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题名	激光与等离子体相互作用下超热电子及自生磁场研究
作者	刘会亚
文献子类	博士
导师	林尊琪
关键词	惯性约束核聚变 Inertial Confinement Fusion, 激光与等离子体相互作用 Laser and Plasma Interaction 超热电子 Suprathermal Electron 电子能谱仪 Weibel Instability Weibel不稳定性 Self-generated Magnetic Field 自生磁场
其他题名	Research on Suprathermal Electrons and Self-generated Magnetic Field in Laser produced Plasmas
英文摘要	高功率激光与等离子体相互作用（Laser and plasma interaction, LPI）产生的超热电子的空间分布特点一直是激光驱动惯性约束核聚变（inertial confinement fusion，ICF）的重要研究内容。当激光在等离子体中传输时，会发生多种参量不稳定性，如受激拉曼散射（Stimulated Raman scattering, SRS）、双等离子体子衰变（Two-plasmon decay, TPD）和共振吸收等不稳定性。在这些不稳定性过程中会加速产生大量超热电子。高能电子会穿过靶丸烧蚀层，导致预热增熵，增加压缩难度，并降低内爆能量增益。对超热电子的空间能谱测量，不仅有利于评估其预热增熵能力，优化靶的参数设计，而且对LPI的基础物理研究和抑制参量不稳定性的研究提供重要的实验依据。本文的工作主要体现在以下三个方面： 首先，完成了对电子空间能谱分布测量的电子能谱仪阵列的方案设计、制作加工和定标工作。此探测器阵列包含19台电子能谱仪，覆盖空间上多个角度，角分辨率为10°，可对20~500keV的电子进行探测。根据实际磁场测量结果和机械加工精度，结合理论上分析，得到了电子谱仪的能量测量分辨率和电子数目测量的相对误，分别为10%和12%。利用电子加速器，对谱仪进行了能量定标工作，定标结果与理论计算一致。此外，描述了一种新型电子谱仪的设计方案。它利用了入射的电子具有聚焦的行为，焦点轨迹为双曲线，减少了谱仪的注入孔的孔径大小引入的系统误差。这种新型谱仪相对传统的180°偏转的方案，能量测量相对误差降低了至少一倍。 其次，开展了激光辐照Al靶的实验研究，探测了超热电子和3ω0/2光谱的空间分布。利用神光II第九路激光装置，采用电子能谱仪阵列和多个光信号收集单元，测量了超热电子和光谱的空间分布。结果表明，超热电子的能谱分布符合麦克斯韦双温分布，拟合电子温度分别约15keV和50keV。得到了超热电子空间发射近似符合高斯分布特点，峰值方向与激光波矢方向相同。统计了10μm厚Al靶实验中激光生成超热电子的转化率，在激光波矢方向π sr空间立体角内的转化率在10-4量级，且随激光功率密度（~1014W/cm2）近似线性增长。对比了不同聚焦方式下，超热电子和TPD特征光谱的转化率，验证了连续相位板对抑制LPI不稳定增长的有效性。根据超热电子产生理论，结合SRS与TPD的色散特点，以及已有的实验数据，分析认为当前激光条件下TPD不是产生超热电子的主要机制。 最后，研究了等离子体中Weibel不稳定性产生的细丝与自生磁场随时间增长的特点。对等离子体中磁场的产生和放大过程的认识，不仅对ICF中超热电子的发射以及传播的研究，还对理解天体物理中高能粒子产生都有重要意义。利用高功率激光辐照CH靶，在等离子体中观测到了密度成丝分布的特征，分析认为Weibel不稳定性是其产生的主要机制。求解不稳定性的色散关系，得到了当前实验参数下的增长率。发现最大增长率对应的模式波长与实验观测到的细丝间距近似一致。利用法拉第旋光效应测得了细丝周围磁场的强度，并研究了随时间的增长率。实验结果显示，细丝周围的磁场强度为数十特斯拉，增长率为0.148ns-1，这与Weibel不稳定性的理论增长率较为一致。此外，对比了两个理论分析模型，研究了细丝的间距随时间变化关系，考虑了细丝间的聚合行为，认为实验观测到的细丝处于非线性增长阶段。; The angular and spectral properties of suprathermal electrons generated by high power laser and plasma interactions (LPI) have long been recognized as critical issues in the development of the laser-driven inertial confinement fusion (ICF). These suprathermal electrons are accelerated by the damping mechanism of the electron plasma waves, which produced by the parametric instabilities in the plasmas, like stimulated Raman scattering (SRS), two plasmon decay (TPD), resonance absorption etc. Some suprathermal electrons can penetrate the ablation shell, and deposit their energy into the main fuel layer, making the fuel preheat and eventually leading to a reduction of the performance of the target in the ICF experiment. The spectrally and angularly resolved measurements of suprathermal electrons are not only important for evaluating the ability to preheat the fuel and optimizing the parameters of the laser pulse and target, but also crucial for the study of the LPI physics and searching the techniques of reducing the impact of parametric instabilities. The major achievements of this thesis are described as follows. Firstly, the key instrument of measuring the properties of suprathermal electrons that is suprathermal-electron spectrometer array was developed and calibrated. It consists of 19 identical electron spectrometers, which are set in three directions with an interval of 10°. Each electron spectrometer was designed with a uniform magnetic field to detect electrons in the range from 20 to 500 keV with imaging plate as the electron detector. The parameters of the spectrometer are given based on the theoretical