Theoretical And Experimental Studies On High Power Laser Induced Liquid Plasma

The physical process of laser and liquid interaction is investigated and a physical model of the breakdown threshold for water is established. The cause of the diffraction effect in the Shadowgraphy and Interferometry caused by laser-induced shock wave is analyzed. The mechanical effects of laser and target interaction in water were studied. The trajectory equation is derived under geometrical optics approximation. Analysis of the shielding effect of the laser plasma, as well as the experiment and numerical calculation on the character parameters is done.At first, based on the rate equation of free electron, two models to calculate the dielectric breakdown threshold of water are established. The results show that the numbercal model is closer to the experimental data than the analytical model. The influence on the breakdown threshold from multi-photon ionization, avalanche ionization and combined loss of electrons are evaluated. On this basis, the evolution of the electron density and the absorption coefficient and energy density of the plasma during the laser pulse were gotten. The comparison between theoretical threshold and the experimental breakdown threshold for water induced by pulse laser of different wave band indicates that the model is consistent with the experimental results in a large range.The method to avoid the diffraction effect in the Shadowgraphy and Interferometry caused by laser-induced plasma and shock wave is proposed. The physical model is established according to the fundamental diffraction theory and the phenomenon and influence of lens diffraction imaging effect is theoretically derived, thus the conclusion that when plasma region and camera have the object-image relationship with the imaging lens the diffraction effect can be overcome is substantiated.By the high-sensitivity optical detection technique based on fiber-coupling beam deflection principle, the plasma shock wave generated in the laser ablation on Al target under water is investigated. It shows that the attenuation distance of the plasma shock wave accrues with the increase of the energy of the laser. An optical fiber pressure sensor basing on light beam deflection has been used to measure the pressure on the surface of the target, and the instantaneous pressure signals of laser plasma shock wave and bubble jetting have been gotten at different laser energy values. The results have indicated that the bubble jetting is as important as the laser plasma shock wave, and more important sometimes.A detailed study on the propagation function of the follow-up laser in the plasma it generated is carried through. According to the refractive index distribution of the laser plasma, the assumption of the expression of laser plasma refractive index is made and thus the trajectory equation describing the propagation of laser in laser plasma is solved. The track map is draw with Matlab and a detailed discussion is done. The Shadowgraphy is used to observe the shielding effect of plasma formed from the laser induced breakdown of water. It is observed that the shielding effect is enhanced with greater laser energy and as the laser self-focusing, laser-induced breakdown of water forms a linear region of plasma.These research results will provide the theoretical and experimental reference to laser processing underwater, the effects and safety of laser medicine,, the propagation character of laser in liquid, the prevention of the harm that laser-induced plasma effect could bring in and the scientific use of laser-induced plasma effect.