中文摘要：

主要介绍了飞秒电子衍射系统的组成及设计指标.包括光电阴极、电子聚焦系统、电子偏转系统、双微通道板（MCP）电子探测器等,并给出了基本的设计思路、设计结果.光电阴极是由位于蓝宝石晶体上面的银膜构成,为了获得足够小的电子束斑以及减小电子上靶时的角度,紧贴栅极后放置一个100μm的小孔,对电子束的形状和大小进行限制.采用磁电子透镜进行聚焦,电子衍射图样由放置在样品后面的双MCP像增强器进行探测.在设计计算时,用Monte Carlo方法对光电子的初能量、初角度以及初位置分布进行抽样,用有限元法计算磁透镜周围的磁场分布,用有限差分法计算阴极和栅极之间以及偏转板之间的电场分布,用四阶龙格-库塔法模拟跟踪大量光电子（3000个）的运行轨迹.统计分析了3000个电子在最佳像面的时间分布、位置分布、方位角以及倾角分布,计算了飞秒电子衍射系统的空间、时间调制传递函数.

英文摘要：

This article has introduces the basic components and characteristics of a femoto-second electron diffraction system, i.e. the photo-cathode, magnetic lens, deflector and detector with doubled MCP etc., and demonstrated our design method and results. The photo-cathode is an Ag film deposition on a sapphire disk and the femtosecond electron pulses are accelerated to 50 keV through an extraction electric field of 10kV/mm between the photocathode and the extraction mesh. After extraction, the electron pulses are reshaped in size by a 100 μm pinhole, which screens out electrons with larger elevation angle and makes the passed electrons flying almost parallel to the axis, which results in a reduced electron beam spot. The beam is collimated with a magnetic lens and positioned to the sample by a pair of deflection plates for diffraction measurements. The electron diffraction images are recorded with a two-dimensional imaging system composed of a Chevron-type microchannel plate and a P20 phosphor screen on an optical-fiber faceplate. The initial position, kinetic energy and angle distribution of the photoelectrons is sampled with Monte Carlo method. The electric field is calculated with finite difference formula, and the magnetic flux of the focus region is calculated with finite elements methods, and the electron trajectory is traced with Runge-Kutta method. We analysized the temporal and spatial distribution of 3000 electrons when they reached the prime plane and give out the temporal and spatial modulation transfer function of the femoto-diffraction system.