calculation and error analysis. By measuring the magnetic field intensity and machine dimensions, maximum total error in the electron energy and number are obtained, which are about 10% and 12%, respectively. Then, they are calibrated using electrons from an accelerator, and the result shows a good agreement with the theoretical calculation. In addition, a new type of electron spectrometer is proposed, which is based on the method of the electron trajectories in the uniform magnetic field focusing on hyperbola curve. This new type electron spectrometer can reduce the error in the electron energy at least one times, compared with the traditional 180° deflection spectrometer. Secondly, the angular and spectral properties of suprathermal electrons together with the optical emission at 3ω0/2 have been measured in the experiments of laser irradiating Al foil targets. These experiments were conducted with the 9th beam at SGII facility. Tens of electron spectrometers and optical signal collectors were used to detect the suprathermal electrons and optical emissions. A double temperature distribution was observed in the electron energy spectra, in which the lower and higher electron temperature were about 15 keV and 50 keV, respectively. The angular distributions of the total suprathermal electron approximately obeyed the Gaussian distribution, peaking along the k vector of the incident laser beam for perpendicular incidence. Moreover, the conversion rate of laser energy into escaped suprathermal electron energy over the π sr solid angle was about 10-4 at the laser intensity of about 1014W/cm2, and increased almost linearly with the laser intensity. By comparing the conversion rate of suprathermal electron energy and 3ω0/2 emission in different laser conditions, the result shows that the smoothing technique of continuous phase plate is an effective way to suppress the growth of instabilities. Furthermore, based on the TPD and SRS theories combined with the recent experimental data, the dominate mechanism of generating suprathermal electron was discussed. Finally, the evolution of filaments and self-generated magnetic field due to the Weibel Instability were studied experimentally in the plasmas produced by nanosecond laser pulses. The study of magnetic field generation and amplification in laser plasmas, is very important for understanding the influences for laser energy absorption and the charged particle transportations. The high speed plasma flows like the jets in the cosmic plasmas produced by high power laser, provides a way to quantitatively inspect the process and profile of WI in laboratory plasmas. According to the characteristic of the filaments observed in the experiment, the Weibel Instability is the most likely generation mechanism. The magnetic field around filamentations was diagnosed by the Faraday rotation technique as about 60 T with an exponential growth rate of 0.148ns-1, which was in good agreement with the calculated features of the Weibel Instability. The separation distances between filaments were also measured, and its temporal evolution, as compared to two existed analytical models, was mostly attributed to the coalescence behavior of filaments in post saturation regime.
学科主题	光学工程
内容类型	学位论文
源URL	[http://ir.siom.ac.cn/handle/181231/31086]
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	刘会亚. 激光与等离子体相互作用下超热电子及自生磁场研究[D].

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