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题名	激光在等离子体冕区小尺度自聚焦成丝研究
作者	康宁
文献子类	博士
导师	林尊琪
关键词	激光与等离子体相互作用 Laser-plasma interactions 小尺度自聚焦成丝 Small-scale filamentation 粒子云网格模拟 Particle-in-cell simulation 双等离子体衰变诊断 Diagnosis of the two-plasmon decay instability
其他题名	Small-scale self-focusing laser filamentation in plasma corona
英文摘要	惯性约束聚变自其概念提出以来，一直处于激光技术与物理研究的前沿。一方面，高功率激光装置的输出能力不断提高，从几千焦耳至美国国家点火装置达到的约两百万焦耳；另一方面，科学界对惯性约束聚变中物理过程的理解也愈发深入。激光与等离子体相互作用过程作为惯性约束聚变过程中的重要环节，其包含了许多非线性效应及多种不稳定性对后续聚变过程造成重大影响，已成为国内外研究的重点。理想激光与等离子体相互作用的分析大多采用均匀激光场与等离子体的模型，与实际条件下非均匀激光场与等离子体分布造成的影响差异较大。本文以对非均匀激光与等离子体相互作用过程中的激光小尺度自聚焦成丝模型开展模拟分析与实验验证为主线，主要包含以下三方面内容： 1. 利用二维粒子云网格算法，对强激光尖峰在等离子体喷流通道中成丝过程开展模拟分析。在模拟中得到了典型的小尺度细丝结构及其内部强度的变化特性；细丝周围的有质动力引起的高密度电子区域会导致细丝内的双等离子体衰变得到增强并扩展至较低密度的等离子体区域发生。模拟结果表明，在非均匀激光与等离子体相互作用中，存在大量的高强度小尺度激光细丝，这些细丝会增强其中的激光等离子体不稳定性并对后续物理过程造成影响。 2. 开展了双等离子体衰变特征光谱角分辨诊断实验研究。实验中通过多光纤探头对靶前后多角度的特征光谱进行测试。实验测得双等离子体衰变阈值远低于理论值，分析认为主要因素来自成丝效应使局部光强提高；激光细丝引起强有质动力下的显著Doppler效应使得实验中特征光谱角分布与原先理论分析结果不一致。该实验结果与模拟中细丝内的极高光强导致双等离子体衰变大幅增强，并诱发强有质动力推动电子形成丝壁等特征相符，支撑了小尺度自聚焦成丝模型。 3. 开展了双等离子体衰变特征光谱时空分辨诊断实验研究。结合光谱仪与Normarski干涉仪，利用狭缝方向相互垂直的两台条纹相机分别测量了平行、垂直于等离子体密度梯度方向特征光谱随时间变化的空间分布。实验测得特征光谱在入射激光功率密度较低时存在明显的时空不连续现象；在沿等离子体密度梯度方向，存在明显的光谱区域扩展；在垂直等离子体密度梯度方向，存在发光区域的空间分离，且发光区域的宽度与小尺度细丝的特征宽度接近。实验结果证明了小尺度自聚焦成丝存在于该实验的激光与等离子体相互作用的过程之中。 综上所述，本文结合模拟与实验，对激光小尺度自聚焦成丝现象的存在性与主要特征进行了探讨，初步验证了成丝模型，这对建立非均匀激光与等离子体相互作用理论有重要参考价值。; Since the concept was proposed, inertial confinement fusion is always at the forefront of laser technology and physics research. On one hand, the output energy of high power laser facility has increased from several kilo-joules to nearly two mega-joules achieved by the national ignition facility; one the other hand, the scientific community has also deepened its understanding of the physical processes in the inertial confinement fusion. The laser-plasma interactions play a key role in the inertial confinement fusion and have become key research objects worldwide, because many instabilities triggered by nonlinear effects are involved in the interactions, which may have a significant impact on the subsequent fusion process. Analysis of the interactions between laser and plasma has been mainly based on uniform incident laser and homogeneous plasma, but, in practice, laser and plasma are extremely non-uniform which may cause much different consequences from the uniform conditions. This dissertation focuses on the verification of small-scale self-focusing filamentation model through simulations and experiments, and mainly includes three aspects as following. 1. By using two-dimensional particle-in-cell simulation program, the interaction between intense laser speckles and plasma channels surrounded by jets was investigated. Typical geometric features and variations on internal intensity of light filaments were observed. Electron walls with relatively high density formed by ponderomotive force were observed around the light filaments, which intensified the two-plasmon decay instability inside them and broadened the electron density range where the characteristic spectra of two-plasmon decay appeared. Simulation results imply that there are many small-scale light filaments in non-uniform laser-plasma interactions, and these filaments could intensify the instabilities inside them and affect subsequent physical process. 2. Angular- and spectral-resolved measurements of three-halves harmonic emissions experiments were conducted. In the experiments, light emissions at different angles around targets were collected by many fiber probes. The measured threshold of the two-plasmon decay instability is much lower than the theoretical value, which is considered as a result of the light enhancement inside filaments; and significant Doppler effect caused by strong ponderomotive force around filaments affected the spectra, resulting in a mismatch between measured data and theory. These experiment results are consistent with the simulations in the phenomena that light could be enhanced a lot inside filaments and, therefore, two-plasmon decay could be intensified and electron walls could be formed by strong ponderomotive force, which supports the small-scale self-focusing laser filamentation model. 3. Temporal- and spatial-resolved measurements of three-halves harmonic emissions experiments were conducted. With the help of a spectrometer and a Normarski interferometer, the variation on intensity of three-halves harmonic emissions over time along the direction parallel and perpendicular to the plasma density gradient was measured by two streak cameras whose direction of slits are orthogonal, respectively. The measured characteristic spectra had obvious spatial and temporal discontinuity when the incident laser power density is low, and the spectra extended to underdense area along the direction of plasma density gradient, while the emission source was divided into several parts in the transverse direction with a typical width similar to that in simulations. These experiment results show that the small-scale self-focusing laser filamentation existed in the laser-plasma interactions of the experiments. In summary, this dissertation investigates the existence and main characteristics of the small-scale self-focusing laser filamentation model through simulations and experiments. The model is preliminarily verified, which has important reference value for establishing a theory of non-uniform laser-plasma interactions.
学科主题	光学工程
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/31035]
专题	中国科学院上海光学精密机械研究所
作者单位	中国科学院上海光学精密机械研究所
推荐引用方式 GB/T 7714	康宁. 激光在等离子体冕区小尺度自聚焦成丝研究[D].

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