Direct Observation of the Injection Dynamics of a Laser Wakefield Accelerator Using Few-Femtosecond Shadowgraphy

We present few-femtosecond shadowgraphic snapshots taken during the nonlinear evolution of the plasma wave in a laser wakefield accelerator with transverse synchronized few-cycle probe pulses. These snapshots can be directly associated with the electron density distribution within the plasma wave and give quantitative information about its size and shape. Our results show that self-injection of electrons into the first plasma-wave period is induced by a lengthening of the first plasma period. Three-dimensional particle-in-cell simulations support our observations.

Figure 1

Left: Gas and electron density profile and focus position (dashed line). (a)–(f): Experimental shadowgrams at various positions in the plasma at a background electron density of $n_e=1.65\times {10}^{19}{\mathrm{cm}}^{-3}$. The vertical lines indicate the on-axis position of the plasma wave’s peaks as deduced from simulated shadowgrams (cf. Fig. 3). (g)–(l): Energy in the electron beam per MeV and spatially resolved in the vertical coordinate corresponding to the above images.

Figure 2

(a) Length of the first plasma period (bubble) as a function of propagation distance $v_{g}t$ taken from the shadowgrams [error bars represent the combination of uncertainty in determining the bubble length ($\pm 0.5\mu \mathrm{m}$) in a single shot reduced by $\sqrt{N}$ where repeat shots are available]. The blue (solid) horizontal line is the expected ${\lambda }_p$ for $n_e=1.65\times {10}^{19}{\mathrm{cm}}^{-3}$. Between $v_{g}t=0.86$ and 1.0 mm (gray shaded area), wave breaking radiation was frequently detected. (b) Wavelength of the second plasma period versus $n_e$ at a fixed position $v_{g}t=1.0\mathrm{mm}$ and corresponding integrated electron bunch charge. Open circles represent measured and averaged data points with the standard error of the mean, and the red (solid) line is according to Eq. (1). (c) Electron beam profiles for different plasma densities averaged over 40–180 shots.

Figure 3

(a) Electron density maps (gray) from the simulations at various positions and contours of the envelope of pump magnetic field $B_z$ (red or solid lines) corresponding to 50% and 75% of the peak. (b) Shadowgrams simulated from the images in Fig. 3.

Figure 4

Bubble length derived from density maps (black squares) and injected charge with energy larger than 20 MeV (red or light gray) from the PIC simulation and maximum instantaneous value $S_{\max }$ of the Poynting vector magnitude (blue or dark grey). Black horizontal line is the expected ${\lambda }_p$ for $n_e=1.7\times {10}^{19}{\mathrm{cm}}^{-3}$.