A preliminary design of a mega-electron-volt (MeV) monochromator with 10⁻⁵ energy spread for ultrafast electron diffraction (UED) and ultrafast electron microscopy (UEM) is presented. Such a narrow energy spread is advantageous in both the single shot mode, where the momentum resolution in diffraction is improved, and the accumulation mode, where shot-to-shot energy jitter is reduced. In the single-shot mode, we numerically optimized the monochromator efficiency up to 13% achieving 1.3 million electrons per pulse. In the accumulation mode, to mitigate the efficiency degradation caused by the shot-to-shot energy jitter, an optimized gun phase yields only a mild reduction of the single-shot efficiency, therefore the number of accumulated electrons nearly proportional to the repetition rate. Inspired by the recent work of Qi et al. (Phys Rev Lett 124:134803, 2020), a novel concept of applying reverse bending magnets to adjust the energy-dependent path length difference has been successfully realized in designing a MeV monochromator to achieve the minimum energy-dependent path length difference between cathode and sample. Thanks to the achromat design, the pulse length of the electron bunches and the energy-dependent timing jitter can be greatly reduced to the 10 fs level. The introduction of such a monochromator provides a major step forward, towards constructing a UEM with sub-nm resolution and a UED with ten-femtosecond temporal resolution. The one-to-one mapping between the electron beam parameter and the diffraction peak broadening enables a real-time nondestructive diagnosis of the beam energy spread and divergence. The tunable electric–magnetic monochromator allows the scanning of the electron beam energy with a 10⁻⁵ precision, enabling online energy matching for the UEM, on-momentum flux maximizing for the UED and real-time energy measuring for energy-loss spectroscopy. A combination of the monochromator and a downstream chicane enables “two-color” double pulses with femtosecond duration and the tunable delay in the range of 10 to 160 fs, which can potentially provide an unprecedented femtosecond time resolution for time resolved UED.

^{10 -5}兆电子伏 (MeV) 单色仪的初步设计介绍了超快电子衍射 (UED) 和超快电子显微镜 (UEM) 的能量扩散。如此窄的能量散布在提高衍射中的动量分辨率的单次发射模式和减少发射到发射能量抖动的累积模式中都是有利的。在单次模式下，我们对单色器效率进行了数值优化，最高可达 13%，实现了每脉冲 130 万个电子。在累积模式中，为了减轻由发射到发射能量抖动引起的效率下降，优化的枪相位只会轻微降低单发射效率，因此累积电子的数量几乎与重复率成正比。受到 Qi 等人最近工作的启发。(Phys Rev Lett 124:134803, 2020), 在设计 MeV 单色器以实现阴极和样品之间的最小能量依赖路径长度差时，成功实现了应用反向弯曲磁体来调整能量相关路径长度差的新概念。由于消色差设计，电子束的脉冲长度和与能量相关的定时抖动可以大大降低到 10 fs 水平。这种单色仪的引入为构建具有亚纳米分辨率的 UEM 和具有 10 飞秒时间分辨率的 UED 向前迈出了重要一步。电子束参数和衍射峰展宽之间的一对一映射使得能够实时无损诊断束能量扩散和发散。⁻⁵精度，实现 UEM 的在线能量匹配、UED 的动量通量最大化和能量损失谱的实时能量测量。单色仪和下游减速机的组合实现了具有飞秒持续时间和 10 到 160 fs 范围内的可调延迟的“双色”双脉冲，这可能为时间分辨的 UED 提供前所未有的飞秒时间分辨率。