Electron Rephasing in a Laser-Wakefield Accelerator

An important limit for energy gain in laser-plasma wakefield accelerators is the dephasing length, after which the electron beam reaches the decelerating region of the wakefield and starts to decelerate. Here, we propose to manipulate the phase of the electron beam in the wakefield, in order to bring the beam back into the accelerating region, hence increasing the final beam energy. This rephasing is operated by placing an upward density step in the beam path. In a first experiment, we demonstrate the principle of this technique using a large energy spread electron beam. Then, we show that it can be used to increase the energy of monoenergetic electron beams by more than 50%.

Figure 1

Schematic representation of the first (a) and second (b) experimental setup. The blade can move in and out the gas jet. The density profile is shown in green in the two cases near the gas jet. (c) Schematic representation of the bubble before and after the density step. The driving pulse (red) generates a bubble with a size $L_{b,1}$, which shrinks ($L_{b,2}<L_{b,1}$) by crossing the density step. The accelerating (green gradient) and decelerating (blue gradient) regions are shown. The electron bunch (purple) reaches the end of the accelerating region before the density step and is shifted back to the accelerating field when crossing the density step.

Figure 2

(a) Experimental angle resolved electron spectra in logarithmic scale without (top panel) and with the shock at 0.7 mm after the gas jet center (bottom panel). (b) Angle integrated electron spectra in logarithmic scale for four positions of the blade.

Figure 3

Electron density in the phase space $(z,E)$ for a gas jet without (a) and with (b) the shock. The green curve shows the longitudinal electric field on the laser axis. Simulated spectra obtained at the exit are shown in (c). The density transition is at $z=0.900\mathrm{mm}$.

Figure 4

Experimental angle resolved electron spectra of shock-injected electrons with no gas flowing from the needle (top panel), and for four different plasma densities in the second jet.