激光尾波场加速高质量电子束的产生研究
文献类型：学位论文
| 作者 | 张志钧 |
| 学位类别 | 博士 |
| 答辩日期 | 2016 |
| 授予单位 | 中国科学院上海光学精密机械研究所 |
| 导师 | 刘建胜 |
| 关键词 | 激光尾波场加速，级联加速，第二尾波周期，准稳相加速，电子束纵向尺寸，能量啁啾，能散压缩，飞秒延时控制 |
| 其他题名 | Studies on the Generation of High Quality Electron Beams from Laser Wakefield Accelerators |
| 中文摘要 | 同步辐射光源和粒子对撞的研究都需要粒子加速器提供的相对论性粒子束。粒子加速器的最基本原理是利用电磁场来提升粒子束的能量。近年来，粒子加速器的许多应用，比如说材料表面处理、医疗诊断、癌症治疗、食品消毒和包括生物、化学、材料科学、粒子和核物理在内的科学研究，对于科学技术进步来说不可或缺。激光等离子体加速器为下一代紧凑经济型加速器的实现提供了可能。超强脉冲激光驱动的等离子体尾波内可维持的电场加速场可达100 GV/m量级，比传统加速器大三个量级。通过这个强电场，激光等离子体加速器在理论上可以在远小于传统加速器的距离之内实现预期的粒子能量增益。这为其成为新一代紧凑型加速器提供了可能。基于等离子体的加速器在过去几十年发展迅速，在实验上使用激光尾波场加速方案可产生准单能的极端相对论性电子束并且可以在厘米量级的加速长度内得到GeV量级的高能电子束输出。此外，在激光尾波场电子加速器中加速的电子可以产生较亮的电磁辐射，且输出的电子也能用于波荡器产生强X射线。由此不仅可在小面积的实验室内实现强的X射线输出，而且可应用与于医院、工业和学院范围的光子学研究。因此，激光尾波场电子加速器的发展和进步不仅对基础科学的研究有重要意义，在实际应用中也前景巨大，是非常值得深入研究讨论的课题方向。 虽然激光尾波场电子加速近年来发展迅速，但相比于传统射频加速器来说，电子束的一些输出参数还是有所不如，比如说电子束能散和发射度。本论文针对激光等离子体尾波场电子加速器的电子束输出参数展开讨论，具体分析了影响电子束输出参数的重要物理过程，在激光尾波场级联加速第二尾波周期高品质梯度注入、准稳相加速、电子束等离子体内纵向尺寸压缩、电子束能散压缩、超短电子束纵向尺寸诊断和飞秒延时调控等方面取得了如下创新成果： 1. 提出了一种通过密度下降沿控制电子束在激光驱动尾波第二周期内注入的新机制。通过一个等离子体密度下降沿控制激光驱动尾波第二周期的电子束注入，可以得到高质量低能量的高质量纵向注入电子束，且其能在较高密度等离子体内获得一定加速。为使该预加速电子束进入第一尾波周期的加速相位进行进一步加速，采用级联电子加速方案通过第二个密度下降沿来改变电子束的相位，随后为等离子体平坦区域使电子束得到稳定加速。二维PIC模拟结果表明这种方案可以在12 mm长密度为9×1017 cm?3的等离子体内得到峰值能量约为1.2 GeV的单能高质量电子束输出，所需脉冲光功率约为77 TW。进一步将加速级优化为多段密度上升的台阶式等离子体段，可以实现电子束的准稳相加速，通过这个方案电子束的峰值能量可以得到50%的提升。 2. 提出了一种减小级联电子加速得到电子束能散的新机制，通过引入一段密度适当的等离子体可以对注入电子束的纵向空间分布进行压缩，从而实现能散为千分之一量级的电子束输出。在这个机制下，三段等离子体分别设计为可实现电子束注入、电子束长度压缩和电子束加速。在注入段捕获注入的电子束在压缩级转移到尾场周期的零相位区域，由于电子束团的Velocity Bunching效应，电子束团的纵向尺寸得到极大压缩。之后电子束团进入加速段，相比于未压缩电子束团，由于电子束团纵向长度得到了极大压缩，该电子束可以在能量空间啁啾分布得到补偿之前加速到更高能量。为了验证这个机制，我们建立了一维理论模型并进行了二维PIC模拟，模拟结果表明在该机制下得到了能散度0.2%且发射度极小的高质量电子束团，同时电子束团电量也不会发生损失。 3. 提出了一种诊断激光等离子体加速产生超短电子束的纵向长度的新方法。在激光尾场电子加速的加速过程中，由于纵向尾场梯度差的存在，激光驱动等离子体波内的电子束团的能散是演化的。且其能散的演化和电子束团纵向长度及所处尾场斜率密切相关。这个方法的基本思路如下：首先，由经历不同加速长度的电子束团能谱，可得到电子束团能散的变化和及其作用加速场的估计值；然后，由于激光尾场电子加速存在的失相效应，电子束团会相对等离子体波向前滑移，因此，电子束团所处尾场斜率可由加速场的变化除以滑移距离得到。然后就能得到电子束团的纵向长度。由于所需的参数都能从电子束能谱得到，激光尾场电子加速器得到电子束纵向宽度可以被测量。这个方法适用于稳定的激光尾场加速器，且目标电子束团可依据电量进行区分。为了研究被加速电子束团的动力学并估算其束长，进行了实验和三维模拟，并讨论了该方法的实验和理论误差。同时在实验上运用自注入激光尾波场级联加速方案获得了高质量的电子束产生，二维PIC模拟揭示了该机制下电子束团加速的物理过程。 4. 提出了一种适用于飞秒量级激光脉冲延时同步监控方法，此方法的两束飞秒脉冲光由同源飞秒脉冲分束而成。通过对两束同源脉冲光进行频谱干涉，由干涉条纹的条纹间距可对两束脉冲光之间的延时进行诊断。这个方法可有效解决飞秒脉冲间的时间抖动问题，同时运用此方法控制测量并监控两同源飞秒脉冲间的延时，可以在激光尾波场电子加速运用于自由电子激光实验中实现在飞秒(fs)精度内控制注入种子脉冲与电子束之间的延时。 |
| 英文摘要 | Synchrotron light sources and colliders are driven by relativistic particle-beams which are provided by particle accelerators, which basically use electromagnetic ?elds to increase the energy of charged particle beams. In recent years, applications of accelerators such as material surface treatment, medical diagnostics, cancer therapy, food safety, as well as scienti?c research including biological, chemical, material science, particle and nuclear physics, they have been indispensable for scienti?c and technological advancements. Laser wakefield accelerators have the potential to be the next generation compact accelerators. It is demonstrated that high accelerating gradients on the order of 100 GV/m, three orders of magnitude greater than conventional accelerators, could be sustained in the plasma wake driven by ultra-intense laser pulse. With these high ?elds, laser wakefield accelerators could achieve the desired particle energy in a much shorter distance than a conventional accelerator. This makes the laser wakefield accelerators promising to be the next generation of compact accelerators. In addition, the electron beams can produce bright radiation during the acceleration process or can be used for the generation of brilliant X-ray radiation in undulators. Not only would this pave the way for the production of brilliant X-ray generation in small-scale labs but also allow for their application in hospitals, industry and university-scale photon research. Therefore, it is meaningful for the studies on the laser wakefield accelerators for their roles in basic physics and the potential applications. Compared with the conventional accelerator, some of the parameters generated from laser wakefield accelerators, such as energy spread and emittance, are worse in somehow. In this thesis, the investigations on the physic processes which impact the generated electron beam parameters are performed. And several novel schemes, including density transition for the second wake, quasi-phase stable acceleration, bunch