飞秒强激光加速电子的理论与模拟研究
文献类型：学位论文
| 作者 | 温猛 |
| 学位类别 | 博士 |
| 答辩日期 | 2010 |
| 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 |
| 关键词 | 相对论激光脉冲 有质动力 激光尾波加速 激光直接加速 |
| 其他题名 | Theoretical and Numerical Studies on Electron Acceleration driven by Femtosecond Intense Laser Pulse |
| 中文摘要 | 现代光源和对撞机的加速器原理和技术在近百年的发展中很可能已接近技术和经济可行性的极限，导致全世界已多年未兴建更高能量的大型加速器。在过去几十年里，物理学家们一直在探索新的加速粒子方案，期望能在短得多的距离内把粒子加速到相当高的能量。超短超强激光脉冲的发展为粒子加速提供了一个全新的思路。随着激光技术的飞速发展，人们已能获得聚焦强度超过1022W/cm2、单脉冲宽度小于10fs的相对论激光脉冲，这在实验和理论上大大推进了粒子加速的研究。用飞秒强激光加速带电粒子，这种新型加速器与传统加速器相比有着巨大的优势，因此近年来受到人们的广泛关注。用激光加速电子是激光加速其它粒子的基础，是我们研究的重点。目前激光驱动电子加速的机制主要分为两种：一种是利用等离子体作为加速介质的间接加速，最典型的就是激光尾波加速；另一种就是激光在真空中对电子直接加速。本论文分别对这两种加速机制做了深入的研究，主要成果如下： 1. 本文研究了电子束在激光尾波中的加速过程对电子束单能性的影响。电子束在三维激光尾波——“空泡”中加速的过程中存在失相的问题，这严重影响了电子束的单能性。为了解决这个问题，我们引入了空间变化的等离子体密度。理论研究发现，空泡前沿的速度和激光的群速度相等，均随着等离子体密度的增大而减小；空泡的纵向尺度会随着等离子体密度的增加而缩小，其缩小的速度与等离子体密度增加的梯度有关。研究等离子体密度对空泡前沿速度和空泡收缩速度的影响发现，设置合适的等离子体密度变化梯度可以使得空泡底部的速度变大，从而让电子束在空泡中合适的位置不断被加速。我们在模拟中找到空泡中最适合电子加速的位置，通过解析计算得到保持电子在该位置需要的等离子体密度空间变化形式。根据理论解析得到的结果，用VORPAL三维程序验证了等离子体密度在空间的变化对电子束单能性的作用。结果发现，通过优化等离子体密度，电子可以在相对于尾波稳定的位置，稳定的电场下加速，使得所获高能电子束的能散度得到有效的降低。 2. 利用激光尾波加速机制，我们模拟研究了用多脉冲来加大高能电子束的电量。在很多应用中，高能电子束的高电量也是人们追求的品质之一。为了得到更大的电量，可以通过增加单束脉冲的焦斑大小来增大空泡的尺寸，以达到增加注入电子束的纵向长度、增大电量的目的。但是单脉冲产生的空泡最大尺寸存在极限。因此，我们采用平行的双脉冲来增加电子束的横向尺寸，使得电量在横向上增大，以达到增加总的高能电子电量的目的。通过研究发现，这样平行传输的脉冲存在一个优化的距离。只要脉冲的间距不小于该距离，得到的高能电子电量就可以与脉冲数成比例地增长。只要等离子体的横向尺寸够大，电子束的电量就可以没有上限。 3. 研究了相对论强度的激光脉冲在真空中直接照射超薄固体靶，电子在激光场的作用下与离子完全分离并加速的情况。我们用理论解析的方法建立了模型，并用该模型描述了激光场中的电子层动力学，推导了电子层密度演化方程。通过计算发现，被激光脉冲从超薄靶中推出的电子在获得高能量的同时保持了非常高的密度。我们从解析上计算了高密度电子层的演化情况，并与一维的PIC模拟进行了比较，得到了非常吻合的结果。我们通过分析发现，激光脉冲可以把初始较厚电子层压缩至更薄、密度更高的状态，这对实际实验中应用的激光很有利。我们还从理论上给出了被加速的电子层离开激光后的空间演化情况，由此我们得出了高能量高密度电子层可以应用的时间尺度。这种激光直接加速得到的高密度电子薄片有很多重用的用途，例如可用来作为反向汤姆逊散射的相对论飞镜，可用来反射脉宽为几个周期的脉冲，并把它压缩到阿秒甚者仄秒的长度，以及用作产生紫外光或者X射线的机制。 |
| 英文摘要 | The Technology and physics of the accelerators that power today’s light sources and colliders have developed and improved nearly a hundred of years, which may well be approaching the limit of what is technologically and economically feasible, and therefore no more much larger and higher energy accelerators have been founded for many years all around the world. In the past decades, people were searching for new ways to accelerate particles, hoping to obtain high energy in a much shorter distance. The rapid development of laser technology has made available ultraintense (1022W/cm2) ultrashort (<10fs) laser pulses, which has promoted not only the theoretical research but also the experimental research of particle acceleration. It has attracted much attention due to the advantages of the new accelerator compared with the conventional accelerator. In this thesis, we focus on the laser driven electron acceleration, which is the foundation of laser particle accelerators. We study the two main mechanisms of the laser driven electron acceleration. One is the indirect laser acceleration with the media of plasma, e.g. the laser wakefield acceleration. The other is direct laser acceleration in vacuum. The main results are given as follows: 1. The electron acceleration process in the laser wakefield is investigated, which dominates the quality of the accelerated bunch. When an electron bunch is accelerated in the three dimentional laser wakefield (“bubble”), the dephasing of the bunch broads its energy spectrum. To solve this