激光烧蚀瑞利-泰勒不稳定性弱非线性气泡加速和尖钉减速机理研究

Inertial fusion energy (IFE) will play an increasingly important role in world future energy. At the present time, the research of inertial confinement fusion (ICF) reaches its full speed-up preparation stage. Laser-plasma interaction is an important research subject in ICF. However, the laser ablative Rayleigh-Taylor instability (RTI) hinders the success of ICF. It is the restricting factor and the scientific risk for ICF ignition with small drive energy. In ICF, the shell undergoes the RTI both in the acceleration and the deceleration phases. At the initial acceleration stage, when the imploding shell is accelerated inward by the low density blow-off plasma, the outer shell ablation front between the shell and the ablated plasma is unstable to the RTI, while at the later stage, when the compressed fuel begins to decelerate the imploding pusher, the inner shell surface between the hot spot and the shell is also prone to the RTI. This inherent physical instability can limit the implosion velocity and, in some cases, even break up the implosion shell. Furthermore, the instability would preclude the hot spot formation resulting in the auto-ignition failure. Therefore, the ICF targets must be designed to keep the RTI growth at an acceptable level. In astrophysics, the RTI plays a central role in the evolutions of many astrophysical phenomena, such as supernova explosion, high Mach number jets, etc. Moreover, in defense research, the ablative RTI is a key factor in high energy density material mixing. Therefore, it is essential to understand and estimate the evolution of the RTI because of its significance in both fundamental research and engineering applications. In this research, we investigate the formation mechanism of the bubble acceleration and the spike deceleration in laser ablative RTI. This research aims at fit empirical formulas for velocities of the bubble and the spike suitable for the ICF target design, and improves the understanding for the formation mechanism of the astrophysics jets and of the high energy density jets in laboratory experiments. This research is focused on the following topics: (1) study the interface width effect and obtain the formulas for the bubble and the spike velocities including the interface width effect; (2) separate the laser ablative effect by comparing the classical RTI and the laser ablative RTI; (3) study the bubble and the spike velocities in different preheating cases and fit the formulas for the bubble and the spike velocities including the preheating effect. Numerical simulation is adopted as the main research method. The theoretical approach will also be used in this research. The simulation results will be compared with the experiment results. This research is expected to improve the understanding of the astrophysical jet formation, of the high energy density laboratory jet-like spike phenomena, and of the hot spot formation in central ignition ICF.

激光与物质相互作用是惯性约束聚变（ICF）的重要研究课题,而激光烧蚀瑞利-泰勒不稳定性(RTI)是实现较小驱动能量点火的制约因素和科学风险。激光烧蚀RTI引起的气泡和尖钉结构会限制内爆速度、延缓点火热斑形成，甚至会导致点火失败。同时烧蚀 RTI是天体物理射流形成的重要机制，是影响国防研究中高能量密度物质混合的关键因素。本课题研究激光烧蚀RTI弱非线性气泡加速和尖钉减速机理，拟合适合ICF 点火靶设计的气泡和尖钉运动速度经验公式，提高对天体物理射流及实验室高能量密度射流形成机制的物理认识。主要研究内容：(1) 研究纯流体RTI中界面宽度效应，得到包含界面宽度效应的气泡和尖钉运动速度公式；(2) 通过对比研究激光烧蚀情况和纯流体情况的差别，分离激光烧蚀因素；(3) 研究预热强度，拟合包含预热因素的气泡和尖钉运动速度经验公式。研究方法主要采用数值模拟，结合理论分析，辅助与实验结果的比对。

烧蚀瑞利-泰勒(RT)不稳定性是影响中心热斑点火惯性约束聚变(ICF)成功的关键科学问题。预热烧蚀RT不稳定性中大量出现射流状长气泡尖钉结构。这种长气泡尖钉结构在ICF加速阶段可以引起壳层破裂引起烧蚀层材料混入热斑，在减速阶段可以延缓热斑的形成，缩小热斑的体积，延后热斑点火，甚至破坏点火热斑的形成。本项目针对ICF内爆实验数值模拟中大量出现的射流状长气泡尖钉结构开展了从分解到综合的深入物理研究。研究表明预热是烧蚀 RT不稳定性射流状长尖钉形成的直接原因；早期预热烧蚀显著地增强密度梯度致稳效应，有效抑制高次谐波的发展，主模增长占优势；中期预热烧蚀引起的烧蚀面速度剪切层宽度效应有效抑制 KH 不稳定性的发展；后期预热烧蚀引起非线性气泡加速最终导致射流状尖钉的形成。我们建立了收缩几何中运动界面和有限厚度流体多界面RT不稳定性弱非线性理论，获得了收缩几何效应和薄壳效应加剧内爆RT弱非线性发展的重要物理认识，形成了皮实的点火内爆需要降低内爆收缩比、增加壳层厚度来控制流体非线性发展的物理思想，研究成果为我国点火靶物理研究提供了重要物理支持。此外，通过分析内爆流体不稳定性各个阶段的增长规律，针对目前美国NIF内爆的实验和点火面临的困难，我们提出了控制热斑界面流体力学不稳定性增长的新物理思想。