longitudinal compression on plasma, energy spread compression, longitudinal length measurement of ultra-short bunch and femtosecond delay control, are proposed and listed as below: 1. We propose a new scheme for controlling the electron injection process in the second period of the laser-driven wake?eld via a downward density ramp. By controlling electron injection into the second period of the laser-driven wake?eld in a downward density ramp, a high-quality low-energy electron beam can be accelerated in a short segment of high density plasma. After a second downward density ramp followed by a low-density plasma plateau, the pre-accelerated electron beam can be seeded into the ?rst period of the laser-driven wake?eld for cascaded acceleration at an optimized phase. A monoenergetic electron beam with peak energy of ～1.2 GeV can be generated from plasma with a length of 12 mm and density of 9×1017 cm?3, driven by a laser pulse with peak power of 77 TW. By modifying the acceleration stage comprising several density-ascending plasma segments, the peak energy of the quasi-monoenergetic electron beam can be ef?ciently increased by about 50% via a quasi-phase-stable multiple-cascade acceleration scheme. 2. We propose a scheme to minimize the energy spread of an electron beam (e-beam) in a cascaded laser wakefield accelerator to the one-thousandth-level by inserting a stage to compress its longitudinal spatial distribution. In this scheme, three-segment plasma stages are designed for electron injection, e-beam length compression, and e-beam acceleration, respectively. The trapped e-beam in the injection stage is transferred to the zero-phase region at the center of one wakefield period in the compression stage where the length of the e-beam can be greatly shortened owing to velocity bunching. After being seeded into the third stage for acceleration, the e-beam can be accelerated to much higher energy before its energy chirp is compensated owing to the shortened e-beam length. A one-dimensional theory and two-dimensional particle-in-cell simulations have demonstrated this scheme and an e-beam with 0.2% rms energy spread and low transverse emittance could be generated without loss of charge. 3. We propose a new method to diagnose the longitudinal length of ultrashort electron bunches generated from a laser wake?eld accelerator via energy spectrum evolution. The energy spectrum of an electron bunch, which is accelerated in a laser driven plasma wave, varies because of the longitudinal dispersion of the wakefield. By analyzing the energy spectra of electron bunches with different acceleration length, the wakefield gradient difference and the wakefield slope of the bunch could be estimated by combining with the slippage between the plasma wave and the electron bunch, thus the electron bunches? longitudinal length could be estimated. In the experiment, we observed that the absolute energy spread experienced a compression at first and then a growth during the acceleration. By applying this new method, the longitudinal length of electron bunches with charge of about 60 pC generated from a laser wakefield accelerator was estimated to be (1.9 ± 1.2) μm, which was in good agreement with three-dimension particle-in-cell simulations. Meanwhile, we carried out a cascaded laser wakefield accelerator with self-injection applied in experiment. High quality electron beams generated in this scheme, and basic processes the target electron beam witnessed were demonstrated with two-dimension simulations. 4. We propose a new method to control the delay between two femtosecond laser pulses split from the same one by spectral interferometry. The delay between the two pulses could be diagnosed from the fringe interval from a spectrometer where the pulses spectral interference is measured, and the delay could be controlled as the feedback of the fringe interval change. The accuracy of this method is on the scale of femtoseconds. In the free-electron-lasers experiment with electron beams from laser wakefield accelerators, this method could be applied to control the delay between the seeding pulse and the electron bunch in precisely. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15958]  |
| 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 | 张志钧. 激光尾波场加速高质量电子束的产生研究[D]. 中国科学院上海光学精密机械研究所. 2016. |
入库方式： OAI收割
来源：上海光学精密机械研究所
浏览0
下载0
收藏0
其他版本
除非特别说明，本系统中所有内容都受版权保护，并保留所有权利。