空间较低能量密度纳秒激光脉冲辐照金属靶的能量耦合瞬态过程及力学效应机理研究

The mechanical effects induced by irradiating target with high power laser can be used in space debris removal. A scientific problem in this field is that, in order to improve thrust performance, if it is possible to heat the target surface rapidly to a high temperature high impulse generation state and keep it for sufficient long time, with using of polarized laser and a nanosecond pulse with special pulse shape which includes a high power leading peak stage and a relative low power tail stage. Polarization sate is selected as an influence factor because it can change surface reflectivity when irradiation target with laser beam. This research plans to resolve the problem by measuring the transient process of laser energy coupling to target and the generated impulse, and study the influence mechanisms of several related factors as well. In the research, measuring of the transient process of energy coupling is one difficult part. For this, we propose to use photon diode array to measure the special distribution of reflected laser power during the laser irradiating process. Based on the energy coupling result and the material specification parameters, a model of the target surface temperature changing process when it is irradiated by a nanosecond laser pulse is establish. The model includes parameters like laser specification, surface temperature and pressure. The calculated results with the model are used to compare with that obtained using a micro-impulse measuring device. The compare results are going to be used to verify the validity of the model and perform further modification to it for optimizing of the laser parameters in a future practice laser propulsion operation in space.

激光辐照物质产生的力学效应可应用于激光空间碎片清除。该领域目前面临的一个科学难题是，能否在较低激光能量密度下，通过对激光偏振态及纳秒脉冲各阶段功率的主动调制，实现对靶材的初期快速加热以迅速达到高温高推力状态，并使其维持较长时间，以优化推进效果。偏振态选择能改变靶面反射率从而影响升温速率。调制的激光脉冲包含一个用于初期快速加热靶材的高功率短脉宽段及一个紧随的用于维持高温状态的较低功率段。本项目拟通过对物质的能量耦合瞬态过程及宏观力学效应的测量解答该难题并研究相关参数产生影响的机理和机制。其中对瞬态过程的测量是该领域的难点，采用光电二极管阵列探测过程中的反射光功率空间分布是一种有效的技术途径。基于该能量耦合过程及材料物性参数，建立包含激光参数、靶面温度和压力的激光辐照下物质升温模型，与用微推力测试装置测得的宏观推力结果进行比较，对模型进行反馈修正并实现对低能量密度条件下空间激光推进方案的优化。

激光推进利用高功率激光辐照物质时在辐照区域引起的高温热气团喷射反冲力对靶面产生推进作用，具有远距离非接触作用、对激光平台无反作用力、平台无工质消耗等优点，在空间碎片清除、失控卫星消旋等方面有巨大的潜在应用前景。然而，在这些应用中，主要受限于天基平台脉冲激光器脉冲能量水平和激光空间远距离传输光束扩散等效应影响，到达目标的激光能量密度较低，难以获得理想的推进性能，这是当前制约上述应用实现的关键问题。针对此问题及空间目标非配合的特点，本项目以脉冲激光烧蚀铝靶的冲量耦合情况及能量吸收瞬态过程为研究对象，通过对激光偏振态及入射角度调节、脉冲功率波形主动调控的方法，实现对激光烧蚀推进性能的提升，阐明通过对以上激光参数调节实现冲量耦合提升的调控机理，完善较低能量密度条件下激光烧蚀金属冲量耦合模型，结合数值模拟研究结果，指导空间激光推进应用方案中对激光参数的设计和优化，提升激光推进性能，提高技术成熟度，为实现激光推进空间碎片清除等应用提供理论指导和技术支撑。