Multi-GeV Electron Acceleration in Wakefields Strongly Driven by Oversized Laser Spots

Experiments were performed on laser wakefield acceleration in the highly nonlinear regime. With laser powers $P<250\mathrm{TW}$ and using an initial spot size larger than the matched spot size for guiding, we were able to accelerate electrons to energies ${\mathcal{E}}_{\max }>2.5\mathrm{GeV}$, in fields exceeding $500\mathrm{GV}{\mathrm{m}}^{-1}$, with more than 80 pC of charge at energies $\mathcal{E}>1\mathrm{GeV}$. Three-dimensional particle-in-cell simulations show that using an oversized spot delays injection, avoiding beam loss as the wakefield undergoes length oscillation. This enables injected electrons to remain in the regions of highest accelerating fields and leads to a doubling of energy gain as compared to results from using half the focal length with the same laser.

Figure 1

(a)–(c) Sample electron spectrometer images: (a) narrow energy spread beam with $\mathcal{E}=(0.90\pm 0.01)\mathrm{GeV}$ and FWHM energy spread $\mathrm{\Delta }\mathcal{E}/\mathcal{E}=4\%$, for $(P,L,n_e)=(215\mathrm{TW},32\mathrm{mm},1.6\times {10}^{18}{\mathrm{cm}}^{-3})$; (b) beam with peak at $\mathcal{E}=1.4\mathrm{GeV}$ and $\mathrm{\Delta }\mathcal{E}/\mathcal{E}=10\%$, for $(240\mathrm{TW},20\mathrm{mm},1.6\times {10}^{18}{\mathrm{cm}}^{-3})$; (c) multi-GeV electron beam, with cutoff at ${\mathcal{E}}_{\max }=2.7\mathrm{GeV}$ for $(240\mathrm{TW},20\mathrm{mm},2.9\times {10}^{18}{\mathrm{cm}}^{-3})$. Note the different color scales in (a)–(c). (d) spectra for (a)–(c).

Figure 2

(a) Average spectra (solid line) and standard deviation (highlighted area) of (red) 6 shots taken with $(P,L,n_e)=(120\mathrm{TW},10\mathrm{mm},2.8\times {10}^{18}{\mathrm{cm}}^{-3})$ and (blue) 4 shots taken with (240 TW, 12.5 mm, $2.9\times {10}^{18}{\mathrm{cm}}^{-3}$); these represent all consecutively taken data for these conditions. (b) Variation of cut-off energy ${\mathcal{E}}_{\max }$ with $L$ at two different plasma densities for $P=120\mathrm{TW}$. The solid lines are parabolic fits to the data. Scaling of electron beam: (c) ${\mathcal{E}}_{\max }$ and (d) beam charge with plasma density for $L=12.5\mathrm{mm}$ and $P=240\mathrm{TW}$. The error bars in (c) and (d) are the standard deviation of the data points.

Figure 3

Results from 3D particle-in-cell simulations: electron energy spectra as a function of propagation distance $x$ for (a) $f/40$ and (b) $f/20$ focusing. (c) $a_0$ as a function of $x$ for $f/20$ (blue) and $f/40$ (red) simulations; inset shows the longitudinal profile of the wakefields ($f/20$ in gray, $f/40$ in black dots) and the independently normalized current profiles of the electron bunches in the moving window $\sim 1\mathrm{mm}$ after injection in both simulations. (d) peak longitudinal electric field in the bubble $E_x$ (dotted lines) and the accelerating field experienced by electrons that reach the highest energies (solid lines). Electrons that gain the most energy are highlighted by solid vertical lines in panels (a) and (b), with their injection $x$ positions highlighted by the shaded regions in (c) and (d).

Figure 4

Scaling of the highest electron energy with laser power for a selection of laser-driven wakefield experiments. Data are from references detailed in Supplemental Material.