Plasma-Density-Gradient Injection of Low Absolute-Momentum-Spread Electron Bunches

Plasma density gradients in a gas jet were used to control the wake phase velocity and trapping threshold in a laser wakefield accelerator, producing stable electron bunches with longitudinal and transverse momentum spreads more than 10 times lower than in previous experiments (0.17 and $0.02\mathrm{MeV}/c$ FWHM, respectively) and with central momenta of $0.76\pm 0.02\mathrm{MeV}/c$. Transition radiation measurements combined with simulations indicated that the bunches can be used as a wakefield accelerator injector to produce stable beams with $0.2\mathrm{MeV}/c$-class momentum spread at high energies.

Figure 1

Setup for density gradient controlled injection using a laser focused on the gas jet downstream edge.

Figure 2

Charge measured by the ICT and Gamma signal versus gas jet Z position with respect to the laser focus. The Gaussian fit to the plasma density indicates the laser focus position in the profile. Correlation of the ICT and phosphor data showed that the MeV bunch charge was $\sim 0.3–1\mathrm{nC}$.

Figure 3

Magnetic spectra at a jet position of $-300\mu \mathrm{m}$, with the laser focus in the density downramp. Sequential shots (A), show the centroid (*) with respect to the average (square) over 45 shots, displaying bunch stability. The contour scale is fixed (see Fig. 4). The integrated magnetic spectrum (B), obtained by scanning the magnet current, shows rms error bars for charge and central energy (top of curve), and FWHM (at half max.). Data points (blue) are connected by a black line.

Figure 4

Electron bunch and transmitted laser profiles. In the electron profile at a jet position of $-300\mu \mathrm{m}$ (A), the black solid contour indicates the small-divergence feature (70% of peak). Dotted (dashed) contours indicate the sizes and displacements of the smallest (largest) area bunches observed. The (*) indicates the centroid, and inner (outer) error bars display rms (peak-to-peak) pointing deviation over 30 shots. The transmitted laser is unaberrated at this location (B), in contrast to the mode when focused upstream (C).

Figure 5

Simulated wake and phase space. The laser [(A), red] drives a plasma density wake ( gray) in the downramp, trapping and accelerating electrons (yellow). Phase space [(B), color bar in Fig. 4] shows formation of an ultrafast, narrow momentum spread bunch at $1.5\mathrm{MeV}/c$, also visible in the momentum spectrum lineout at right.