超快电子衍射（UED）利用超短电子束团探测物质的超快结构变化过程，在物理、化学、生命科学、材料科学等领域内诸多基础问题的研究中有重要作用。在传统的基于直流高压加速技术的 keV UED 中，较低的电子动能及由此引起的较强的空间电荷效应限制了可研究样品的范围和时间分辨能力。近年来提出的基于光阴极微波电子枪的MeV UED能够克服以上限制从而显著提高UED性能。本论文围绕MeV UED中诸多物理和技术上的挑战开展理论及实验研究。我们对MeV UED中的主要物理问题，包括衍射过程和束流动力学行为进行了理论分析，建立了计算机粒子跟踪模拟方法开展定量研究，探讨了MeV UED的逆空间分辨率和时间分辨率对参数的依赖关系以及如何优化。我们注意到MeV UED中空间电荷效应导致衍射环峰位偏移的现象，并研究了空间电荷效应在横向和纵向上随电子动能的不同比例关系，结果表明keV UED中是纵向空间电荷效应限制了系统性能而MeV UED中横向空间电荷效应更为显著MeV UED中电子束团的长度与束团从阴极到样品的飞行时间存在关联，为此我们从基本定义出发，推导出MeV UED时间分辨率的具体形式，并给出优化方法，结果表明在现有技术条件下MeV UED的时间分辨率可达100 fs水平。在理论分析与模拟优化的指导下，我们搭建了MeV UED原型装置，获得了各主要低阶衍射环峰位能够清晰分辨的累积衍射样斑。通过进一步的模拟和实验优化，获得了单发的、逆空间分辨率更高的衍射样斑。MeV UED还可以工作在新颖的连续时间分辨（CTR）模式，我们对该模式进行了系统的理论、模拟和实验研究。对于多晶样品，我们获得了各主要低阶衍射环的条纹可以清晰分辨的累积样斑，具备皮秒级时间分辨率。对于单晶样品，我们获得了单发的高质量样斑，即利用单个电子束团就能以约200 fs的时间分辨率捕捉几 ps 内结构的连续变化过程。CTR模式完全有可能实现100 fs的时间分辨能力，将我们对超快过程的观察能力提升到新的水平。
Ultrafast electron diffraction (UED), which employs ultrashort electron bunches to probe ultrafast structural evolutions, is a very powerful tool in physics, chemistry, biological and material sciences. Conventional keV UEDs are based on direct-current high voltage acceleration technique, in which the relatively low kinetic energy and therefore the relatively strong space charge (SC) effects limit the range of samples and the temporal resolution. Recently, it has been proposed that MeV UED, which is based on a photocathode radio-frequency (rf) gun, may overcome above-mentioned limitations thus dramatically enhance UED performances. This dissertation is devoted to investigate several theoretical and technical challenges of MeV UED.We began with theoretical analysis of the dominating physics in MeV UED, including the diffraction and beam dynamics of the electron bunch, and then established a start-to-end particle tracking algorithm to perform quantitative investigation. We showed how the reciprocal space resolution and the temporal resolution of MeV UED depend on the relevant parameters and how to optimize them. We observed that the SC effects induced shifts of the peaks of diffraction rings. We found that the SC effects have different scalings with the electron kinetic energy in the transverse and longitudinal directions, which explains that in keV UED it is the longitudinal SC effects that limit the system performance while in MeV UED the transverse SC effects are much more evident. In MeV UED, there is correlation between the electron bunch length and the time-of-flight from the cathode to the sample, since they both depend on the rf gun phase when the electron bunch is launched from the cathode. We derived the expression of the temporal resolution of MeV UED, and discussed how to optimize it. The temporal resolution of MeV UED can approach 100 fs level with state-of-the-art hardware performances.Based on theoretical and simulation optimizations, we built a prototype MeV UED system. We obtained accumulative diffraction patterns (DPs) in which the peaks of several main low-order rings are clearly distinguishable. With further simulation and experimental optimizations, we achieved single-shot DPs of even higher qualities.MeV UED can also be operated in a novel so-called continuously time-resolved (CTR) mode. We investigated this scheme theoretically, in simulation and with experiment. With a poly-crystalline sample, we obtained CTR patterns in which the streaks of several main low-order rings are clearly resolved, and are of a ps level temporal resolution. More excitingly, with a single-crystalline sample, we achieved single-shot, high quality CTR DPs, which allowed the structural evolutions within several ps to be continuously resolved with a ~200 fs resolution. CTR mode is capable of boosting the temporal resolution to 100 fs, thus truly allows us to observe ultrafast phenomenas with unprecedented capabilities.
