| 题名 | 相对论飞秒强激光加速电子研究 |
| 作者 | 徐建彩 |
| 学位类别 | 博士 |
| 答辩日期 | 2013 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 沈百飞 研究员 |
| 关键词 | 超短超强激光脉冲 等离子体 电子加速 |
| 其他题名 | Electron Acceleration Driven by Femtosecond Relativistic Intense Laser Pulse |
| 中文摘要 | 1979年，Tajima和Dawson提出了激光尾场加速电子的概念。基于超短超强激光与等离子体相互作用，激光尾波场可以获得高于传统加速器上千倍的加速度。这一理论引起了人们广泛的兴趣并得到了深入的研究。另外，激光啁啾脉冲放大（CPA）技术的应用，使得激光强度大幅度提高。近十年来，激光脉冲强度达到相对论强度，脉冲宽度短至数十甚至数飞秒(fs， s)，基于激光尾波场加速的电子加速实验研究迅速发展。目前，电子束能量达到GeV( eV)以上，同时电子束能谱从连续谱提高到准单能能谱，并且电子束稳定性也大大提高。本论文基于激光尾波场加速电子这一概念，做了以下几方面的工作： 1. 通过Particle in Cell (PIC)粒子模拟研究发现，当激光脉冲的焦斑尺寸远大于等离子体波长而且激光功率几百倍于相对论自聚焦所需的临界功率时，激光脉冲驱动的等离子体波具有特殊的结构。空泡尺寸大大增加，且不完全中空，空泡的横向尺寸取决于激光光斑尺寸。大尺寸的空泡可以捕获数十纳库(nC， C)的电子束，将其加速到百MeV（ eV）。在等离子体密度为 ，激光光斑尺寸为60 μm，激光功率超过140TW(1012 W)的条件下，获得加速的相对论能量电子束出等离子体时，能量高于5 MeV的电子束超过45 nC。不断自注入到复杂的空泡中获得加速的电子束成为一个高效率的大电量电子束源。 2. 当注入到空泡的电子数目过多后，空泡的尾波场就会被严重地改变，从而阻止了空泡周围背景电子的后续注入。在二维PIC模拟中我们观察到了这种空泡的过载效应，并运用一维激光尾波场理论对其进行了解释。同时，模拟发现在电子束注入的过程中，如果等离子体密度分布存在一个合适的下降沿，则可以抑制空泡的过载效应，从而获得更多数目的高能电子。 3. 电子的控制注入是提高电子束质量的重要方法。气体激波注入是有效的控制注入方法之一，其方法简单但在实验上获得了很好的电子束输出。与其他控制注入方法相比，气体激波注入方法获得的电子束在能量的可调谐性，稳定性，可重复性都有大幅提高。并且，电子束的绝对能量发散度只有～5 MeV。电子束磁场以及等离子体波的在线测量允许我们对电子束的产生以及加速过程进行追踪。气体密度下降沿处的自发辐射使得其位置很容易观测。结果发现：在整个激光与气体相互作用过程中，电子束只注入一次，即在激波前沿处注入，整个过程中电子束自注入没有发生。 |
| 英文摘要 | In 1979, Tajima and Dawson proposed the concept of laser wakefield acceleration. There is a strong wakefield behind an ultrashort intense laser pulse, which can accelerate particles to high energies in a very compact way. The laser wakefield can generate a longitudinal accelerating field of gigavolt per meter(GV/m), thousands of times larger than that of the conventional accelerator. The chirped pulse amplification(CPA) and subsequent developments make the laser pulse short to fs scale with relativistic intensity, which promotes the rapid development of electron acceleration research, specially in last ten years. Laser wakefield electron acceleration has been now beautifully experimentally demonstrated: electrons energy over GeV, quasi-monoenergetic electron spectrum, and high stability and reproducibility. This thesis has the following parts, all of which are based on the theory of laser wakefield electron acceleration. 1. By particle-in-cell (PIC) simulation, it has been found that a large complex bubble forms instead of a normal bubble with a sphere shape, when the focus size of a laser pulse is much larger than the plasma wavelength and the laser power is hundreds of times larger than the critical power required for the relativistic self-focusing. The transversal size of the bubble depends on the laser spot size. Due to the large bubble size, several tens nano-Coulomb electrons are trapped and accelerated to multi-hundred MeV. When the plasma density is and the laser pulse has a spot size of 60 μm, the charge of the energetic electron bunch with energy above 5 MeV exceeds 45 nC. Electrons continuously self-injected into such a complex bubble serve as an efficient source of the high-charge electron bunch. 2. Overloading effects of a high-charge self-injected electron bunch on bubble wakefield have been studied in the bubble regime. After many electrons have been trapped into the bubble, the wakefield is strongly modified, which prevents further injection of the background electrons. These effects are directly observed in two-dimensional PIC simulations, and are explained by one-dimensional wake theory. In order to obtain much more energetic electrons, it is suggested to use a decreasing density profile of the plasma in the electron acceleration process. 3. High-quality electron bunch strongly depends on how electrons are injected and get trapped in the plasma wave. Shock-front injection offers a simple but powerful method to inject electrons into the plasma wave, which is triggered by a sharp density jump generated from the gas shock. Compared to other injection methods for the relativistic electron generation, the stability, reproducibility and energy-tunability of electron bunches are significantly improved. Moreover, they have a low absolute energy spread of 5 MeV. Real-time observation of the electron bunch and the plasma wave allows us to get more details of the electron injection and acceleration process. Due to the self-emission of the sharp density jump, the shock-front position can be well defined. There is no electron bunch signal until the shock-front position. The injection only occurs at the shock-front position. Afterwards the electron bunch charge keeps constant. There are no new electrons injected into the plasma wave. Self-injection does not take place during the whole interaction. |
| 语种 | 中文 |
| 内容类型 | 学位论文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15752]  |
| 专题 | 上海光学精密机械研究所_学位论文 |
| 推荐引用方式 GB/T 7714 | 徐建彩. 相对论飞秒强激光加速电子研究[D]. 中国科学院上海光学精密机械研究所. 2013. |
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