Spectroscopic Diagnosis Of Air Plasma Induced By Pulsed CO₂Laser

When a high power laser is focused on any material, an optical breakdown of the material will be occurred, and produces the high temperature and high pressure plasma, accompanying with a strong shock wave and spark. This laser induced plasma also attracts researchers’attention, and is applied widely in the aerospace, nuclear energy, industrial monitoring, environment pollution, biological engineering and other fields. With a deeper knowledge of the related mechanism and characteristics of laser induced plasma, these applications of laser induced plasma will get more and more in-depth development.In this dissertation, the laser induced air plasma was generated by the pulsed CO2laser focused in air. The theories and the experiments of laser induced air plasma breakdown, expansion, spectral radiation process had been investigated, and the energy deposition interaction between the pulsed laser and plasma was analyzed at the same time.Firstly, we investigated the optical breakdown and expansion of air plasma generated by pulsed CO2laser in both theory and experiment. Because of the small single photon energy of the CO2laser, the initial free electron produced from the multiphoton absorption ionization could be ignored, while the initial free electron was all produced by the avalanche ionization in laser field. The axial and radial laser induced air plasma expansions were detected by the sequential acquiring method of ICCD camera. Combined with the spherical laser supported detonation wave model, air plasma expansion processes with different focal length and different incident laser energy were investigated, the experimental results showed that air plasma had bigger volume and higher expansion rate when using the longer focal length and higher incident laser energy, and the termination of laser supported detonation wave was closely related to the laser power density in focusing area, the higher laser power density, the later termination of laser supported detonation wave. In addition, based on the measurement of the area of the plasma wave front, we calculated the thrust at the position in front of the plasma wave, which showed that the thrust generated by long focal length of plasma was bigger than the short focal length.Subsequently, we studied the laser induced air plasma by the spectroscopic diagnosis in both theory and experiment. Based on the assumption that the plasma in the local thermodynamic equilibrium state, and combining the Saha ionization of air plasma equilibrium equation, we calculated the equilibrium composition and continuous spectrum of air plasma radiation. With the increase of temperature, ionization state of air particle became more and more high, corresponding to the degree of ionization of air plasma also became more and more high, and the single atomic structure of the particle occupied absolute proportion of plasma, while the molecular structure of the particle were almost negligible. In addition, with the increase of electron temperature and electron density, the intensity of continuous spectrum would strengthen gradually; however, bremsstrahlung dominated the continuous spectrum in the air plasma.Furthermore, based on the emission spectra theory of the local thermodynamic equilibrium plasma, assuming the air plasma in optical thin, we established line spectra model and continuous spectra model of air plasma, and carried on the fitting of the emission spectra of air plasma to obtain the electron temperature and electron density in plasma.On the experiment, in order to obtain the evolution of the laser induced plasma in the stage of laser pulse duration, we built a time and space resolution of laser induced air plasma emission spectra diagnosis experiment platform, and realized the axial and radial directions respectively by the plasma emission diagnosis. By using time-resolved emission diagnosis device, different incident energy of laser induced air plasma emissions were measured, the results showed that the ion line spectrum radiation generated by higher energy laser could maintain longer time, and plasma produced by higher energy laser showed more intense continuum radiation.In addition, the emission spectra of the axial and radial air plasma were diagnosed by using the imaging spectrometer slit coupling method. In this space-resolved emission spectra diagnosis experiment, we found that the intensity of air plasma increased gradually against the direction of incident laser, and the spectral distribution of the radial plasma showed a "separation" phenomenon to the periphery. Then, the spatio-temporal evolution of the electron temperature and electron density in air plasma were calculated by the line spectra model we established, the results showed that air plasma would generate a layer which was opaque for CO2laser, and the laser energy could not penetrate this critical layer to arrive the plasma center, therefore, the reflected laser would heat the plasma outside this critical layer again, leading to the separation of the plasma radiation intensity on the radial direction.