真空中脉冲激光烧蚀掺杂聚合物冲量规律的数值与实验研究

Laser ablation propulsion technology has special advantages in micro-spacecraft launch and space propulsion, and doping absorbers (e.g. metal, carbon) into polymers is an important way to enhance the performance of propellants. However, because the detailed ablation mechanism of doped polymer and the shielding effect of ablated products in vacuum are not very clear, present researches have not reached a consensus on the mechanism of impulse generation and performance characteristics. Combining numerical simulations and theory analysis, in this project an integrated model which contains laser energy deposition, doped polymer ablation and ablated products flow will be established, based on the mecro-distribution of dopants in the polymer. And numerical simulations of ablation and impulse generating process will be investigated, considering the different ablation mechanisms of dopants and polymers and the enhancement of shielding effect by the dopant. Meanwhile, with the TEA CO2 laser which has pulse energies of hundreds joule, experimental study for the impact of the species, proportion and scale of the dopants and the energy fluence on the specific impulse and impulse coupling coefficient would be conducted. By the comparison between simulative and experimental results, the model and the computational method would be refined, and the impulse generating mechanism of doped polymers and the characteristics of propulsion performance in vacuum would be confirmed, which will make the interaction between laser and matters more clear.

激光烧蚀推进在微小航天器发射及空间推进中具有独特优势，其中以聚合物掺杂增强吸收物质（如金属、石墨）作为工质是推进性能改进的重要方向。由于目前掺杂聚合物在真空中的烧蚀机制、产物屏蔽效应等尚不明确，其冲量产生机理及推进性能规律尚未获得一致看法。本项目采用理论建模和数值模拟手段，基于掺杂物在聚合物中的细观分布，建立包含激光能量沉积、工质烧蚀、产物流动的一体化模型，考虑掺杂物与聚合物的不同烧蚀机制、掺杂物引起产物屏蔽效应的增强作用，开展烧蚀和冲量产生过程的数值模拟；同时利用最大500J脉冲能量的TEA CO2激光器，开展掺杂物种类、比例、尺寸及激光能量密度对比冲和冲量耦合系数影响的实验研究；通过综合数值和实验结果，完善模型和计算方法，确定真空中烧蚀掺杂聚合物的冲量产生机理及推进性能规律，深化激光与物质相互作用的认识。

激光烧蚀推进技术采用脉冲激光烧蚀聚合物产生瞬态冲量，这一推进方式因具有可控性好、安全性高等优势而受到关注。烧蚀对象的性质对冲量产生过程有重要影响，进而影响到以冲量耦合系数、比冲表征的推进性能。其中，聚合物掺杂金属是一项重要的改进方向。. 本项目以真空条件下掺杂金属聚合物为研究对象，通过数值模拟和实验测量研究了其在脉冲激光辐照时烧蚀产生冲量的过程和规律。建立了激光烧蚀聚甲醛的热-化学耦合模型，采用掺杂聚合物细观模型以模拟烧蚀过程，着重考虑引入掺杂物造成的激光吸收增强和掺杂物/聚合物界面处的能量沉积差异，及由此导致的非均匀加热、相变、热解和机械剥蚀；建立了烧蚀产物流动模型以模拟其飞散过程，着重考虑产物飞散时对激光的吸收，及由此引发的离解、电离和对后续激光的屏蔽效应。在此基础上，调整入射激光能量密度及掺杂物的比例、物性参数，模拟和分析了不同条件下的冲量产生过程及比冲、冲量耦合系数等性能参数。采用TEA CO2激光器配合真空实验舱及冲量摆测量系统，建立并完善了激光烧蚀推进真空测量平台。调整掺杂物比例等影响因素，测量单脉冲激光作用下掺杂聚甲醛产生的质量迁移和脉冲冲量，由此获得推进性能参数以验证计算结果。结果表明：（1）金属掺杂物可增强烧蚀产物对激光吸收，减缓对靶材的烧蚀，使更多激光能量沉积于烧蚀产物中，有利于增大飞散速度；（2）金属掺杂物在动态烧蚀过程中易产生低速的机械剥蚀现象，降低质量利用效率；（3）高掺杂比例下掺杂物的质量损失以剥蚀为主，导致低的推进性能；低掺杂比例情况下，由于掺杂聚合物可在较低的能量密度下有效吸收入射激光，从而在一定程度上改善推进性能。. 项目研究对于深化激光与物质相互作用的认识、加深激光烧蚀冲量产生机理的理解有着重要意义，其研究成果对于航天推进、工业加工、惯性约束核聚变等多种领域的应用均具有参考价值。