Tunable high-repetition-rate femtosecond few-hundred keV electron source

Using three-dimensional particle-in-cell simulations, we demonstrate that femtosecond few-hundred keV electron pulses can be produced at a high repetition rate by tightly focusing few mJ few-cycle radially polarized laser pulses in a low density gas. In particular, we show that the laser pulse parameters and gas density can be optimized to cover the full 100-300 keV energy window that characterizes ultrafast electron diffraction imaging experiments. The active development of high-power laser sources promises routine operation at 1 kHz and above, allowing time-resolved electron diffraction on the femtosecond time scale.