We investigate the production of electron beams from the interaction of relativistically-intense laser pulses with a solid-density ${\mathrm{SiO}}_2$ target in a regime where the laser pulse energy is $\sim \mathrm{mJ}$ and the repetition rate $\sim \mathrm{kHz}$. The electron beam spatial distribution and spectrum were investigated as a function of the plasma scale length, which was varied by deliberately introducing a moderate-intensity prepulse. At the optimum scale length of $\lambda /2$, the electrons are emitted in a collimated beam having a quasimonoenergetic distribution that peaked at $\sim 0.8\mathrm{MeV}$. A highly reproducible structure in the spatial distribution exhibits an evacuation of electrons along the laser specular direction and suggests that the electron beam duration is comparable to that of the laser pulse. Particle-in-cell simulations which are in good agreement with the experimental results offer insights on the acceleration mechanism by the laser field.

• Received 24 April 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.235001