Femtosecond Probing of Plasma Wakefields and Observation of the Plasma Wake Reversal Using a Relativistic Electron Bunch

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.

Figure 1

The schematic drawing of the probing of a laser-driven plasma wakefield using a relativistic electron beam.

Figure 2

Snapshot of the wake, the deduced electric field structure of the wake, and comparison with simulations. (a) Raw image of the probe density $n$. (b) Density modulation $\delta n/n_0$. (c) and (d) The $(y,z)$ plot of the reconstructed $E_z$ and $E_y$ field, respectively. (e) and (f) The axial and transverse ($z=242\mu \mathrm{m}$) lineouts of $E_z$, respectively. (g) and (h) The axial (at $y=-10\mu \mathrm{m}$) and transverse ($z=300\mu \mathrm{m}$) lineouts of $E_y$. The blue solid lines show the experimental data, while the red dashed lines are from simulations. The transverse lineouts are taken at positions where the agreement between the simulations and the experiment is the closest. The error bars arise from the fluctuating energy, pulse width, and correlated divergence of the probe beam.

Figure 3

(a) Propagation of wakefield in an increasing plasma density ramp. Different lines correspond to the axial lineout of density modulation of the probe at different times. The blue dots mark the center of the first wake period and is fitted by the red line with a slope $dz/dt=c$ confirming that it is also the position of the wake-driving laser pulse. (b) The retrieved initial density profile of the plasma density up ramp of the gas jet seen by the laser pulse using data in (a). Color tables in (a) and (b) are the same. The spread in data is indicative of the shot-to-shot reproducibility of the gas jet density profile.

Figure 4

(a) Experimental wakefield snapshots at different time delays at a particular $z$ location with a range of $\mathrm{\Delta }z=140\mu \mathrm{m}$ for $0<t(\mathrm{ps})<4.6$. The color tables in all the shots have been adjusted to highlight the wake wavelength variation. (b) Evolution of wakefield wave number $k$ versus time over the (detectable) lifetime of the wake for a lower density plasma up ramp than in (a). The blue dots are experimental measurements and the red solid line is the best fit to the data. The open circles and red dashed line are reflections of the measurements and fitting curve in the time region where $k$ becomes negative. The axial lineouts of the wakefield amplitude (in arbitrary units) for five different time delays are shown in the insets.