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.