Multi-GeV Electron Beams from Capillary-Discharge-Guided Subpetawatt Laser Pulses in the Self-Trapping Regime

Multi-GeV electron beams with energy up to 4.2 GeV, 6% rms energy spread, 6 pC charge, and 0.3 mrad rms divergence have been produced from a 9-cm-long capillary discharge waveguide with a plasma density of $\approx 7\times 10^{17}{\mathrm{cm}}^{-3}$, powered by laser pulses with peak power up to 0.3 PW. Preformed plasma waveguides allow the use of lower laser power compared to unguided plasma structures to achieve the same electron beam energy. A detailed comparison between experiment and simulation indicates the sensitivity in this regime of the guiding and acceleration in the plasma structure to input intensity, density, and near-field laser mode profile.

Figure 1

Typical laser spatial profiles in a vacuum with laser energy 16 J for (a) focus, (b) $z=10.4\mathrm{m}$ downstream of focus, and (c) $z=9\mathrm{cm}$ downstream of focus. The near field (b) shows an approximately top-hat profile. In (d) the guided mode is shown for plasma density $7.5\times 10^{17}{\mathrm{cm}}^{-3}$. Well-confined high-quality modes were observed for densities as low as $2\times 10^{17}{\mathrm{cm}}^{-3}$. The color scale is the same for (c) and (d).

Figure 2

Laser pulse radial fluence profile evolution through the waveguide for different plasma density and initial fluence profiles for input laser energy 15 J. In each image the color scale represents the fluence (each normalized to the peak fluence at $z=0$) and the dashed line is two times the radius at which the fluence drops to $1/e^2$ of the on-axis value. For (a) the initial laser pulse radial profile is Gaussian and the preformed channel is defined by $r_m=93\mu \mathrm{m}$ and $n_e=4\times 10^{17}{\mathrm{cm}}^{-3}$. In (b), (c), and (d) the near field of the laser pulse has a top-hat profile. For the same density and channel depth as in (a), (b) shows that the guiding is less efficient for the top-hat case. For (c) the combination of a self-guiding and a preformed plasma channel ($r_m=81\mu \mathrm{m}$) mitigates diffraction over the full length of the plasma. Without a preformed plasma channel [panel (d)], self-guiding reduces the laser spot size for the first 2 cm but the laser diffracts strongly before the exit of the channel.

Figure 3

Measured (left panel) and simulated (right panel) optical spectra at the output of the $500\text{−}\mu \mathrm{m}$-diam capillary as a function of plasma density for input laser energy $\approx 7.5\mathrm{J}$ and pulse duration $\approx 40\mathrm{fs}$. The spectral response of the detection system was applied to the simulated data.

Figure 4

Evolution (a) of the peak normalized laser field strength, $a_0(z)$ (red plot), in a PIC simulation for a top-hat laser pulse with an energy of 16 J focused at the entrance of a 9-cm-long plasma channel. The on-axis density (black dashed line) has a plateau density of $n_e=7\times 10^{17}{\mathrm{cm}}^{-3}$, and the matched radius is $r_m=81\mu \mathrm{m}$. The wakefield (electron density) at various longitudinal locations is shown in (i)–(iv).

Figure 5

Energy spectrum of a 4.2 GeV electron beam measured using the broadband magnetic spectrometer. The plasma conditions closely match those in Fig. 2. The white lines show the angular acceptance of the spectrometer. The two black vertical stripes are areas not covered by the phosphor screen.