All-Optical GeV Electron Bunch Generation in a Laser-Plasma Accelerator via Truncated-Channel Injection

We describe a simple scheme, truncated-channel injection, to inject electrons directly into the wakefield driven by a high-intensity laser pulse guided in an all-optical plasma channel. We use this approach to generate dark-current-free 1.2 GeV, 4.5% relative energy spread electron bunches with 120 TW laser pulses guided in a 110 mm-long hydrodynamic optical-field-ionized plasma channel. Our experiments and particle-in-cell simulations show that high-quality electron bunches were only obtained when the drive pulse was closely aligned with the channel axis, and was focused close to the density down ramp formed at the channel entrance. Start-to-end simulations of the channel formation, and electron injection and acceleration show that increasing the channel length to 410 mm would yield 3.65 GeV bunches, with a slice energy spread $\sim 5\times {10}^{-4}$.

Figure 1

Schematic of truncated-channel injection scheme. (a) Setup: channel-forming (red) and drive (orange) beams were coupled into the gas target. The input mode, output mode, optical, and electron spectra were measured on every shot. Inset: measured transverse fluence profiles of the drive laser at focus and at the exit of the HOFI channel. (b) Measured axicon longitudinal intensity profile, (c) calculated initial electron temperature profile, and (d) calculated density profile of the truncated HOFI plasma channel 3.5 ns after arrival of the channel-forming pulse. In each panel, the black curve shows the relative magnitude of each variable along the optical axis.

Figure 2

(a) Left axis: injection probability, $p_{\mathrm{inj}}$, as a function of $\mathrm{\Delta }z=z_{\mathrm{ch}}-z_{\mathrm{f}}$ for all (blue circles) and guided (green squares) shots. Right axis: variation of calculated electron bunch charge $Q_b$ with $\mathrm{\Delta }z$, from PIC simulations. Inset (gray): simulated longitudinal phase-space distribution of the electron bunch at the channel exit for $\mathrm{\Delta }z=0.8\mathrm{mm}$. (b) Examples of the measured, angularly resolved, electron spectra (left) and guided drive beam profile (right) for $\mathrm{\Delta }z=0.8\mathrm{mm}$; black crosses show the drive input position. Also shown is a comparison between the measured and simulated spectral density. (c) Plots as in (b) for $\mathrm{\Delta }z=-11.2\mathrm{mm}$, for cases where the drive pulse was misaligned (upper) or aligned (lower) to the channel.

Figure 3

Variation with $\delta r$ of the spectra of the output drive and electron beams. (a) $⟨\lambda ⟩$ for $\mathrm{\Delta }z=0.8\mathrm{mm}$. The size of data points corresponds to the relative fluence of the measured guided mode. The result from start-to-end simulations is shown in gray. (b) Mean electron energy, ${\mu }_E$, for guided shots for TCI (purple squares) and ionization injection (blue circles); bins containing only a single event use “$+$” markers. For the ionization injection dataset, $\mathrm{\Delta }z=-11.2\mathrm{mm}$ and $n_{e0}=(2.2\pm 0.1)\times {10}^{17}{\mathrm{cm}}^{-3}$. (c) Average ratio ${\sigma }_E/{\mu }_E$ of accelerated electron bunches. For all plots the data points with error bars are averages over $10\mathrm{\mu }\mathrm{m}$-wide bins [45], with the error bars showing one standard error.

Figure 4

(a) Laser spot size (top) and TCI bunch charge (bottom) during propagation for input modes of different sizes. (b) Electron bunch longitudinal phase space at four different propagation distances. The longitudinal current profile at $z=410\mathrm{mm}$ is shown in blue. (c) Relative energy spread of the TCI bunch (${\sigma }_E/{\mu }_E$, blue), the mean relative slice energy spread (red), and bunch chirp ($h$, purple).

Figure 5

Results of fluid simulations of the longitudinal variation of the total density $n=n_{\mathrm{e}}+n_{\mathrm{H}}$ for three simulations with $T_{\mathrm{e}}(r,z-\delta z)$ with $\delta z=0,-100,-200\mathrm{\mu }\mathrm{m}$. The initial gas density profile, $n_{\mathrm{gas}}$, assumed in these simulations is represented by the black dashed line.

Figure 6

Results of fluid simulations showing the temporal evolution of the total density $n=n_{\mathrm{e}}+n_{\mathrm{H}}$ for (a) the longitudinal expansion at the start of the channel and (b) the transverse expansion of the bulk that forms the guiding structure. The delays plotted vary between 0 and 5 ns in steps of 1 ns. The delay used in the experiment, $\mathrm{\Delta }\tau =3.5\mathrm{ns}$, is represented by the black dotted line.