Investigation of high-intensity laser-plasma interaction in multi-picosecond laser pulse length regime

Intense (I
_laser
>1018 W/cm
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
_c
~n
_c
) density region, and ponderomotive piling-up of electrons that leads to a sharp interface at relativistic critical density γn
_c
. Both of these eventually result in large amplitude increase and volume broadening of the electrostatic potential for electron acceleration.