Femtosecond probing of plasma wakefields and observation of a plasma wake echo using a relativistic electron bunch
C. J. Zhang, J. F. Hua, Y. Wan, C.-H. Pai, B. Guo, J. Zhang, Y. Ma, F. Li, Y. P. Wu, H.-H. Chu, Y. Q. Gu, X. L. Xu, W. B. Mori, C. Joshi, J. Wang, and W. Lu
Physical Review Letters 119, 064801 (2017)
We show that a high-energy electron bunch can be used to capture the instantaneous longitudinal and
transverse field structures of the highly transient, microscopic, laser-excited relativistic wake with
femtosecond resolution. The spatiotemporal evolution of wakefields in a plasma density up ramp is
measured and the reversal of the plasma wake, where the wake wavelength at a particular point in space
increases until the wake disappears completely only to reappear at a later time but propagating in the
opposite direction, is observed for the first time by using this new technique.