Investigation of Thrust Performance and Physical Phenomena in Laser Propulsion

为了建立实现激光推进系统推力性能的标度规律，研制了连续波激光推进器。利用光学诊断技术研究激光持续等离子体（LSP）的物理现象，如激光吸收和加热过程、电离和激发现象、传热和能量耗散过程。此外，设计并制作了1kw级激光推进器，通过改变推进剂种类、流量、激光输入功率等工作参数来测量推力和性能，并研究了推力性能与上述物理现象的关系。首先通过推力测量得到了连续波推进器的比冲和能量转换效率。并对其推力性能的基本特性进行了研究。其次，对激光等离子体进行光学诊断，获得等离子体的电子温度、等离子体密度及其空间分布等特性；第三，研究了LSP特性对推力性能的影响。结果表明，高温区越平坦，推力性能越好。采用多聚焦聚光透镜可以扩大LSP的尺寸，特别是在轴向上，获得了最佳的性能。此外，对振荡现象的分析研究表明，稳定运行需要对振荡进行控制。最后，成功地证明了以氩气为工作气体时的比冲为300 s，对应于以氢气为工作气体时的比冲大于1000 s。它的比冲是化学火箭推进的三倍。

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.