Generation of Quasimonoenergetic Electron Bunches with 80-fs Laser Pulses

Highly collimated, quasimonoenergetic multi-MeV electron bunches were generated by the interaction of tightly focused, 80-fs laser pulses in a high-pressure gas jet. These monoenergetic bunches are characteristic of wakefield acceleration in the highly nonlinear wave breaking regime, which was previously thought to be accessible only by much shorter laser pulses in thinner plasmas. In our experiment, the initially long laser pulse was modified in underdense plasma to match the necessary conditions. This picture is confirmed by semianalytical scaling laws and 3D particle-in-cell simulations. Our results show that laser-plasma interaction can drive itself towards this type of laser wakefield acceleration even if the initial laser and plasma parameters are outside the required regime.

Figure 1

Experimental setup. The laser pulse is incident from the left. Nonlinear Thomson scattered light was measured perpendicular to the laser direction. Collimated electron beams were accelerated in the laser direction and analyzed by an electron spectrometer. Also in the forward direction, transmitted laser light was analyzed. The insets show an image of the nonlinear Thomson signal from the relativistic channel and a forward Raman spectrum, respectively.

Figure 2

Single-shot measurements: Raw data as recorded on the bent IP (insets) and extracted electron spectra. (a) Exponential spectrum with $T_{\mathrm{eff}}=8\mathrm{MeV}$; (b) double peak at 23 and 36 MeV; (c) monoenergetic peak at 47 MeV.

Figure 3

Zoom of a typical image of the scintillating screen at optimum conditions. A narrow electron beam is visible. The width of 1 mm at 120 mm distance between focus and screen corresponds to an opening angle of $<10\mathrm{mrad}$. The well-collimated beam sits on top of a pedestal of electrons with a broad divergence of approximately 20° (inset).

Figure 4

PIC results of the laser intensity (nonaveraged energy flux) during interaction with the underdense plasma. The snapshots are separated by 45 fs, with the Gaussian density profile of the gas jet centered around $z=0$. The laser pulse is incident from the left and creates a relativistic plasma channel (a), is relativistically self-modulated (b), and only the first fragment survives  (c) and is strong enough to accelerate quasimonoenergetic electrons (d).

Figure 5

Electron density and spectrum for the last snapshot in Fig. 4. The diameter of the bubble is about $8\mu \mathrm{m}$, and the remaining laser pulse comprises only a few cycles and is located at the front (right side) of the bubble. The electrons inside the bubble are forming the peak in the spectrum. The spectrum includes all electrons inside the simulation box.