Fast electron production in intense laser-plasma interaction of multi-picosecond time scales*

Intense (I$_{\mathrm{laser}}$\textgreater 10$^{\mathrm{18}}$ W/cm$^{\mathrm{2}})$ laser-plasma interaction offers a very efficient source of fast electrons at relativistic energies, which can be used for fast-ignition inertial confinement fusion, ultra-short x-ray sources and heating matter to warm dense states. We report theoretical and particle-in-cell simulation results for characterization of fast electron source produced from intense laser interaction with solid targets at the time scale of multi-picosecond and energy scale of kilojoule. A substantial increase in both fast electron average energy and laser-electron conversion efficiency has been observed when the laser pulse length was extended from 1 to 10 picoseconds. The enhanced electron acceleration is attributed to a significant thermal preplasma expansion on several picosecond time scale that forms a long flat ``shelf'' at near-critical (0.1n$_{\mathrm{c}}$ - n$_{\mathrm{c}})$ density region, and ponderomotive piling-up of electrons that leads to a sharp interface at relativistic critical density $\gamma $n$_{\mathrm{c}}$. Both of these eventually result in large amplitude increase and volume broadening of the electrostatic potential for electron acceleration. \\[4pt] [1] D. R. Welch et al., Phys. Plasmas 13, 063105 (2006).