激光尾波场电子加速器相关物理问题的研究
文献类型：学位论文
| 作者 | 李文涛 |
| 学位类别 | 博士 |
| 答辩日期 | 2014 |
| 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 锁相加速 激光多丝 可控电子注入 电子束诊断 |
| 其他题名 | Study on the Physical Issues of the Laser Wakefield Electron Accelerator |
| 中文摘要 | 超强超短激光的持续迅猛发展，为人类提供了前所未有的全新实验手段与极端物理条件。激光聚焦强度在过去的十多年里已提高了7-8 个量级，实验室条件下的聚焦强度已经能够达到1021 W/cm2 乃至1022 W/cm2 量级，激光脉冲的时间尺度已小于10 fs，接近激光光场的单个振荡周期。如此强的激光光场可将物质迅速电离形成等离子体，光和物质的相互作用也进入了强相对论非线性光学的全新范畴。超强超短激光驱动的等离子体尾波场电子加速器，突破了传统射频加速器的加速电场极限，提供了超高加速梯度，极大的降低了加速器的规模和成本，将会在众多领域如自由电子激光、生物医学高分辨率成像、材料检测及高能粒子物理研究中得到广泛应用，并将带来巨大的经济效益和社会效益。因此，激光等离子体尾波场电子加速器不仅是强场激光物理领域的重要基础物理研究，有重大的科学意义，也具有重要的应用前景。 本论文针对激光等离子体尾波场电子加速器的若干主要问题进行了较为系统的研究，在级联电子加速、电子的锁相加速、激光的导引、可控注入方案和电子束诊断等方面取得了如下创新成果： 1. 开展了超强超短激光驱动的尾波场级联电子加速实验研究，获得了0.8GeV的准单能电子束。分析了采用电离注入的尾波场级联加速中的电子注入相、激光传输、焦斑匹配等问题，为级联尾波场电子加速实验提供了参数选择依据。设计了级联电子加速中的高能电子束的诊断系统，并对级联尾波场电子加速的实验数据进行了能谱分析。 2. 开展了基于梯度注入方案的可控级联电子加速实验研究，通过调节加速级的等离子体密度，实现了对电子束注入相位的优化，并获得了能量为500MeV、能散为3%的单能电子束。通过将加速级增长到5mm，获得了峰值能量为1.3GeV的准单能电子束。创造性的建立了一套具有时间分辨能力的，用于测量尾波场中高能电子束环形磁场的法拉第旋光探测装置，该装置可以同时测量多个位置的磁场信号。通过将测量点设置在加速级的入口处和中央处，实现了对电子束注入状态和加速过程的实时诊断。该装置对于探索加速过程的物理图像和指导实验操作都具有重要意义。 3. 实验获得了突破理论极限的1.8GeV高能电子束，并结合理论分析、数值模拟，建立了一个合理解释该实验结果的物理模型。在该模型中，激光强度的衰减对于电子加速的相位锁定和高增益获取发挥了主要作用。我们给出了在均匀等离子体中实现电子加速相位完美锁定所对应的激光强度演化公式，分析了突破能量增益理论极限所需的物理条件，并针对进一步将能量增益提高到3GeV以上提供了建议方案。 4. 在激光尾波场电子加速实验中，观测到了由于激光模式变化而导致的激光多丝现象，通过分析不同参数下出射激光的光斑成像变化，还原了激光多丝现象在等离子体内的演化过程。验证了在电离注入机制下，该多丝现象所导致的双电子束加速效应。通过实验分析了激光多丝对加速电子束质量的影响，发现在级联加速模式下，多丝过程弱化了激光的自导引效果并缩短了导引距离，其电子束参数，如能量、能散度和发散角等均不如单丝情况下所得到的电子束。 5. 提出了一种可直接控制电子束注入相位的电子注入机制，该机制首次提出利用远红外激光脉冲作为激光尾场电子加速器中的注入激光，直接将电子束推入到尾波场中，电子的注入相位可以通过调节两束脉冲之间的延时而进行控制。结合PIC模拟，我们分析了该机制的注入过程、改变延时所影响的电子束的各项指标，如能量、能散、电量和发散角等，同时还分析了在该机制下密度分布的影响。现有理论结果表明，在合适的注入相位下，该机制甚至能够获得粒子载荷效应所决定的最大电量注入。 6. 提出了一种飞秒电子束诊断仪的设计方案，该设计装置除了拥有一般的电子能谱仪和电荷测量仪的功能以外，还可以通过测量相干衍射辐射导致的电子束偏转角来对电子束的长度进行测量。 |
| 英文摘要 | The rapid development of ultra-short and ultra-intense laser provides new experimental means and extreme physical conditions as never before. The peak focusing laser intensity in the past decade has been improved 7-8 of magnitude, reaching 1021 W/cm2 even up to 1022 W/cm2 under laboratory conditions, and also the laser pulses with duration less than 10 fs are available, which is close to a single cycle of oscillation of the laser light field. Such a strong laser field can quickly ionize materials to plasma, leading the laser-matter interaction to the highly relativistic nonlinear optical category. The electron accelerators driven by ultra-intense ultra-short laser pulses can greatly reduce the size and cost of the accelerators due to its ultra-high acceleration gradient, and it will be widely used in many applications, including free electron laser, high resolution imaging in biomedicine, material characterization, and high energy physics, providing great economic and social benefits. The electron acceleration driven by ultra-intense ultra-short laser pulses not only is the important basic research with great scientific significance in the area of high field laser physics but also has important application prospect. In this thesis, the systemic investigation on the physical issues of the laser plamas wakefield electron accelerator are performed. These issues include the cascaded electron accelerator, the phase-locking accelerating, the laser multiple filamentation process, the controllable electron injection scheme and the diagnosis of the electron beam. The related innovative results are as follows, 1. Studies on the all-optical cascaded laser-wakefield accelerator were carried out and a 0.8 GeV quasi monoenergetic electron beam was generated. The design for the parameters of the cascaded acceleration with ionization-induced injection as well as injection phase, laser propagation, and focus matching were performed. The electron energy spectrometer for high energy beams was developed and the data from this system were analyzed. 