高能量激光尾场电子加速的相关研究
文献类型：学位论文
| 作者 | 刘佳琦 |
| 文献子类 | 博士 |
| 导师 | 李儒新 刘建胜 |
| 关键词 | 激光尾场电子加速 Laser Wakefield Acceleration 级联加速 Cascaded Acceleration 毛细管放电等离子体通道 Capillary Discharge Waveguide 电子注入 Injection of Electron 电子束脉宽测量 Electron beam duration measurement |
| 其他题名 | Studies on the Generation of High-Energy Electron beams from Laser Wakefield |
| 英文摘要 | 高功率激光的发展，尤其是啁啾脉冲放大技术的发明，使超强超短激光的功率密度得到极大的提升，达到了相对论强度领域。探索超强超短激光与物质的相互作用过程的规律，为研究粒子物理提供了新方法。其中，利用超强超短激光在等离子体中驱动尾波场进行电子加速的方法在1979年首次被提出。相比于传统加速器，等离子体能承受更高的电场强度，不受材料耐压击穿的限制。尾波场中的轴向加速电场可达~100 GV/m量级，比传统加速器中的加速电场高出约三个量级。理想情况下，激光尾波场能够在比传统加速器小三个量级的加速长度内将电子束加速至相同能量。由于具有超高的加速电场，激光尾波场加速被寄希望于研制下一代紧凑型加速器。经过近四十年的研究，基于激光等离子体尾波场的多种加速方案在实验和理论中都得到了充分的验证和研究，相关的辅助控制方案和电子束参数测量方案也随之被提出和验证。这些控制和测量方法提高了被加速电子束的品质，甚至使激光驱动等离子体尾波场加速所得电子束的某些参数和传统加速器中的电子束可比。另外，由激光驱动等离子体尾波场加速得到的电子束也被作为电子源，应用于多种波荡器方案中产生相干和非相干的同步辐射。通过控制电子束源的参数或者电子束的横向振荡过程，同步辐射可被产生且波长在一定范围内可调谐。 本论文立足于通过激光等离子体尾波场电子加速方案获得高品质和高能量电子束。论文利用实验研究、理论分析和数值模拟等方法，对等离子体密度分布和尾波场动力学过程进行深入的分析，在高品质电子束的获得、毛细管放电等离子体通道诊断、等离子体通道导引激光尾场电子加速、电子束脉宽测量等方面取得了以下的成果： 1. 利用两段喷嘴结构，将第一段喷嘴后壁插入第二段喷嘴的气流中，通过控制气流密度的细微分布，产生了尺寸约两百微米的高密度区域。同时运用级联加速、准相位匹配加速和电子能散啁啾补偿等多种技术，获得峰值能量~500 MeV、RMS能散~1%、RMS发散角~0.2 mrad、电量为10-80 pC、重复率极高的高品质电子束。电子束的能量可通过改变加速级的长度实现准线性的调节。此外，由于该电子束的发散度极小，电子束的六维相空间亮度比过去LWFA方案中获得的电子束亮度高了数倍，与传统直线加速器输出的电子束亮度可比。 2. 通过激光光斑在通道内的尺寸振荡曲线测量充氢气毛细管放电等离子体通道的匹配光斑尺寸。保持放电电流和充气气压一定，不同长度充气放电毛细管波导中的径向等离子体密度分布可视为相同。因此，测量不同长度毛细管出口处的光斑尺寸可恢复出激光光斑在通道内的尺寸振荡曲线，以此计算出匹配光斑尺寸。相比于以往的方案，这种方法避免了多解问题和激光发散引入的测量误差。 3. 利用烧蚀毛细管放电得到了稳定的等离子体通道，并用其导引激光进行尾场电子加速的实验。在毛细管中充入气压极低的气体作为种子等离子体，可稳定毛细管放电波导的延时抖动，延长毛细管的寿命。通过对比单段毛细管和两段混合材料毛细管放电波导导引的激光尾场电子加速结果，实验验证了毛细管放电波导中的级联加速效果。利用电离注入的方案，得到了峰值能量最高为3.2 GeV的高能量电子束。 4. 提出了利用等离子体通道尺寸箍缩结构实现电子离轴注入的方案，并在二维和三维PIC模拟中得到验证。通道箍缩结构改变电子注入的阈值，离轴位置越远的电子注入阈值较低。在通道尺寸扩展区，电子从横向离轴位置注入。离轴注入的电子在加速过程中伴随着离轴振荡，因此可用作电子源产生振荡辐射。根据模拟结果，通道箍缩引入的电子振荡周期更短，相比于利用激光离轴入射的方案，其产生的辐射能量更高。 5. 从实验设计和理论验证角度讨论了一种法拉第旋转自相关法测量飞秒量级电子束的脉宽。利用电子束在激光尾场中加速时产生的角向强磁场改变侧向探测光的偏振，并使偏振改变的光进入干涉仪产生干涉条纹。由于干涉条纹的区域尺寸只依赖于磁场的区域尺寸，与探测光的脉宽无关，通过测量干涉条纹的区域尺寸可反推出电子束的脉宽。结合一定倍率的放大系统，该方法的测量精度能够到1 fs。; The development of high power laser, especially the invention of the chirped pulse amplification technique, has greatly improved the intensity of the ultra-short and ultra-fast laser, which has reached the relativistic region. Exploring the interaction between the high intensity laser and medium paves a possible way to study the particle physics. The plasma wake, which is driven by an intense laser, was proposed as electron accelerators for the first time at 1979. Compared to the conventional accelerators, the plasma can sustain a higher electric filed, which is not limited by the collapsing threshold of the wall materials. The accelerating field in the driven wake is of the order of 100 GV/m, which is almost 3 orders of magnitude higher than that in the conventional accelerator. Theoretically, the electron beam can be accelerated to the same energy in the wakefield accelerator within a distance of 3 orders of magnitude shorter than that in the conventional accelerator. Because of the ultra-high accelerating field, the laser wakefield accelerator has been considered as next-generation compact accelerators. In the recent 40 years, many schemes based on the laser wakefield acceleration has been verified and explored in experiments, theories and numerical simulations. And related controlling and measurement techniques have also been proposed and proved. The parameters of the accelerated electron beams have been improved by these techniques, and some of them can even be comparable to those in conventional ones. Moreover, electron beams obtained in the laser wakefield accelerator have been applied to different kinds of undulators for coherent or incoherent synchrotron radiation as electron beam sources. By controlling the electron beam parameters or transverse oscillation, the generated synchrotron radiation can be generated and tunable in certain regions. This thesis is aimed at obtaining high-quality and high-energy electron beams in the laser wakefield accelerator. By the means of experimental research, theoretical analysis and simulation study on the plasma density distribution and