Numerical Investigation On Laser Plasma Interaction Mechanism And Air-breathing Laser Propulsion

For an air-breathing laser propulsion lightcraft, the mechanism of formation and evolvement of plasma absorptive wave in lightcraft due to laser and air interaction, and the operating process were investigated by means of numerical computation and simulation in this dissertation.The governing equations were established under different conditions and cases, and the initializing conditions were also given. Physical and chemical models of thermo-chemical nonequilibrium flow and equilibrium flow in laser propulsion were analyzed and discussed. Park's reaction model with three-temperature and 11 air species was chosen to solve thermo-chemical nonequilibrium flow. Gupta's high temperature equilibrium air model was chosen to solve thermo-chemical equilibrium flow. Then, some corresponding energy exchange mechanisms and transport models were discussed.Some numerical methods such as the finite difference NND scheme and finite volume NND scheme were analyzed in detail. The uncoupled method and coupled one used to solve thermo-chemical nonequilibrium flow were discussed. The dot implicit method used to solve source terms of species and energy in chemical reaction was studied. Then, the LU-SGS implicit method was given to solve the non-viscous flux.The ignition model of plasma in numerical computation was discussed. The governing equations based on thermo-chemical non-equilibrium model with three-temperature and 11 air species were discretized numerically by finite difference NND scheme. The inverse bremsstrahlung absorption and plasma resonance absorption were considered when laser is transmitted in air plasma. The corresponding computing codes were implemented to simulate the formation and evolvement of plasma absorptive wave at one dimension plane/ cylindrical/ spherical wave. The variations of some key parameters in plasma absorptive wave were studied in detail. An interesting phenomenon was found, that is, when laser is focused, the maximum electron density decreases with the progress of plasma absorptive wave, but the maximum electron temperature increases. Two simplified models based on the characteristics of plasma absorptive wave were proposed to solve the absorptive process and aggradation of laser energy: 1) the limited electron density model. 2) the velocity model of plasma absorptive wave.The operating process of laser propulsion lightcraft was computed based on the limited electron density model. In order to improve the computational efficiency, the absorptive process and aggradation of laser energy was solved by the thermo-chemical non-equilibrium model with three-temperature and 11 air species, and the followed flow field was solved by the high temperature equilibrium air model. The governing equations uncoupled with the source terms of species and energy were discretized numerically by finite volume NND scheme. The corresponding two dimensional programs were implemented. The time when the thermo-chemical non-equilibrium model was switched to equilibrium air model was studied. The results show that thermo-chemical non-equilibrium effect has significant influence on the propulsion performance of lightcraft. The predicted coupling coefficients for the lightcraft by simulation are coincident reasonably well with those of tests done by Schall. In addition, the influence of some laser parameters such as single pulse energy and pulse width on the propulsion performance of lightcraft was also discussed.The velocity model of plasma absorptive wave was extended to spheral and two dimensional cases. The operating process of laser propulsion lightcraft in different incident laser was simulated based on high temperature equilibrium air model. The corresponding two dimensional programs were also implemented. The influence by velocity coefficient on the flow field parameters and the propulsion performance of lightcraft was studied. The flow field characteristic was analyzed when the focused laser has two- dimensional distribution feature. The results show that the space distribution of laser may have significant influence on the flow field.