Xu Xinlu, Li Fei, Tsung Frank S, Miller Kyle, Yakimenko Vitaly, Hogan Mark J, Joshi Chan, Mori Warren B
SLAC National Accelerator Laboratory, Menlo Park, CA, USA.
Department of Electrical Engineering, University of California Los Angeles, Los Angeles, CA, USA.
Nat Commun. 2022 Jun 11;13(1):3364. doi: 10.1038/s41467-022-30806-6.
The longitudinal coherence of X-ray free-electron lasers (XFELs) in the self-amplified spontaneous emission regime could be substantially improved if the high brightness electron beam could be pre-bunched on the radiated wavelength-scale. Here, we show that it is indeed possible to realize such current modulated electron beam at angstrom scale by exciting a nonlinear wake across a periodically modulated plasma-density downramp/plasma cathode. The density modulation turns on and off the injection of electrons in the wake while downramp provides a unique longitudinal mapping between the electrons' initial injection positions and their final trapped positions inside the wake. The combined use of a downramp and periodic modulation of micrometers is shown to be able to produces a train of high peak current (17 kA) electron bunches with a modulation wavelength of 10's of angstroms - orders of magnitude shorter than the plasma density modulation. The peak brightness of the nano-bunched beam can be O(10A/m/rad) orders of magnitude higher than current XFEL beams. Such prebunched, high brightness electron beams hold the promise for compact and lower cost XEFLs that can produce nanometer radiation with hundreds of GW power in a 10s of centimeter long undulator.
如果能在辐射波长尺度上对高亮度电子束进行预群聚，那么处于自放大自发辐射状态的X射线自由电子激光（XFEL）的纵向相干性就能得到显著改善。在此，我们表明，通过在周期性调制的等离子体密度斜坡/等离子体阴极上激发非线性尾波，确实有可能在埃尺度上实现这种电流调制电子束。密度调制开启和关闭尾波中电子的注入，而斜坡则在电子的初始注入位置与其在尾波内的最终俘获位置之间提供了独特的纵向映射。结果表明，将斜坡与微米级的周期性调制结合使用，能够产生一系列高峰值电流（17千安）的电子束团，其调制波长为十几埃——比等离子体密度调制短几个数量级。纳米群聚束的峰值亮度可比当前的XFEL束高O(10A/m/rad)几个数量级。这种预群聚的高亮度电子束有望用于制造紧凑且成本更低的XFEL，这种XFEL能在几十厘米长的波荡器中产生数百吉瓦功率的纳米辐射。
