太空化学激光推进中等离子体时空演化特性及能量转化机理研究

Space laser propulsion can propel the spacecraft to reach the magnitude level of light velocity in short time due to friction-free acceleration under space environment, and thus is expected to achieve the destination for human to explore the universe and interstellar space. As a new technology, current space laser propulsion is still dependent on the interaction between laser and propellant. Meanwhile, several urgent issues, i.e., selecting applicable high-power laser and enhancing the energy conversion efficiency are mainly concerned. In this project, the chemical laser with high output power and good beam quality is used as the energy source of space laser propulsion. Moreover, the energy conversion mechanism is studied. The energy absorption and conversion in the laser propulsion are relevant to plasma characteristics and the induced airflow field, and are influenced by external conditions such as laser wavelength and propellant property. Therefore, the experimental investigation and numerical simulation on the interactions between chemical lasers with various wavelengths and different working mediums are proposed. In the experiment, the spatiotemporal evolution characteristics of plasma and airflow field are measured to determine the absorption mechanism and conversion efficiency of energy. In the simulation, a coupled multi-physics model is developed to reveal the transfer process of energy parameters including the temperature, pressure, momentum and mass, etc. The rationality of the results will be verified by the comparison between experimental diagnostics and model predictions, and then the energy conversion mechanism will be further analyzed. This project is able to provide the necessary theoretical basis and guidance for the establishment and design optimization of high-efficiency space laser propulsion system.

太空激光推进可使飞船在太空中实现无摩擦加速并在短时间内达到光速量级水平，有望满足人类探索宇宙及星际空间的迫切需求。对当前的太空激光推进而言，高性能激光器的选择应用以及提高能量转化效率是亟需解决的问题。基于此，本项目拟采用输出功率大以及光束质量优良的化学激光作为激光光源并展开能量转化机理的研究工作。由于激光推进中能量的吸收转化与等离子体及其流场特性相关，同时受激光波长、工质特性等外界条件的影响，所以本项目将对不同化学激光波长、不同工质特性下等离子体产生及输运过程进行实验和理论模拟研究。主要对等离子体特征参量及流场特性进行时空演化测量，明确能量的吸收机制及转化效率，并结合多物理场数值模拟，揭示复杂物理过程中温度、压力、动量和质量等参数的传递方式。将实验及理论模拟结果相互验证，进一步分析其能量转化机理，为高效太空激光推进体系的研发以及设计优化提供必要的理论依据和指导。

激光推进通过激光与工质的相互作用产生等离子体羽流进行驱动，具有理论比冲大、有效载荷比高、推力动态范围广等优势，有望在飞行器姿态调整、轨道控制及深空探测任务中发挥重要作用。激光推进作为一项新兴技术，明确能量转化机理并提高推进效率是亟需解决的问题。基于此，我们开展了激光与金属及聚合物工质相互作用等离子体及其动力学特性的研究工作，并主要对不同激光能量及气压条件下产生的冲量及耦合系数、等离子体时空演化图像、特征参数、烧蚀形貌和质量进行了诊断测量。实验发现，相同能量下等离子体密度均随气压的增加而升高，但烧蚀深度及质量却逐步减少，这是因为高密度等离子体通过逆韧致吸收机制形成了较强的激光屏蔽效果。此外，大气压下，随着激光能量的增加，等离子体逐步脱离工质表面且持续较短时间，而近真空条件下，等离子体剧烈膨胀并快速淬灭，这些因素导致其产生的冲量及耦合系数较低。中间气压范围内，等离子体持续较长时间并保持与工质作用，进而有效提高了体系的动力学性能。不过，在相同的外部条件下，激光与聚合物工质相互作用产生的等离子体持续时间、电子密度及温度相较于激光与金属工质作用而言更高，这主要是不同的能量吸收机制所导致的。聚合物具有较低的热传导系数和烧蚀阈值，激光与其作用后产生了较大的烧蚀深度及质量，形成了体吸收机制及高能量、高密度等离子体。而金属具有良好的导热性和较高的烧蚀阈值，激光与其作用后产生的烧蚀深度及质量较小，形成了面吸收并导致电离蒸发气体的能量减弱，因此，电子密度及温度相对较低。聚合物较高的电子温度能够增强其电子在传播过程中的激发和电离能力，从而使等离子体持续时间相较于金属进一步延长，并使聚合物工质产生的冲量及耦合系数增加。材料的物化性质和外部条件变化决定了激光能量吸收机制及等离子体特性，进而影响了动力学行为。该项目研究为提高激光推进性能提供了可靠的理论依据。