Electron-Yield Enhancement in a Laser-Wakefield Accelerator Driven by Asymmetric Laser Pulses

The effect of asymmetric laser pulses on electron yield from a laser wakefield accelerator has been experimentally studied using $>{10}^{19}{\mathrm{c}\mathrm{m}}^{-3}$ plasmas and a 10 TW, $>45\mathrm{f}\mathrm{s}$, $\mathrm{T}\mathrm{i}:{\mathrm{A}\mathrm{l}}_2{\mathrm{O}}_{\mathrm{3}}$ laser. The laser pulse shape was controlled through nonlinear chirp with a grating pair compressor. Pulses (76 fs FWHM) with a steep rise and positive chirp were found to significantly enhance the electron yield compared to pulses with a gentle rise and negative chirp. Theory and simulation show that fast rising pulses can generate larger amplitude wakes that seed the growth of the self-modulation instability, and that frequency chirp is of minimal importance for the experimental parameters.

Figure 1

Pulse duration from SSA (dashed line) vs compressor setting. The insets show the laser pulse envelope ($I(t)/I_0$) at two relative compressor separations corresponding to positively and negatively skewed pulses ($b=\pm 0.6$) of duration (${\tau }_{\mathrm{F}\mathrm{W}\mathrm{H}\mathrm{M}}=76\mathrm{f}\mathrm{s}$).

Figure 2

Forward laser spectra vs compressor setting. The energy was $\sim 460\mathrm{m}\mathrm{J}/\mathrm{\text{pulse}}$ and the duration $\ge 50\mathrm{f}\mathrm{s}$. The laser was focused on the gas jet upstream edge.

Figure 3

Electron yield for cases 1 and 2 and pulse duration (dashed line) vs compressor setting. Clear asymmetry in electron yield is observed vs compressor setting for case 1. The energy was $\sim 460\mathrm{m}\mathrm{J}/\mathrm{\text{pulse}}$ and the duration $\ge 50\mathrm{f}\mathrm{s}$.

Figure 4

(a) Plasma wave potential $\delta {\varphi }_{\pm }$ (solid curves) excited by pulse envelope $a_{\pm }(\zeta )$ (dashed curves) for positive (red curves) and negative (blue curves) skewed pulses with ${\omega }_{p0}{\tau }_{\mathrm{F}\mathrm{W}\mathrm{H}\mathrm{M}}=6$ for $2\times {10}^{18}{\mathrm{c}\mathrm{m}}^{-3}$ density (gas jet ramp). (b) The solid curve is the ratio of the Fourier components (evaluated for ${\omega }_p$ at peak intensity) of the RFS seeds ($a_{\pm }\delta {\varphi }_{\pm }$) with pulse shapes of (a), as a function of density. Dashed curve is the maximum of $[a_{+}\delta {\varphi }_{+}(\zeta )]$.