Shock excitation and cooling stage in the laser plasma induced by a Q-switched Nd : YAG laser at low pressures

An experimental study has been carried out on the dynamical process taking place in the secondary plasma generated by a Q-switched Nd:YAG laser (80 mJ , 8 ns) on a copper target at reduced pressure. Accurate dynamical characte rization of the cross-sectional view of the plasma has been made possible b y the unique combination of a plasma confinement configuration and the time -resolved measurement technique. In addition to reaffirming the role of the blast-wave mechanism in the generation of secondary plasma, an analysis of the time-resolved spatial distributions of emission intensities and the ti me-resolved spatial distributions of temperature was made. As a result, the occurrence of two-stage emission processes, the "shock excitation stage" a nd "cooling stage," has been proved. For instance, at 2 Torr it is shown th at the emission process is initiated by a brief shock excitation process (s imilar to 1 mu s) and followed by a longer cooling process (similar to 3 mu s). The experimental results concerning the characteristics of the plasma can be well understood by considering the two-stage processes.