We propose and demonstrate a novel scheme to produce ultrashort and ultrastable MeV electron beam. In this scheme, the electron beam produced in a photocathode radio frequency (rf) gun first expands under its own Coulomb force with which a positive energy chirp is imprinted in the beam longitudinal phase space. The beam is then sent through a double bend achromat with positive longitudinal dispersion where electrons at the bunch tail with lower energies follow shorter paths and thus catch up with the bunch head, leading to longitudinal bunch compression. We show that with optimized parameter sets, the whole beam path from the electron source to the compression point can be made isochronous such that the time of flight for the electron beam is immune to the fluctuations of rf amplitude. With a laser-driven THz deflector, the bunch length and arrival time jitter for a 20 fC beam after bunch compression are measured to be about 29 fs (FWHM) and 22 fs (FWHM), respectively. Such an ultrashort and ultrastable electron beam allows us to achieve 50 femtosecond (FWHM) resolution in MeV ultrafast electron diffraction where lattice oscillation at 2.6 THz corresponding to Bismuth A_{1g} mode is clearly observed without correcting both the short-term timing jitter and long-term timing drift. Furthermore, oscillating weak diffuse scattering signal related to phonon coupling and decay is also clearly resolved thanks to the improved temporal resolution and increased electron flux. We expect that this technique will have a strong impact in emerging ultrashort electron beam based facilities and applications.

中文翻译：

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用双弯曲消色差压缩机在MeV超快电子衍射中突破50飞秒的分辨率壁垒。

我们提出并证明了一种产生超短且超稳定的MeV电子束的新颖方案。在该方案中，在光电阴极射频（rf）枪中产生的电子束首先在其自身的库仑力下膨胀，利用该库仑力在束纵向相空间中施加正能量chi。然后，光束通过具有正纵向色散的双弯曲消色差透镜传输，在该处，具有较低能量的束尾处的电子遵循较短的路径，从而追赶束头，从而导致纵向束压缩。我们表明，通过优化的参数集，可以使从电子源到压缩点的整个电子束路径是等时的，从而使电子束的飞行时间不受rf幅度波动的影响。借助激光驱动的太赫兹偏转器，束压缩后20 fC光束的束长度和到达时间抖动分别约为29 fs（FWHM）和22 fs（FWHM）。这种超短且超稳定的电子束使我们能够在MeV超快电子衍射中获得50飞秒（FWHM）的分辨率，其中在不校正短期定时抖动和长时间校正的情况下，可以清晰地观察到对应于铋A_ {1g}模式的2.6 THz晶格振荡。定时漂移。此外，归因于时间分辨率的提高和电子通量的提高，与声子耦合和衰变有关的微弱散射信号的振荡也得到了明显解决。我们希望该技术将对新兴的基于超短电子束的设施和应用产生巨大影响。