降低电子源的发射度可以减小X射线自由电子激光器的规模和提高汤姆逊散射X射线装置的亮度，因此发射度问题一直是光阴极微波电子枪的研究热点。当发射度补偿原理被应用于光阴极注入器后，低热发射度的光阴极、激光整形和高梯度的电子枪成为了降低发射度的关键。目前光阴极热发射度的实验结果与理论预测有较大的差距，为了进一步理解热发射度，本论文对金属阴极的热发射度进行了实验研究。此外，清华大学现有光阴极微波电子枪的加速梯度只有~75MV/m，成为了降低电子束发射度的瓶颈之一。为满足清华大学汤姆逊散射X射线装置和上海软X射线自由电子激光装置对电荷量为0.5~1nC，发射度为1~2mm.mrad的高亮度电子束的需求，加速梯度急需提升至~100MV/m，因此本论文在现有的光阴极微波电子枪基础上，通过结构及工艺优化，研制了新的光阴极微波电子枪。我们将粗糙度热发射度与光电激发后电子剩余动能形成的热发射度进行了不相关叠加，结合总的热发射度和量子效率对电场的关系，设计了新的实验方法，国际上首次测量出了铜阴极的在线逸出功、场增强因子和粗糙度热发射度，并与铜阴极表面形貌的离线测量的分析结果基本吻合。铜阴极在线逸出功和文献值的偏离，以及50MV/m下0.92mm.mrad/mm的粗糙度发射度，不仅解释了铜热发射度大于Dowell理论值的原因，更说明了减小阴极表面粗糙度对热发射度的意义。镁阴极的热发射度实验值小于理论值，不能用粗糙度热发射度来解释。通过深入分析光电激发的原理，我们发现Dowell的热发射度理论的各向同性假设并不适用于表面光电发射。镁阴极的逸出功低于表面等离激元能量，使得激光垂直入射时也能通过表面等离基元激发表面发射。通过测量镁阴极在不同量子效率下的热发射度变化和斜入射下的矢量光电效应，我们佐证了镁阴极的光电效应是由表面等离子体主导的表面发射。镁阴极的表面光电效应打破了Dowell对金属阴极热发射度随着量子效率提高而增大的预测，解释了镁阴极低热发射度的原因。本论文在BNL型电子枪基础上，改进了腔内易打火的结构，降低了零模场的激发和π模场的非轴对称性，研制了新的光阴极微波电子枪。新电子枪经过高功率老练达到了112MV/m，通过改进阴极处理工艺，暗电流也达到了国际同类电子枪的水平。论文还利用Panofsky-Wenzel定理分析了多极场和基模场发射度之间的关联，功率单馈的粒子动力学效应，并与模拟结果取得了吻合。关键词：光阴极微波电子枪；热发射度；粗糙度；表面发射；多极场

It will help downsize the X-ray Free Electron Laser (XFEL) facility and increase the X-ray brightness of Thomson scattering if the electron source emittance could be improved, which has been a hot topic in photocathode RF gun study for decades. After the application of emittance compensation in photoinjectors, low thermal emittance, laser shaping and high gradient gun become the keys to low beam emittance. The thermal emittance measurement results have been inconsistent with theoretical predictions, and not been explained yet. We experimentally studied the thermal emittance of metal photocathodes (Copper and Magnesium), and mechanisms of thermal emittance formation were analyzed. Besides, the peak gradients of the Tsinghua Photocathode RF guns were limited to ~75 MV/m due to RF breakdowns, and become the bottleneck of emittance optimization. In order to get the emittance of an high charge beam (0.5 ~ 1 nC) below 2 mm.mrad, which is required by both the Tsinghua Thomson scattering (TTX) facility and the soft X-ray FEL (SXFEL) project of China, the gun gradient of 100 MV/m or more is necessary, so we studied the structure optimization of the BNL/KEK/SHI type photocathode RF gun to increase the gun gradient.In the thermal emittance study of Copper cathode, the thermal emittance due to roughness effect and residual photon energy are assumed to be uncorrelated and added quadratically. The electric field dependence of total thermal emittance and quantum efficiency is used to fit work function, field enhancement factor and roughness emittance. The experiment results revealed a work function (4.16 eV) much lower than literature value (4.65 eV) and roughness emittance (0.92 mm.mrad/mm at 50 MV/m) was measured for the first time. The cathode surface morphology was characterized under white light interferometer, and roughness emittance theorectical calculation with the actual surface morphology is consistent with emittance fitting results. The experiment not only explains the previous inconsistencies between measured thermal emittance and theory calculations, but also indicates the role of surface roughness in thermal emittance formations.The Mg thermal emittance cannot be explained by roughness emittance because measured thermal emittance is smaller than theory prediction (Dowell's equation). By reviewing the mechanism of photoexcitation, it is found the assumption in Dowell's theory, i.e. isotropic distribution of electron momentum after photoexcitation, is not right in surface photoemission. Surface photoemission can be excited by surface Plasmon even when the UV light (266 nm) is normal incident on the Mg cathode. By laser cleaning, thermal emittance of ~0.5 mm.mrad/mm is measured with QE variation as much as a factor of 10. Besides, vectorial photoelectric effect is measured at oblique incidence, and vector ratio of 1.5 is measured. Both experiment results are consistent with surface photoemission excited by surface Plasmon. Surface photoemission breaks Dowell's prediction of higher thermal emittance with higher QE for a metal cathdoe, and explains that Mg is a low thermal emittance cathode.Based on the BNL/KEK/SHI type gun, the new Tsinghua photocathode RF gun was optimized to reduce RF breakdown, zero mode excitation and nonaxisymmetry of π mode field, and reached ~112 MV/m with a RF pulse of 1.5 μs after ~20 hours' high power conditioning. By improving the cathode processing technique, the dark current is also greatly reduced and of the same level as that of LCLS gun. The correlation between multipole field emittance and monopole field emittance, and the beam dynamics due to single RF feed are both analyzed by Panofsky-Wenzel theory, and the analysis results are demonstrated by PARMELA simulations.