Nanosecond Laser Plasma In Water Shielding Effect Of The Acoustic Spectrum Analysis

The laser-induced plasma sound waves spectrum and laser-produced plasma shielding effect caused by interaction between the high-power laser and liquid materials are investigated both with theory and experiments, as well as numerical simulation and calculation of characteristic parameters.After analyzing the process of plasma formed by laser breakdown near the work-piece and the factor affecting coupling between the laser and work-piece under water confinement regime, the breakdown plasma model is presented to analyze the plasma shielding effect by an electron rate equation together with a Maxwell's wave equation. The properties of the transmitted peak laser power density and laser pulse duration under different incident laser power density are then investigated.Based on the physical process of laser ablation and plasma formation and expansion, a self-closed mathematical model under water confinement regime is presented. The breakdown plasma model is used to study the pressure changes on the front surface of the work-piece with different incident power density of laser.By the confined plasma model, the changing properties of the transient number densities of atoms and electrons in plasma are investigated, as well as the reason why the peak plasma pressure for 532 nm is higher than that for 1064nm.Through the optical detection technique based on fiber-coupling beam deflection principle, the plasma and its shock wave caused by high-power laser interacting with the fluid materials are studied. Based on the experiment, analysis is given to show how the laser-induced plasma shock waves attenuate to sound waves. The results tell that the distance of the laser-induced plasma shock waves attenuating to sound waves increases in linear trend as incident laser energy increases.Laser-induced plasma sound waves under different conditions are measured by the hydrophone underwater. And the signals of sound waves are analyzed based on Fourier Transform. The investigation results show that the peak spectrum distribution of laser-induced plasma have few differences under different conditions. However, the peak of frequency has some relations to incident energy of the laser, the attenuation properties of the laser underwater and the surface ionization of metals.These research results in this paper will help to build physical and mathematical models in the process of laser shock peening under the water confinement regime and also provide theoretical and experimental references for the applications of laser-induced sound waves.