Near-GeV Acceleration of Electrons by a Nonlinear Plasma Wave Driven by a Self-Guided Laser Pulse

The acceleration of electrons to $\simeq 0.8\mathrm{GeV}$ has been observed in a self-injecting laser wakefield accelerator driven at a plasma density of $5.5\times {10}^{18}{\mathrm{cm}}^{-3}$ by a 10 J, 55 fs, 800 nm laser pulse in the blowout regime. The laser pulse is found to be self-guided for 1 cm ($>10z_R$), by measurement of a single filament containing $>30\%$ of the initial laser energy at this distance. Three-dimensional particle in cell simulations show that the intensity within the guided filament is amplified beyond its initial focused value to a normalized vector potential of $a_0>6$, thus driving a highly nonlinear plasma wave.

Figure 1

Spectrally dispersed electron beams at the exit of the magnetic spectrometer for gas jet lengths (a) 3, (b) 5, (c) 8, and (d) 10 mm, at a plasma density of $n_e=(5.7\pm 0.2)\times {10}^{18}{\mathrm{cm}}^{-3}$ with $(10.0\pm 1.5)\mathrm{J}$ of laser energy on target. (e)–(h) Show lineouts of (a) to (d), that have been deconvolved to give the electron number per solid angle and relative energy spread $dE/E$. All plots are normalized to unity.

Figure 2

Scaling of electron beam energy as a function of plasma density. The maximum achievable peak energy for the 5 mm ([red] squares) and 10 mm nozzle ([blue] circles) and the average peak energy for the 10 mm nozzle (dashed [blue] line) are plotted for $(10.0\pm 1.5)\mathrm{J}$ on target. The dotted [black] and solid [black] lines show the predictions by the nonlinear scaling law [8] for $a\simeq 3.9$ and $a\simeq 6.0$. The rms energy stability is 40% and 15% for the 10 mm nozzle at 5.7 and $6.3\times {10}^{18}{\mathrm{cm}}^{-3}$ averaging over 14 and 7 shots, respectively. (inset) Maximum observed electron energy as a function of plasma length. Error bars are explained in the text.

Figure 3

(a) Typical interferogram obtained by transverse probing. (b)–(h) background subtracted 2D images of the laser mode for various conditions: (b) in vacuum at the position of optimal focus $z=0$; (c) in vacuum at $z=10\mathrm{mm}$; (d)–(h) at $z=10\mathrm{mm}$ with plasma $n_e=(5.7\pm 0.2)\times {10}^{18}{\mathrm{cm}}^{-3}$ and input laser powers of: (d) 180 TW, (e) 60 TW, (f) 30 TW, (g) 20 TW, and (h) 7 TW.

Figure 4

Simulation for the density profile of the 10 mm nozzle at $n_e=5.5\times {10}^{18}{\mathrm{cm}}^{-3}$ (a) plasma density (blue) and injected bunch energy (red); (b) longitudinal momentum of injected electrons. Lineout (right axis) gives corresponding energy spectrum (arb. units).