Self-Guided Laser Wakefield Acceleration beyond 1 GeV Using Ionization-Induced Injection

The concepts of matched-beam, self-guided laser propagation and ionization-induced injection have been combined to accelerate electrons up to 1.45 GeV energy in a laser wakefield accelerator. From the spatial and spectral content of the laser light exiting the plasma, we infer that the 60 fs, 110 TW laser pulse is guided and excites a wake over the entire 1.3 cm length of the gas cell at densities below $1.5\times {10}^{18}{\mathrm{cm}}^{-3}$. High-energy electrons are observed only when small (3%) amounts of ${\mathrm{CO}}_2$ gas are added to the He gas. Computer simulations confirm that it is the $K$-shell electrons of oxygen that are ionized and injected into the wake and accelerated to beyond 1 GeV energy.

Figure 1

A schematic of the experiment showing the $f/8$ laser beam, a blowup of the gas cell, the Michelson interferometer (100 fsec probe duration), the laser exit-spot diagnostics taking reflections from a $5\mathrm{\text{−}}\mu \mathrm{m}$-thick pellicle and an uncoated wedge, the dipole magnet, and two successive image plates (IPa and IPb). The image plates show actual experimental results and reveal, in this case, three distinct features in the electron spectrum labeled (i), (j), and (k). Electrons pass through a $40\mathrm{\text{−}}\mu \mathrm{m}$-thick aluminum optical dump and a $63\mathrm{\text{−}}\mu \mathrm{m}$-thick Mylar vacuum window (neither shown). Also shown are two electron trajectories and dimensions. Inset: Interferogram, the Abel-inverted on-axis density (solid line), and estimated complete $n_e$ profile (dashed line). The up-ramp at the cell entrance is about 0.5 mm and down-ramp at the cell exit is estimated to occur over 2 mm, the size of the entrance and exit apertures on the gas cell, respectively. The laser is polarized along $\widehat{x}$.

Figure 2

(a) The laser spot at the exit of the gas cell. White curves show a lineout through the peak and a Gaussian fit to the lineout. (b) Spectrally—and spatially—resolved (in $\widehat{y}$) laser spot at the exit of the gas cell. The raw-data spectrum up to 920 nm has been suppressed by $10\times$. (c) Lineout along the spatially narrow portion of the spectrum of (b) corrected for the quantum efficiency of the camera and for the 830-nm long-pass filter used to reduce the level of unshifted laser light on (b) (solid line) and original laser spectrum (dashed line). An identical long-pass filter was also used to record the image in (a).

Figure 3

(a) Raw electron data from IPa with corresponding ${\theta }_x$ and energy scales. Three clear features are labeled (i), (j), and (k). Location of lineout for $D_x(\theta )$ for finding ${\sigma }_x$ (white arrows, see text). (b) Electron spectra for the three features of (a). Uncertainty in ${\theta }_{0y}$ indicated by shaded region for curve (k) [negligible for curves (i) and (j)].

Figure 4

Results from a three-dimensional PIC simulation for 80 TW and a $15\mu \mathrm{m}$, diffraction-limited spot. Laser propagates from left to right. (a) Contour maps of the envelope of the laser field [orange-white, right colorbar, normalized to $(mc{\omega }_p)/e$] and the electron-density contributions (normalized to $n_e$) from He, C, and the $L$-shell electrons of O (grey, center colorbar) and from the $K$ shell of O (blue, left colorbar) at a distance of 1.75 mm into the simulation. The longitudinal position where the $L$-shell ($K$-shell) O electrons were ionized [dashed lines (solid line)]. (b) Momentum spectrum of $K$-shell O electrons from the simulation after 1 cm propagation in the plasma.