【作者】 张凌；

【导师】 唐志平；

【作者基本信息】 中国科学技术大学 ， 工程力学， 2008， 博士

【摘要】 激光推进是利用高功率激光与物质相互作用产生的高温高压等离子体反喷产生推力驱动飞行器前进的一种新概念推进技术。固态工质由于其易加工,可操作性强,成为烧蚀模式的首选工质形态。本文就固态工质的烧蚀模式激光推进进行探讨,采用数值模拟和实验等手段,深入研究该推进模式的物理机制和推进力学效应,得到了一些有意义的结果。建立了激光辐照下固态靶蒸汽等离子体形成的动态烧蚀耗能模型和光线跟踪激光能量等离子体吸收模型。结合等离子体理想状态方程,采用WENO空间差分格式和Runge-Kutta时间离散格式编写了二维轴对称流体力学程序LDAP-2D,该程序能够完整地计算激光作用下固态单原子靶的动态烧蚀、蒸汽等离子体喷射以及流场演化的整个过程。大大提高了流场和推进性能的计算精度。跟踪脉冲激光能量在等离子体流场中的吸收过程,发现微秒量级脉宽时间内激光被吸收的过程与流场膨胀发展相耦合,使得激光吸收过程出现振荡现象。追踪脉宽时间内激光能量的转化规律,以及激光熄灭后流场膨胀过程中流场内能和动能转化的规律发现,短波长利于将激光能量向流场动能转化并维持流场中较高的动能转化率,提高激光推进的能量利用率。固定入射激光功率,研究脉冲能量、功率密度等对推进效应的影响。激光脉冲能量增大,工质靶获得的冲量增大;同一功率密度下,脉冲能量增大,冲量耦合系数减小;比冲随脉冲能量的增大而增大。同一脉冲能量下,冲量耦合系数随功率密度的增大呈指数衰减;比冲随功率密度的变化不大。针对应用于激光推进的激光器的波长——1.06μm、1.315μm、3.8μm以及10.6μm进行研究,冲量耦合系数随波长的增大而变小,比冲随波长增大而变大。考虑综合推进效应,波长适中（1.315μm和3.8μm）的激光比较理想。对铝、碳、铁和铜四种工质靶材进行数值模拟,发现原子量适中的工质材料综合推进效应较好,追踪其能量转化过程发现,短脉冲时,逃逸速度大不利于提高动能转化率和激光推进的能量利用率。同一功率密度较长脉宽下,流场动能和内能出现振荡,动能转化率可以多次达到最大,但是对推进效应贡献不大。几十到几百纳秒量级的脉宽对材料的综合推进效应较好。对激光辐照固态工质材料产生的等离子体进行光谱诊断。记录了六种材料（铝、铜、钢、石墨、PVC和POM）的发射光谱,研究了它们随脉冲能量的变化规律。在局部热平衡假定下,由铝和铜特征谱线的相对强度得到了二者的电子温度,范围为（7.9-25）×10³K;由谱线展宽得到铝等离子体的电子密度在（4-9）×10¹⁷cm^-3范围内。还利用相机B门拍摄到真空中六种材料在烧蚀瞬间的喷射形貌。对六种材料在大气环境下进行推进效应的实验研究。结果表明,所有材料单脉冲作用下的冲量耦合系数都略高于双脉冲;冲量耦合系数Cm随功率密度增大先增大后减小;聚合物靶材的冲量耦合系数要普遍高于金属材料,且对功率密度的变化更为敏感;Delrin（POM）材料是性能优异的待选工质。本文还对相关文献中的实验研究进行综述,将其中的实验结果汇编成数据库,供以后的研究者查找。更多还原

【Abstract】 Laser propulsion is a new concept propulsion technology by which the aircraft is promoted utilizing the counterforce generated during the injection of plasma with extremely high temperature and pressure which formed by interaction between high-power laser and propellant. Solid matter is the most promising state of propellants for its easy processing and maneuverability. In this dissertation, it focuses on the ablation mode laser propulsion with solid propellant. Physical and mechanism of this mode have been studied through numerical simulation and experiment. Some significant results are achieved.The energy consumption model for plasma formation in vapor through the dynamic ablation of solid propellants and laser energy absorption model introduced by beam tracing method are built in this dissertation. A program named LDAP-2D for 2D axis symmetrical hydrodynamic is made using WENO spatial difference scheme and Runge-Kutta temporal discretization scheme and combining the ideal state equations of plasma. The interaction between laser and monatomic solid matter could be studied by this program including the dynamic ablation of solid matter by laser, the formation and injection of vapour plasma, the evolution of flow field which is favorable to study the physical mechanism and mechanical efficiency of ablation mode laser propulsion. It will enhance the precision of numerial simulation for flow field.It is found that the absorption of laser by vapor plasma is coupled with the evolution of flow field and the oscillation of absorption shows up at pulse width inμs order. It is also found that short wavelength is beneficial to transform the laser energy to the dynamic energy of flow field, more energy deposited in condensed target favors propulsion efficiency and larger v_e makes against improving the propulsion efficiency.Study on that the laser energy and intensity effects on propulsion efficiency at laser intensity great than threshold intensity shows that the impulse gained by propelling target increases with the pulse energy, Cm decreases with the laser energy at the same intensity, while Isp increases with it. Cm is in exponential decay with laser intensity at the same energy, while Isp changes not much with it.Study on the wavelength of the four lasers candidates （1.06μm, 1.315μm, 3.8μm, 10.6μm） effect on propulsion efficiency shows that Cm decreases with the laser intensity at each wavelength and decreases with the wavelength at the same laser intensity. At the first three wavelengths, Isp decreases a little then increases back with laser intensity, and increases with wavelength, while it almost decreases linearly with intensity. So that the two wavelength 1.315μm, 3.8μm are better choice regarding to Cm and Isp synthetically.Study on C, Al, Cu, Fe propellants target shows that propellant which has moderate atom is good choice, pulse duration of tens or hundreds of ns benefit the synthetic propulsion efficiency, it will oscillate between innet energy and kinetic energy in flow field while not helping improving the propulsion efficiency.We record the emission spectrum of Al, copper Graphite, PVC, POM plasma. The space- resolved and the power density-depended spectrum are studied. Under the local thermo equilibrium supposition, we obtain the electron temperature and the electron density with regard to Al and copper from the relative intensity and broadening of characteristic spectrum. Their dependence on laser power-density and spacial variation are also studied. The obtained electron temperature of Al and Cu is in the range of （7.9-25）×10³K and electron density of Al is in the range of （4-9）×10¹⁷cm^-3. At the same time, we take the photos of instant ablation of the six materials in vacuum by the camera’s B shutter and analyze the ejected matter.The propulsion efficiency of the six material in air is studied. The results show that all of the material have a little greater Cm under single pulse than two repeated pulses, Cm increases with laser intensity and then decreases after reaching the highest Cm, polymer material have sensitive variation with laser intensity and Delrin shows wonderful character for propulsion. Many related experiments are summarized in this dissertation and a database of the results of experiments is built.更多还原