Near-GeV Electron Beams at a Few Per-Mille Level from a Laser Wakefield Accelerator via Density-Tailored Plasma

A simple, efficient scheme was developed to obtain near-gigaelectronvolt electron beams with energy spreads of few per-mille level in a single-stage laser wakefield accelerator. Longitudinal plasma density was tailored to control relativistic laser-beam evolution, resulting in injection, dechirping, and a quasi-phase-stable acceleration. With this scheme, electron beams with peak energies of 780–840 MeV, rms energy spreads of 2.4‰–4.1‰, charges of 8.5–23.6 pC, and rms divergences of 0.1–0.4 mrad were experimentally obtained. Quasi-three-dimensional particle-in-cell simulations agreed well with the experimental results. The dechirping strength was estimated to reach up to $11\mathrm{TeV}/\mathrm{mm}\mathrm{/m}$, which is higher than previously obtained results. Such high-quality electron beams will boost the development of compact intense coherent radiation sources and x-ray free-electron lasers.

Figure 1

Schematic of the experimental setup. (a) A perforated baffle was inserted upright into the flow of a supersonic gas nozzle. A 100–120 TW, 25-fs laser pulse polarized along the horizontal direction was focused onto the gas target and propagated through the hole of the baffle. The spectra of the generated electrons were measured with a 1.1-T magnetic spectrometer. (b) The shock wave in the shadow graph is circled with a dashed line. The optical axis is at zero. (c) The retrieved on-axis plasma density profile ranges from the center of the baffle. The evolution of the laser beam in vacuum (dashed line) and in the plasma (red shaded area) are also shown, respectively.

Figure 2

Measured $e$ beams energy spectra. (a) The raw energy spectra of 15 shots with energy spreads ranging from 2.4‰–4.1‰. The color map represents the charge density ($\mathrm{pC}/\mathrm{MeV}/\mathrm{mrad}$) on the detector. (b) The spatially integrated energy spectrum of the 13th shot in (a). The peak energy of this shot is 817 MeV, and the rms relative energy spread is 3.3‰. The filled area represents the charge of the beam corresponding to 10.6 pC within a threefold relative energy spread. The inset shows a magnification of shot 13 rotated 90° clockwise. The rms divergence is 0.25 mrad.

Figure 3

Simulated evolution of various parameters. (a) Normalized vector potential ($a_0$), wake phase velocity (${\beta }_p$), and plasma density. Here, ${\beta }_p$ (blue line) is calculated at the rear of the bubble where the longitudinal electric field is zero. It reached the minimum near the end of the density down ramp. The evolution of $a_0$ (red line) can be separated as an oscillation period (blue shaded area) and a nearly matched period (red shaded area). (b),(c) The absolute energy spread (AES; red line) has a maximum of 4.5 MeV corresponding to a relative energy spread (RES) of 1.7% and a minimum of 2.4 MeV corresponding to an RES of 4.6‰. The red dashed lines indicate the locations where the dechirping process started and completed. The green dashed line shows the fitted of the mean longitudinal wakefield slope (MLWS).

Figure 4

Snapshots of electron density distribution in $r-\xi$ plane, where $\xi =z-ct$, the on-axis plasma field lineouts (green line), and the phase space dynamics of the $e$ beam at different distances in the PIC simulation. (a) The $e$ beam resides in a highly nonlinear accelerating field just after injection. (b) As the laser defocuses, the electron density at the rear of the bubble decreases. A sawtoothlike accelerating field with a gradual negative slope is formed. (c) The beam-loading effect helps sustain the relative energy spread during the final acceleration process. (d) The $e$ beam approaches dephasing. (e)–(h) The corresponding phase space dynamics and the localized longitudinal wakefield of the $e$ beams for (a)–(d). The dashed line represents the fitted chirp of the $e$ beam. The electron spectra (red shaded area) and the current profile (blue shaded area) of the entire $e$ beam are also shown.