激光烧蚀掺杂金属聚合物羽流屏蔽特性数值研究

Numerical investigation on shielding properties of the laser ablation plume of polymer doped metal

激光烧蚀推进中,烧蚀羽流对入射激光的屏蔽效应是影响推进性能的重要因素;当采用掺杂金属聚合物作为工质时,易电离金属掺杂物的引入,使屏蔽效应更加明显.本文建立了激光烧蚀掺杂金属聚合物羽流飞散及电离、屏蔽模型,计算了3—40 J/cm^2激光烧蚀掺杂微米铝颗粒聚甲醛工质的比冲,与实验数据对比表明模型能够有效模拟掺杂聚合物羽流的屏蔽特性.获得了不同激光能量密度下的电子数密度、吸收系数分布及屏蔽系数时间变化曲线.结果表明:低激光能量密度(5 J/cm^2)时,羽流屏蔽效应以未完全分解聚合物短链对入射激光能量的吸收为主;高激光能量密度(20 J/cm^2)下,羽流电子数密度逐步增大至1020m^(-3),形成明显的等离子体吸收,屏蔽系数的时间变化特征复杂.本文对掺杂金属聚合物烧蚀羽流的屏蔽特性进行了定量研究,可为激光烧蚀推进性能优化提供参考.

For laser ablation propulsion, the shielding effect of ablation plume on the incident laser is an essential factor affecting the propulsion performance. When the polymer doped metal is utilized as the propellant, the shielding effect would be more significant because the metal dopant is easily ionized. In order to study the shielding effect of ablation plume on the incident laser energy, a laser ablation model with taking into account the plume expansion, ionization and the shielding effect is built in the present work. For the polyoxymethylene doped aluminum particles irradiated by a laser with a fluence of 3–40 J/cm^2, the specific impulse of laser ablation is calculated, and the consistency of the numerical results with the experimental data demonstrates the availability of the model. Furthermore, the effects of both the incompletely decomposed polymer chains and the plasma induced by laser ablation on the incident laser are considered. The time variations of electron number density distribution under different laser fluences are calculated based on the laser induced ionization model. Subsequently, the absorption coefficient distributions and the time variations of shielding coefficient under different laser fluences are obtained. The results show that at a low laser fluence（ 5 J/cm^2）,the electron number density is small, so the plume shielding effect is dominated by the laser energy absorption of the small polymer chains which are not completely decomposed. While at a high laser fluence（ 20 J/cm^2）, small polymer chains are almost completely decomposed into atoms even plasma, hence the shielding effect is dominated by the plasma since the electron number density in the plume increases up to 1020m^（-3）, and the complicated characteristic in the time variation of shielding coefficient appears. The quantitative analysis results obtained in the present work can be helpful for optimizing the performances of laser ablation propulsion.