Opitcal Diagnoses And Mechanism Ivestigation Of Nanosecond Laser Induced Air Plasma

In this dissertation, the laser induced gas breakdown, laser supported detonation wave (LSDW) and propagation mechanisms were systemically investigated in experiment and theory. The evolution of the electron density and the gas density behind the shock wave, as well as the gas plasma collision were investigated.First of all, an interferometry system based on Mach-Zehnder interferometer for laser plasma diagnoses was established. It has ability to automatically acquire the experimental data. The hingh quality interferogram was aquired. The method for the processing of the laser interferogram was proposed and high resolution imagings of the 3D distribution of the plasma refractive index were achieved.A method for determining the breakdown time of the air was proposed, and further to determine the quantitative relation of the breakdown time and the breakdown intensity. It was found that the transmissivity is inversely proportional to the incident pulse energy approximately. The arithmetic product of the breakdown time and the breakdown intensity is fixed with small pulse energy. However, with increasing the pulse energy, when a threshold was achieved, the breakdown time became saturation. This phenomenon was explained with the avalanche ionization mechanism and the concept of the equivalent pulse was proposed.Based on the shadowgraph and point scan technology, a method of measurement of the LSDW velocity was proposed. The characteristics of the plasma two-dimensional shielding were investigated. And then the LSDW velocity as a function of the pulse energy and time was obtained by the plasma two-dimensional shielding image processing. The mechanism of the formation of the LSDW was explained with the Taylor model, and the theory agrees with the experiments results well.The evolution of the plasma electron density 3D distribution was investigated in experiment. The results show that the plasma channel has been formed in the early stage of laser induced air plasma, and the plasma expanding velocity rapid attenuation accelerates the plasma channel collapses. The evolution of the gas density 3D distribution behind shock wave front was also investigated. The result shows that, at the initial stage, the gas density was increasing at first and then decreasing, and the evolutionary process of the gas density from non-uniform density distribution to the uniform distribution duo to the anisotropic expansion of the plasmaLaser induced gas plasma collision was investigated in experiment. From the results, the enhancement gas density behind the shock wave front and the electron density were observed at the collision zone. We think that the enhancement of the gas density behind the shock wave was induced by the formation of the gas stagnation layer, while the enhancement of the electron density was induced by the attraction of the positive ions which were stagnated by the gas molecules.The results of this dissertation will facilitate the theoretical study of laser induced gas breakdown, laser applications and the related development of detection technology.