Investigation of Thrust Performance and Physical Phenomena in Laser Propulsion

本文研制了一台连续波激光推力器，为实现激光推进系统建立了推力性能的定标律。利用光学诊断技术研究了激光维持等离子体（LSP）的物理现象，如激光吸收和加热过程、电离和激发现象、热传递和能量耗散过程。此外，设计并制作了一台1KW级激光推力器，通过改变推进剂种类、流量、激光输入功率等工作参数，测量推力和性能，研究推力性能与上述物理现象之间的关系，首先通过推力测量获得了CW推力器的比冲和能量转换效率。其次，对激光等离子体进行了光学诊断，获得了等离子体的电子温度、密度等特性及其空间分布，再次，研究了激光等离子体特性对推力性能的影响。结果表明，高温区较平坦的叶型具有较高的推力性能。使用多焦点聚光透镜获得了最佳性能，该透镜扩大了LSP的尺寸，特别是在轴向方向上。此外，对振荡现象的分析研究表明，稳定的操作将需要控制的振荡。最后，成功地演示了300秒的比冲与氩气作为工作气体，这相当于超过1000秒的比冲的情况下，氢气。与化学火箭推进相比，它的比冲大三倍。

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.