Observation of Laser Power Amplification in a Self-Injecting Laser Wakefield Accelerator

We report on the depletion and power amplification of the driving laser pulse in a strongly driven laser wakefield accelerator. Simultaneous measurement of the transmitted pulse energy and temporal shape indicate an increase in peak power from $187\pm 11\mathrm{TW}$ to a maximum of $318\pm 12\mathrm{TW}$ after 13 mm of propagation in a plasma density of $0.9\times {10}^{18}{\mathrm{cm}}^{-3}$. The power amplification is correlated with the injection and acceleration of electrons in the nonlinear wakefield. This process is modeled by including a localized redshift and subsequent group delay dispersion at the laser pulse front.

Figure 1

(Top) FROG traces and (bottom) Wigner transforms with temporal profiles (black lines) for (left to right) $n_e=(0,0.9,1.8,2.3)\times {10}^{18}{\mathrm{cm}}^{-3}$ and a nozzle diameter of 15 mm. The Wigner transforms and temporal profiles have been corrected for diagnostic dispersion, representing the pulse at the plasma exit. The fs FWHM pulse durations are displayed by each pulse.

Figure 2

Experimental and simulated (a) transmitted laser energy fraction, (b) pulse duration (FWHM), and (c) peak pulse power and maximum observed electron beam energy (red circles) versus plasma density for a 15 mm nozzle diameter. The gray shaded regions indicate rms error (statistical and measurement errors) of a moving average of the data points. The red dashed line in (b) is the instrument limit of the FROG for time-bandwidth limited pulses. The solid black lines are calculated from our pulse evolution model.

Figure 3

Laser depletion (a) and pulse compression (b) as functions of the areal density (top axis) and the interaction length normalized by the depletion length (bottom axis). Experimental measurements (markers) are averaged over a $0.25\times {10}^{18}{\mathrm{cm}}^{-2}$ bin width for three different nozzle diameters [inset in (a)]. Simulated values (lines) are given for different electron densities [inset in (b) in units of $10^{18}{\mathrm{cm}}^{-3}$].

Figure 4

(a) On-axis electron density map (image) in a frame moving at $v_{\varphi }=v_g-v_{\mathrm{etch}}$ and peak $a_0$ of the laser (red line), as functions of propagation distance in a simulation with $n_e=4\times {10}^{18}{\mathrm{cm}}^{-3}$. The first maxima of the plasma wave is overlaid with a black dashed line. (b) Wigner transform (blue-red) of the laser pulse overlaid with the temporal intensity profile at $z=0.7L_{dp}$ (3.9 mm). The red horizontal line shows the initial laser frequency.