Qi Fengfeng, Ma Zhuoran, Zhao Lingrong, Cheng Yun, Jiang Wenxiang, Lu Chao, Jiang Tao, Qian Dong, Wang Zhe, Zhang Wentao, Zhu Pengfei, Zou Xiao, Wan Weishi, Xiang Dao, Zhang Jie
Key Laboratory for Laser Plasmas (Ministry of Education), School of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, China.
Collaborative Innovation Center of IFSA (CICIFSA), Shanghai Jiao Tong University, Shanghai 200240, China.
Phys Rev Lett. 2020 Apr 3;124(13):134803. doi: 10.1103/PhysRevLett.124.134803.
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.
我们提出并演示了一种产生超短且超稳定的兆电子伏特电子束的新方案。在该方案中，光电阴极射频（rf）枪产生的电子束首先在其自身的库仑力作用下膨胀，在此过程中，正的能量啁啾被印刻在束流纵向相空间中。然后，该束流被送入具有正纵向色散的双弯消色差磁体，在那里，能量较低的束团尾部电子沿着较短路径行进，从而赶上束团头部，导致纵向束团压缩。我们表明，通过优化参数集，从电子源到压缩点的整个束流路径可以实现等时性，使得电子束的飞行时间不受射频幅度波动的影响。利用激光驱动的太赫兹偏转器，测量得到束团压缩后20 fC束流的束团长度和到达时间抖动分别约为29 fs（半高宽）和22 fs（半高宽）。这种超短且超稳定的电子束使我们能够在兆电子伏特超快电子衍射中实现50飞秒（半高宽）的分辨率，其中清晰地观察到对应于铋A_{1g}模式的2.6太赫兹晶格振荡，而无需校正短期定时抖动和长期定时漂移。此外，由于时间分辨率的提高和电子通量的增加，与声子耦合和衰减相关的振荡弱漫散射信号也得到了清晰分辨。我们预计该技术将对基于超短电子束的新兴设施和应用产生重大影响。