2. We demonstrated experimentally the seeding-phase control for a two-stage laser wakefield accelerator with gradient injection. By optimizing the seeding phase of electrons into the second stage, electron beams beyond 0.5 GeV with a 3% rms energy spread were produced over a short acceleration distance of 2 mm. Peak energy of the electron beam was further extended beyond 1 GeV by lengthening the second acceleration stage to 5mm. We have constructed the platform for the measurement of the magnetic field via magneto-optical Faraday polarimetry. Unlike the reported similar platforms, our platform is able to measure more than one spot in one shot. By placing the spots in the entrance and middle of the second stage, we were able to monitor the processes of electron seeding and acceleration in the second stage. 3. Electron beam with energy higher than the energy limitation was obtained in the experiment. A physical model was advocated to explain this abnormal phenomena. It is found that the evolution of the laser intensity played a major role in the enhancement of the energy gain. The expressions for the complete phase locking in the uniform plasma were also given. By comparing the calculated optimal laser intensity evolutions with the PIC simulation results, we found that the well guided laser pulse is most suitable to phase-lock the electron beam in the intensity-decaying region. Suggestions for the higher energy gain were made according to this expression. These findings may be very important for the GeV or even TeV range LWFA. 4. We have observed the multiple filamentation of the laser pulse in the LWFA. By analyzing the variation of the transmitted laser spots of different density, we were able to restore the multiple filamentation process in the plasma. We have demonstrated the two electron beams acceleration accompanied with the process. In the single-stage LWFA, the influence of the multiple filamentation on the electron beam quality is not obvious, while in the cascaded LWFA, most of the parameters, such as energy, energy spread and divergence, of the single filamentation are better than that of the multiple filamentation. 5. We have proposed a new optical injection scheme by using the PIC simulations and analysis. Far-infrared laser is used in the LWFA as an injection pulse, pushing electrons into the wakefield. The output electron beam energy and charge can be controlled by adjusting the injection phase which is determined by the delay distance between two laser pulses. Besides, this scheme also provides large electron charge and small absolute energy spread. All these advantages make it a promising option for the cascaded LWFA injector, opening new perspectives for stable GeV electron acceleration. 6. We have proposed a diagnosis equipment for the fs electron beams from LWFA. This equipment is designed to measure the charge, energy spectrum and length of the electron beams. The measurement of the beam length is based on the electron beam rotation effect due to the coherent diffraction radiation. |
| 语种 | 中文 |
| 源URL | [http://ir.siom.ac.cn/handle/181231/15851]  |
| 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 | 李文涛. 激光尾波场电子加速器相关物理问题的研究[D]. 中国科学院上海光学精密机械研究所. 2014. |
入库方式： OAI收割
来源：上海光学精密机械研究所
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