Density-transition based electron injector for laser driven wakefield accelerators

We demonstrate a laser wakefield accelerator with a novel electron injection scheme resulting in enhanced stability, reproducibility, and ease of use. In order to inject electrons into the accelerating phase of the plasma wave, a sharp downward density transition is employed. Prior to ionization by the laser pulse this transition is formed by a shock front induced by a knife edge inserted into a supersonic gas jet. With laser pulses of 8 fs duration and with only 65 mJ energy on target, the accelerator produces a monoenergetic electron beam with tunable energy between 15 and 25 MeV and on average 3.3 pC charge per electron bunch. The shock-front injector is a simple and powerful new tool to enhance the reproducibility of laser-driven electron accelerators, is easily adapted to different laser parameters, and should therefore allow scaling to the energy range of several hundred MeV.

Figure 1

Sketch and shadowgraph image of the target setup (a). A razor blade is introduced laterally into a supersonic gas jet (undistorted gas jet edges: white dashed lines) in order to generate a shock front (blue line). The shadowgraph image was taken during the experiments showing the nozzle tip, the razor blade (slightly tilted), and the plasma produced by the driving laser. Inside the plasma the tilted shock front can be discerned and a small, bright spot indicative of electron injection is visible. (b) Measured electron density along the laser propagation axis.

Figure 2

A few shots representative for those 10% of all the shots with lowest energy spread for self-injection (a) and injection at a density transition (b). The horizontal axis in each image corresponds to the transversal electron beam size; the vertical axis shows electron energy. For self-injection the parameters of this fraction are $29.7\pm 1.2\mathrm{MeV}$ energy, $8\pm 5\%$ energy spread, $10.0\pm 2.3\mathrm{mrad}$ divergence, and $3.1\pm 1.0\mathrm{pC}$ charge. For injection at the density transition, these values are $24.3\pm 0.9\mathrm{MeV}$, $4\pm 0.5\%$, $7.3\pm 0.5\mathrm{mrad}$, and $1.8\pm 0.5\mathrm{pC}$, respectively.

Figure 3

Variation of electron energy with the position of the density transition. The grey dashed line shows a linear fit yielding an acceleration gradient $\simeq 190\mathrm{GV}/\mathrm{m}$.

Figure 4

State of the accelerator as obtained from PIC simulations shortly before (upper panel), immediately after (middle panel), and $\sim 85\mathrm{fs}$ after (lower panel) the laser pulse has crossed the density transition. White arrows indicate the position of the density transition; grey lines show the contour where 13.5% of maximum laser intensity is reached. The normalized instantaneous laser intensity is shown in a rainbow color map. The upper panel shows a strongly driven plasma wave that does not break. Strong injection triggered by the shock front is evident in the middle panel. The strong electric field of the injected electron bunch destroys the remaining plasma wave thus impeding injection in more than one wave-trough. This leads to a fully developed bubble structure (lower panel) that accelerates the injected electrons to several 10 MeV energy. Since the bubble is fully loaded with electrons, no further injection occurs.