Transversely Excited Atmospheric (TEA) CO2 laser pulses were used in order to generate an optical breakdown in a variety of mono- and polyatomic molecules using different focusing powers. The dependence of the spark kernel geometry and the transmitted pulse shapes on the focusing power as well as the pressure, molecular weight, and ionization energy of the gases was investigated in detail. Partial removal of the transmitted pulse tail in the 0.05–2.6 μs range together with shortened spikes in the 10–60 ns range has been observed by applying a 2.5 cm focal length lens for all the gases. At higher focal lengths, this effect is only incompletely observed for He gas. Spatial-temporal analyses of the laser beams and the relevant plasma plumes indicate that this behavior is due to the drop in the plasma density below the critical level, before the laser pulse tail is completed.
The evolution of laser-induced plasma for an aluminum target in a helium ambient gas at different pressures of 100, 300, 500, 700, and 1000 mbar is numerically studied. A thermal model of laser ablation is utilized for calculation of plasma parameters which comprise heat conduction, Euler equations, Saha–Eggert equations, Knudsen layer boundary condition, mass and energy balance relations, and optical shielding effects. In addition, in order to determine the temporal parameters of aluminum's plasma, the hydrodynamic equations are computed for calculation of the plasma absorption due to inverse Bremsstrahlung and photoionization. A CO2 laser pulse at a wavelength of 10.6 μm with different pulse durations of 50 and 100 ns is irradiated on laser induced Al plasma for truncation of the transmitted CO2 laser pulse. The laser intensities irradiated on the Al sample for producing plasma and generation of a shortened pulse are considered as 1016, 1017, and 5 × 1017 W/m2. Furthermore, for validation of the theoretical calculations, some experimental results are presented. Results showed that higher helium gas pressures caused the critical density attained at earlier delay times which caused the CO2 laser beam became efficiently truncated. Moreover, it is concluded that pulse duration has an inverse relation with ambient gas pressure and laser intensity which means that the higher gas pressure or laser intensity induces less pulse duration.
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