作为一种新型的绿色高能液体推进剂，二硝酰胺铵（ADN）以其密度比冲高燃速可调等优势，成为了先进空间化学推进技术的一个热门发展方向。目前ADN基空间发动机已经取得了一定的在轨飞行经历，在推动其向更大推力量级发展、满足更严格的性能指标要求的过程中，迫切需要进一步的提高推力器的性能、拓展推力器的工作模式。随着近年来等离子体助燃领域的快速发展，逐渐形成了较为公认的助燃机理，即由局部压力以及温度升高、丰富的激发态粒子、流场电场的混合所引起的热效应、动力学特性以及输运特性的增强从而改善燃烧效果，将等离子体应用至高能离子液体推进剂领域内有望成为解决上述问题的重要途径。

本工作综合运用了多种光学诊断方式，聚焦于等离子体与ADN基推进剂的相互作用问题，系统的研究了由激光烧蚀ADN基推进剂产生的等离子体特性，对电子温度、电子密度进行了初步探究；通过引入氩气（Ar）载气解耦了等离子体与ADN基推进剂相互作用的输运特性，以混合比为参数讨论了推进剂与载气的混合程度，建立了原子谱线发射强度比与混合比之间的线性关系，实现了混合比的实时、原位测量；提出适用于辅助液体推进剂分解的等离子体应用准则，进一步优化了其产生方式，将等离子体源从激光转变为可持续性能量输入的交流电源，将所发展的定量测量方式应用于放电等离子体中，从等离子体参数、燃烧稳定性以及热解产物等方面证实了等离子体对于推进剂分解的促进作用。

首先，对激光烧蚀ADN基液体推进剂产生的等离子体特性进行了初步研究。基于发射光谱（OES）获取了等离子体的演化过程，通过玻尔兹曼斜线法和斯塔克展宽法分别确定了电子温度和电子密度。在液体推进剂环境中产生的电子弛豫时间为纳秒量级，而促进推进剂的分解要求时间尺度更长的等离子体能量输入，随后对等离子体产生方式进行了优化。

之后，使用交流电源作为等离子体源，在不同的输入功率以及气压工况下，确定了反常辉光放电、丝状放电以及电弧放电三种放电模态，综合基于OES得到的具有时空分辨特性的电子参数，以及基于平面激光诱导荧光（PLIF）获得的亚稳态原子空间分布信息，探讨了模态转换过程中发挥主导作用的不稳定性机制。对比了Ar、Ar与常温推进剂、Ar与预热推进剂三种放电介质的光谱图，常温工况下的放电介质光谱图中出现了H，OH谱线，预热工况下的放电介质在等离子体作用下出现了C，N₂，N₂⁺，C₂，NH，CH和CN等多种原子、离子、自由基的谱线，除了从光谱角度的直观说明外，预热工况下大幅减小的电子密度也侧面验证了等离子体对于推进剂分解的促进作用。

同时，为了能够解耦等离子体的输运机制，采用推进剂与载气的混合比表征混合的均匀程度，基于激光诱导击穿光谱（LIBS）诊断方式，评估了激光能量对于解离阈值的影响，在不同气压工况下建立了H/Ar、O/Ar、N/Ar三种原子谱线发射强度比与混合比之间的线性关系，并通过谱线半高宽确定当地压力，比较了三种发射强度比的灵敏度，实现了推进剂在载气环境中混合比的原位、实时测量。验证了所构建的混合比测量方案的技术可行性，该方案具有一定的普适性，可扩展至更大的气压工况以及混合比范围内，使得燃烧室内混合比的诊断成为可能，对于以提高混合均匀度为指标的推力器设计具有指导意义。

最后，参考星用正样的小推力量级ADN基发动机的结构、尺寸，设计了等离子体辅助型ADN基推力器，搭建了由高速相机、近红外光谱仪和中红外光谱仪组成的光学诊断系统。考虑到ADN基推进剂较为特殊的分解与燃烧进程，为了对比放电等离子体对于推进剂分解、燃烧反应进程的影响选取了NH₃、H₂O、CO₂三种热解和燃烧产物进行辐射测量。发现同等工况在放电等离子体的作用下，推进剂第一步热解产生的NH₃辐射强度减弱而H₂O辐射强度增强，同时作为完全燃烧产物的CO₂辐射强度大幅提高，得出了放电等离子体可以加快推进剂分解、燃烧进程的结论。同时基于本征正交分解（POD）的方法对燃烧室内的火焰结构进行分析，发现放电等离子体的存在减少了燃烧过程中的高频脉动，此外其加快了推进剂的反应速率，在燃烧室上游观察到了更加明显的火焰结构，减少了火焰向下游喷管处的移动。

Ammonium dinitramide (ADN), as an environmental-friendly energetic liquid propellant, has become to a new innovative research direction of advanced chemical space propulsion area due to its advantages of high specific impulse and adjustable combustion rate. At present, ADN-based aircraft engine has already been launched and operated in orbit for several times. It is emerged more technical requirements on the thruster performance as well as operating mode improvement while the promoting ADN-based thruster to a larger scale of thrust and satisfy stricter criteria. By the relentless march of plasma-assisted combustion technology, the enhancement mechanism has been accepted at present from three aspects: thermal, kinetic, transport effect caused by increased local pressure and temperature, number density of excited species and flow and electric field. To apply plasma in the energetic liquid propellant area is expected to become an important way to solve the above problems.

