Shock Excitation and Cooling Stage in the Laser Plasma Induced by a Q-Switched Nd:YAG Laser at Low Pressures

Abstract

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 characterization of the cross-sectional view of the plasma has been made possible by 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 time-resolved spatial distributions of temperature was made. As a result, the occurrence of two-stage emission processes, the “shock excitation stage” and “cooling stage,” has been proved. For instance, at 2 Torr it is shown that the emission process is initiated by a brief shock excitation process (∼ 1 μs) and followed by a longer cooling process (∼ 3 μs). The experimental results concerning the characteristics of the plasma can be well understood by considering the two-stage processes.

Plasma emission induced by an Nd-YAG laser at low pressure on solid organic sample, its mechanism, and analytical application

“…This general feature is closely related to the shock-wave excitation model reported previously. 15 It should be noted that the time taken to reach its maximum is perceptibly longer in the lower surrounding air pressure ͑1.0 µs versus 0.6 µs͒. This result certainly supports our explanation of a more effective compression process in higher pressure surrounding air.…”

“…This result is in complete contrast with the intensity variation observed in the metal sample, in which the emission intensity increases slightly from 2 to 10 Torr, followed by a slow decline from 10 to 760 Torr. 15 This distinct pressure-dependent characteristic can be explained by the weaker ablation process induced from a softer solid target. Consequently the compression process was not sufficiently effective at low air pressure.…”

A Comparative Study of Pressure-Dependent Emission Characteristics in Different Gas Plasmas Induced by Nanosecond and Picosecond Neodymium-Doped Yttrium Aluminum Garnet (Nd:YAG) Lasers

“…This is basically similar to that observed in the case employing ns laser and in good agreement with the shock wave model. 59 This result has thus demonstrated the occurrence of a new transient feature at the very early stage in addition to the later intensity time profile that exhibits the shock wave-induced emission characteristics in the plasma generated by the ps laser, resembling the same basic feature shown in Fig. 8a for ns laser-induced plasma emission.…”

“…10 shows, in the case using the ns laser, a steep rise of the H emission intensity up to its maximum at 150 ns before its equally fast downturn, followed by continued decrease at much lower pace, in good agreement with the proposed scenario consisting of short excitation and long cooling stages in the shock wave model. 59 In the case of the ps laser, the intensity time profile apparently consists of two components. The first one exhibits an even sharper spike in the early stage of the laser irradiation.…”

<title>Excitation mechanism of hydrogen emission in the laser-induced atmospheric plasma in water sample</title>

“…Similar behavior is reported in our previous works conducted on copper sample at reduced gas pressure of around 2 Torr. 27 order to make clear the excitation mechanism of the atmospheric plasma generated in water, it is also very useful to know how the temperature of the atmospheric plasma changes with time after the initiation of the laser bombardment. The plasma temperatures were measured by the Boltzmann two-line method, assuming validity of the Boltzmann distribution in the plasma as described in our previous works.27 Fig.…”