problem, we can optimize the density gradient of background plasma. As known from the theoretical study, with a positive plasma density gradient, the velocity of the bubble’s front, which is the same as the group velocity of the laser pulse, becomes smaller. At the same time, the length of the bubble decreases with the velocity related to the plasma density gradient. With the balance of the two velocities, the velocity of the bubble’s base can be increased, allowing the electron bunch accelerated at a stable phase in the bubble. We can find the best phase in the simulation, then calculate the optimized plasma density profile from the the analytical model. Three-dimensional simulations were performed. By optimizing the density gradient of background plasma, the electron bunch can be stably accelerated at the back of the bubble, with its energy spread restrained. 2. We investigate the large amount of electrons acceleration in laser wakefield with two laser pulses simulatively. A large amount of electrons are needed in many applications. In order to increase the number of accelerated electrons, we can enlarge the focus spot of the pulse to increase the size of the bubble, in which the length of the bunch can be extended with its charge increased. But there is a limit for the enlargement of one bubble. Alternatively, we can also use more pulses to increase the width of the electron bunch, or increase the number of the bunch, and more energetic electrons can be obtained. We found an optimized distance between the pulses for the linear increasing of the electrons by more laser pulses. There can be no limit for the amount of the electrons with wider plasma and more pulses. 3. Irradiation of ultra-thin solid foils by high-contrast laser pulses at relativistic intensities is investigated, in which the foil electrons are completely separated from ions. We use an analytical model to describe the dynamics in the laser field, and the electron layer evolution. By the calculation, we find the electron layer, expelled from the foil by the laser, can obtain relativisitic energy, while keeping high density. The analytical model well reproduces corresponding particle-in-cell simulations. As a particular result, we have identified a regime of layer compression related to initial acceleration inside the ion volume. It is found that foils somewhat expanded initially are superior to foils of same areal density, but thinner and denser, which is good news to consider laser pulses with different envelops. Also an analytical formula is given for longitudinal layer expansion after it disconnects from the driving laser field and is freely propagating. It provides the scaling of the longitudinal decay and may help to better understand the transport of dense electron bunches. The electrons accelerated in this way form a dense electron layer that can be used as a relativistic mirror for Thomson backscattering. It is used to reflect few-cycle probe pulses, to compress them to atto- and zepto-second duration, and to shift their spectra to the VUV- and X-ray regime. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15642]  |
| 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 | 温猛. 飞秒强激光加速电子的理论与模拟研究[D]. 中国科学院上海光学精密机械研究所. 2010. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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