Combined optical diagnosis technologies were applied in this study to focus the interaction between ADN-based liquid propellant and plasma. Plasma produced by laser ablated inside propellant, its parameter such as electron temperature and electron density were explored on the first stage. Argon gas (Ar) was introduced to a constant volume chamber in order to decoupling the transport effect of enhancement mechanism. Mixture ratio, as a characterization of mixing uniformity, was invoked to discuss the local distribution of propellant in the carrier gas. Linear correlations between mixture ratio and emission intensity ratio of atomic spectral lines were established to achieve in-situ and real-time measurement. Optimized generation methods of plasma based on a selection criterion, specifically, change the plasma source from laser to AC power supply because discharge has a more sustainable energy input process. Developed quantitative measurements were applied to the discharge plasma and the promotion effect of plasma on the propellant decomposition was confirmed from plasma parameters and pyrolysis products point of views.   

First, preliminary research on the characteristics of plasma produced by laser ablation inside ADN-based liquid propellant was carried out. Evolution of plasma spectra were obtained by means of Optical Emission Spectroscopy (OES). Electron temperature and electron density were determined by Boltzmann slope method and Stark broadening method, respectively. The electron relaxation time generated in the liquid propellant environment is in the order of nanoseconds which is smaller than time scale of common chemical reactions. The production type of plasma was optimized because propellant decomposition requires a longer time scale of plasma energy input.

Then, AC power supply was used as the plasma source. Three discharge mode: abnormal glow discharge, filament discharge and arc discharge were observed within selected input power and chamber pressure range. Combining the electronic parameters with time-space resolution derived from emission spectra and the spatial distribution information of metastable atoms obtained based on PLIF, the instability mechanisms that played a leading role during mode transition processes were discussed. Spectra comparisons between three different discharge media: only Ar, Ar and ADN-based propellant under room temperature, Ar and preheated ADN-based propellant were put forward. While the spectra from only Ar case were composed of atomic and ionic spectral lines of Ar, room temperature case appeared H, OH spectral lines, preheated propellant case even displayed bunch of excited species such as C, N₂, N₂⁺, C₂, NH, CH and CN. An obvious reduction of electron density under preheated propellant case also verified the role of plasma in promoting the course of decomposition concurrently.

At the same time, mixture ratio between propellant and carrier gas was used to characterize mixing uniformity for the purpose of decoupling the transport effect of enhancement mechanism. The impact of laser energy on the dissociation threshold was evaluated based on Laser-induced Breakdown Spectroscopy (LIBS) method. Emission intensity of H, O, N, Ar atomic spectral lines were calculated as well as intensity ratios of H/Ar, O/Ar and N/Ar so that the linear correlations between these intensity ratios and mixture ratios were established with chamber pressure range of 40-100 kPa. Sensitivity of above three emission intensity ratios was analyzed and compared. With the information about local pressure extracted from Full Width at Half Maximum intensity (FWHM), in situ and real-time measurement of propellant mixture ratio in carrier gas environment was realized. The technical feasibility of the mixture ratio measurement scheme is verified, which has universality to a certain degree so that it can be extended to higher pressure conditions or larger mixture ratio range. This model experiment scheme has guiding significance for the design of thruster to improve mixing uniformity as well as making it possible to diagnose the mixture ratio in the combustion chamber of ADN-based thruster.

Finally, referring to the structure and size of real ADN based engine with minor thrust, a plasma assisted ADN based thruster was designed. Optical diagnostic system consisting of a high-speed camera, a near-infrared spectrometer and a mid-infrared spectrometer was built. Considering the special decomposition and combustion process of ADN based propellant, NH₃, H₂O and CO₂ were selected for radiation measurement in order to compare the impact of discharge plasma. It was found that with the action of discharge plasma, the radiation intensity of NH₃ (produced from the first step of propellant pyrolysis) is reduced while the radiation intensity of H₂O is increased, and the radiation intensity of CO₂ as the product of complete combustion is greatly increased. Conclusions of discharge plasma can accelerate the process of ADN-based liquid propellant decomposition and combustion had been bring forth. Proper Orthogonal Decomposition (POD) method was utilized to investigate the flame structure in the combustion chamber. The presence of discharge plasma had reduced high-frequency pulsation in the combustion process, in addition, it accelerated the reaction rate of the propellant. A more obvious flame structure was observed in the upstream of the combustion chamber, in other words, the movement of the flame to the downstream nozzle had been diminished.