plasma wake dynamics, this thesis has been focused on obtaining the high-quality electron beams, diagnostic for the capillary discharge waveguide, obtaining the high-energy electron beams in the capillary-discharge-guided laser wakefield accelerator and measuring the electron beam duration as listed below: 1. Precisely manipulating the gas-flow distribution to produce a ~200 micrometer long density bump using two gas jet stages, in which the rear wall of the first gas jet is inserted into the flow of the second gas jet. In combination of the cascaded laser wakefield acceleration, quasi-phase acceleration and energy chirp compensation techniques, high-quality electron beams have been generated at high repetitive rate with peak energy of ~500 MeV, RMS energy spread of ~1%, RMS divergence of ~0.2 mrad, charge between 10-80 pC. The peak energy of the accelerated beam could be tunable by changing the lengths of the acceleration stage. Additionally, because the emittance is tiny, the beam brightness in the 6D phase space is several times higher than those in former researches, which is comparable to that in the linear conventional accelerator. 2. Measuring the matched spot size of a gas-filled capillary discharge waveguide based on the spot size oscillation of a collimated laser inside the channel is proposed. With the discharge electric current and gas-filled pressure fixed, the radial density profiles are considered identical for discharged capillaries of different lengths. By measuring the spot sizes at the capillaries’ exits, the spot size oscillation trace inside the channel is retrieved, and the matched spot size could be determined. Compared to former researches, this scheme avoids the inaccuracy introduced by multiple solution problem and laser divergence. 3. The cascaded laser wakefield acceleration guided in a hybrid capillary discharge waveguide is investigated. By filling the capillary with ultra-low pressure gas as ignition seed, the delay jitter between the ignition signal and discharge is stabilized, and the life time of the capillary becomes longer. By comparing the electron beams accelerated in single and two-stage hybrid capillary, the cascaded acceleration in the plasma channel is proved. Using ionization injection in the cascaded laser wakefield acceleration experiment, the peak energy of the electron beam can reach 3.2 GeV. 4. Off-axis injection of the electrons using a pinch structure is proposed and proved in 2D and 3D PIC simulations. The pinch structure changes the injection threshold of the electrons, making it lower in a more off-axis position. As a result, the electrons from off-axis positions are easier to be captured by the bubble. Because it undergoes a transverse oscillation during acceleration, the injected electron beam could be applied to generate synchrotron radiation as electron source. The simulation results indicate that the radiation energy is higher because the oscillation duration is shorter compared to the schemes using an off-axis incident laser in a plasma channel. 5. A Faraday-rotation Self-interference measurement for the electron beam duration is discussed from the aspects of experimental design and theoretical analysis. The vertically propagating probe laser polarization is rotated by the strong azimuthal magnetic field generated by the accelerated electron beam in the laser wakefield accelerator, and the polarization-rotated light is then incident into a Michelson-type interferometry for self-interference. Since the region size of the interference fringes only depends on the sizes of the magnetic field domain, which is independent of the pulse duration of the probe laser, the electron beam duration can be determined from the region size of the interference fringes. With a proper magnification, the resolution of this method can be reduced to less than 1 fs. |
| 学科主题 | 光学 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/31054]  |
| 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 | 刘佳琦. 高能量激光尾场电子加速的相关研究[D]. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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