基于断裂力学的KDP晶体表面缺陷对其激光损伤性能影响机制研究

When KDP crystals are illuminated by nanosecond laser, damage induced by the pre-existing surface defects may occur and will adversely affect the optical transmission property and the lifetime of the crystal. Thus, laser induced damage mechanism of the surface defects is an imperative scientific problem, which is closely related to the realization of laser fusion successfully. Based on the knowledge of the mechanical property of KDP crystal, this subject is going to investigate the deformation and fracture behavior of the crystal material during the laser damage process. A constitutive model of KDP crystal under the condition of high strain rate is going to be established, and it will lay the foundation for simulating the laser damage process. Then, the shock wave energy at the initial stage of the laser damage process is going to be measured, and the result will be input into the fracture simulation model for reproducing phenomenon such as material deformation, cracks propagation and material spalling that occurs in the laser damage process. Finally, influence of the size, the fracture/ductile character and the distribution of the surface defects on the laser induced damage threshold and the growth behavior of the damage zone will be analyzed, and evaluation index of the KDP surface defects will be proposed. The goal of this subject is to reveal the influencing mechanism of surface defects on the laser damage property and provide guidelines for improving the performance and extending the lifetime of KDP crystal. Thus, it is of very great scientific significance and practical value to carry out this research.

纳秒激光作用下KDP晶体表面缺陷极易导致晶体材料发生激光损伤，严重影响晶体的光学传输特性和使用寿命，是关系到能否顺利实现激光聚变点火而亟需解决的重要科学问题。本课题从KDP晶体基本力学性能出发，对激光作用下晶体表面缺陷诱导损伤过程中材料的变形及断裂行为进行研究：建立高应变率条件下KDP晶体的本构模型，为理论模拟激光诱导损伤过程奠定基础；根据激光损伤初始阶段冲击波能量测量结果，建立冲击波载荷下缺陷诱导损伤过程的断裂仿真模型，再现激光损伤过程中KDP晶体的变形、裂纹扩展及材料剥落等现象；综合激光损伤实验及仿真结果，明确表面缺陷尺寸、脆塑特性及分布特征等参数对激光损伤阈值及损伤增长行为的影响，并据此提出KDP晶体表面缺陷评价指标。课题旨在深入揭示表面缺陷对KDP晶体激光损伤性能的影响机制，为提高晶体使用性能和寿命提供理论与实验依据，对其进行研究具有重要的科学意义与实用价值。

紫外纳秒脉冲激光作用下，磷酸二氢钾（KH2PO4, KDP）晶体元件常在远低于其本征击穿阈值的条件下发生激光损伤；并且，与元件内部的体损伤相比，KDP晶体的表面损伤更易在后续激光辐照下发生扩展从而缩短晶体元件的使用寿命。因此，KDP晶体表面损伤已成为制约惯性约束核聚变（Inertial Confinement Fusion, ICF）驱动装置负载能力进一步提升的瓶颈问题。.针对KDP晶体表面缺陷，分别使用原子力显微、荧光显微及能谱分析等测试技术对其进行表征评价，并根据缺陷理化特性的差异对其进行分类。在此基础上，对各类缺陷的激光损伤特性进行实验研究，获得紫外纳秒激光作用下各类表面缺陷微观形貌的变化情况及其激光损伤阈值，并据此提出导致KDP晶体表面发生低阈值损伤的典型损伤前驱体类型。综合KDP晶体表面创成过程及不同类型缺陷的理化特性，分析了KDP晶体飞切加工表面典型缺陷的形成原因，并提出相应的缺陷抑制措施。.在明确了KDP晶体表面典型损伤前驱体的基础上，进一步对纳秒脉冲激光作用下晶体表面损伤过程进行建模分析。基于对纳秒激光诱导KDP晶体损伤机理及损伤过程的认识，提出可将纳秒激光损伤这一复杂的多物理场耦合问题简化为爆炸问题进行研究的思路。为了对损伤过程中晶体材料断裂及损伤坑形成过程进行准确模拟，通过Hopkinson冲击实验结合冲击过程建模仿真的方法确定较高应变率条件下KDP晶体的未知力学性能参数。随后，建立纳秒激光诱导KDP晶体表面损伤过程的等效微爆炸仿真模型，分析引起晶体表面损伤性能产生差异的主要原因，并获得缺陷类型、爆炸能量对损伤坑尺寸、冲击波速度及材料喷溅速度等物理量的影响规律。.最后，以KDP晶体表面典型缺陷为对象开展激光损伤实验，并采用时间分辨泵浦探测技术（Time-resolved pump and probe, TRPP）对损伤过程中涉及的超快现象进行观测。将实验测得的冲击波传播及材料喷溅规律与仿真结果进行对比，验证仿真结果的准确性，同时也证明采用爆炸仿真方法来研究纳秒激光损伤过程的思路是可行的。