High-Brightness High-Energy Electron Beams from a Laser Wakefield Accelerator via Energy Chirp Control

By designing a structured gas density profile between the dual-stage gas jets to manipulate electron seeding and energy chirp reversal for compressing the energy spread, we have experimentally produced high-brightness high-energy electron beams from a cascaded laser wakefield accelerator with peak energies in the range of 200–600 MeV, 0.4%–1.2% rms energy spread, 10–80 pC charge, and $\sim 0.2\mathrm{mrad}\mathrm{rms}$ divergence. The maximum six-dimensional brightness $B_{6\mathrm{D},n}$ is estimated as $\sim 6.5\times 10^{15}\mathrm{A}/{\mathrm{m}}^2/0.1\%$, which is very close to the typical brightness of $e$ beams from state-of-the-art linac drivers. These high-brightness high-energy $e$ beams may lead to the realization of compact monoenergetic gamma-ray and intense coherent x-ray radiation sources.

Figure 1

(a) Experimental layout of the cascaded LWFA using two gas jets. (b) The measured plasma density profile with a “density bump” between the two-segment plasmas.

Figure 2

Measured $e$-beam energy spectra and 6D brightness $B_{6\mathrm{D},n}$. (a) Angle-resolved energy spectra of a series of high-quality $e$ beams with peak energies in the range of 530–580 MeV. (b) Angle integrated energy spectra of the $e$ beams corresponding to (a). (c) The statistic 6D brightness $B_{6\mathrm{D},n}$ of the $e$ beams corresponding to (a).

Figure 3

Measured energy spectra of high-quality $e$ beams while adjusting the length of the second-segment plasma ($l_2$) from 2.0 to 4.0 mm. Angle resolved (a) and integrated (b) spectra of tunable $e$ beams with peak energies ($E$) ranging from 250 to 550 MeV, respectively.

Figure 4

Particle-in-cell simulations illustrating the cascaded electron acceleration scheme in different buckets of a laser-driven wakefield. Plasma density profiles applied, evolutions of normalized intensity $a$ (in red) and the corresponding phase velocity ${\beta }_p$ of the second bucket (in blue) for the cases with (a) and without the density bump (b), respectively. The dashed blue line corresponds to the variation of the phase velocity caused by the density bump alone. FSI (yellow area) and SSI (green area) show the regions where first self-injection and second self-injection occur, respectively. (c)–(f) The snapshots of electron density distribution in the $x\text{−}z$ plane for different $x$ position. (g) Evolutions of the peak energy of the accelerated $e$ beams for the cases with (dashed line in blue) and without the density bump (solid line in blue), respectively. Also shown is the evolution of the absolute energy spread (solid line in red) of the target $e$ beam in the case with the density bump. The light blue area represents the region where the target $e$ beam witnesses a wakefield with a negative slope. (h) Energy distribution of the target $e$ beam along $x$ at $x=1.51\mathrm{mm}$ and the corresponding on-axis wakefield $E_x$. The $e$ beam is located in the negative-slope region of the wakefield.

Figure 5

Energy spectrum of the generated $e$ beam for the case with the density bump.