of Nd-YAG laser at low pressure

The characteristics of the laser plasma which was induced by focusing a Q-s w Nd:YAG laser pulse repeatedly at a fixed position on a glass sample were examined under the reduced pressure of approximately 2 Torr. It was observe d that at the initial irradiation the secondary plasma could not be generat ed when the pulse energy was set below around 30 mJ, while the primary plas ma was produced distinctly. However, after several shots of irradiation the secondary plasma was suddenly generated and this generation continued with an almost constant emission intensity. This fact can be interpreted on the basis of the confinement effect due to the wall of the crater produced by the pre-irradiation, which is solely responsible for the generation of the shock wave. This fact provides strong support for the shock wave model prev iously proposed by us to explain the mechanism of secondary plasma formatio n.