Studies On The Mechanism Of Laser Induced Material Breakdown And The Formation Of Plasma

In this paper, a theoretical model is developed to describe the laser ablation on metal target in air. And the breakdown thresholds of the materials are investigated.First, a comprehensive introduction is given, focusing on the factors which influence the breakdown threshold of a certain material. Considered of the atmospheric ionization machanism, avalanche ionization and multiphoton ionization, some numerical calculations are done to define the trend along which the threshold goes with different electronic intensity at initial state. By the comparison between the thresholds of the targets in air and the air near targets, it is proved that the atmospheric ionization could be neglected in the former case.An one-dimension hydromechanics model is presented for the laser ablation of aluminium target in an ambient gas at 1 atm. The model considers the plasma as a special kind of flux and describes the process of the thermal conduction, the expansion of the mixture of Al vapor and air, the plasma formation and the plasma shielding. The ionization of Al and N in air, the viscosity of the mixture, the inverse bremsstrahlung absorption process and photoionization process, and the back flux onto the target are taken into consideration simultaneously. The thermal distribution, the velocity of the vapor and the plume, the ionization degree and the number intensity of electron can all be obtained from the model. The model can also be used to predict trends in target and plume(plasma) characteristics in the vicinity of the target breakdown.By the self-developed optical detection techniques based on interference principle and deflection principle, the laser induced breakdown thresholds on aluminium targets are investigated separately. Based on the analysis to the signals, it is found that by the deflection method, we can detect the occurrence of laser plasma expediently. A new criterion, whether the signal reverses, is proposed to judge whether the laser plasma comes into being or not.The model and the experimental results provide valuable reference to the control of the parameters of the medium, the target and the laser beam during the laser ablation process due to the lack of available experimental data in the vicinity of the breakdown.