Investigation of Thrust Performance and Physical Phenomena in Laser Propulsion

为了建立实现激光推进系统的推力性能定标规律，研制了一台连续波激光推进器。利用光学诊断技术研究了激光持续等离子体(LSP)的物理现象，如激光的吸收和加热过程、电离和激发现象、热传递和能量耗散过程。此外，设计并制造了一台1kW级激光推进器，通过改变推进剂种类和流量、激光输入功率等工作参数来测量推力和性能，并考察了推力性能与上述物理现象的关系。其次，对激光等离子体进行了光学诊断，得到了电子温度、等离子体密度及其空间分布等等离子体特性；第三，研究了激光等离子体特性对推力性能的影响。结果表明，高温区外形越平坦，推力性能越高。多聚焦聚光透镜的性能最好，它扩大了LSP的尺寸，特别是在轴向上。此外，对振荡现象的分析研究表明，稳定的运行需要控制振荡，最终成功地演示了以Ar为工作气体的300 S的比冲，相当于在氢气情况下S的比冲超过1000。与化学火箭推进相比，其比冲大了三倍。

In this work a continuous wave (CW) laser thruster has been developed in order to establish scaling laws of thrust performance for realizing laser propulsion system. Optical diagnostic techniques were used to investigate physical phenomena of laser sustained plasmas (LSP), such as laser absorption and heating processes, ionization and excitation phenomena, heat transfer and energy dissipation processes. In addition, a 1Kw-class laser thruster was designed and fabricated to measure thrust and performance by varying the operating parameters such as propellant species and flow rate, laser input power and to examine the relation between thrust performance and the physical phenomena described above.At first specific impulse and energy conversion efficiency of the CW thruster were obtained by thrust measurement. Also, fundamental characteristics of its thrust performance were investigated.Secondly, optical diagnostics of laser plasmas were conducted to obtain plasma properties such as electron temperature, plasma density, and their spatial distributions.Thirdly, influence of LSP properties on thrust performance was examined. It was found that the flatter profile of the high temperature region leads to the higher thrust performance. The best performance was obtained with a multi-focused condensing lens, which expands the size of the LSP, particularly in the axial direction. Moreover, an analytical study on oscillation phenomena suggested that the stable operations would require the control of the oscillation.Finally specific impulse of 300 s with argon as the work gas was successfully demonstrated, which corresponds to specific impulse of more than 1000 s in case of hydrogen gas. It resulted in three times larger specific impulse compared with chemical rocket propulsion.