Experimental study on the dynamics and parameters of nanosecond laser-induced aluminum plasma

In this paper, impulse measurement, spectral diagnostics, temporal evolution images and target ablation are employed to investigate the dynamic behaviors and parameters of Nd:YAG nanosecond laser-induced aluminum plasma at different pressures and laser fluences. The impulses and coupling coefficients generated by laser ablation increase firstly and then reduce with the decrease in pressure for the laser fluences of 17.22 J cm(-2) and 20.94 J cm(-2), but they only drop at 0.5 Torr for a laser fluence of 6.19 J cm(-2). The fast exposure images captured by the high-speed camera and ICCD show that the plasma plumes present the separation at atmospheric pressure and expansion near vacuum, but last longer time at pressures of 150 Torr and 22 Torr. The duration and dynamic property of plasma are responsible for the impulse, as well as are mainly dependent on the characteristics of plasma parameters. Therefore, the electron density and electron temperature are obtained by Stark broadening method and Boltzmann plot, respectively. The change in the electron density is proportional to pressure and laser fluence. The variation trend of the crater depth and ablative mass with pressure is opposite to that of the electron density, which is ascribed to the plasma shielding effect caused by the inverse bremsstrahlung absorption mechanism. Besides, the electron temperature varies inversely with pressure in the detection range due to various recombination processes. This study provides an insight for clarifying the energy conversion mechanism and improving the laser propulsion performance.