Femtosecond 240-keV electron pulses from direct laser acceleration in a low-density gas.

We propose a simple laser-driven electron acceleration scheme based on tightly focused radially polarized laser pulses for the production of femtosecond electron bunches with energies in the few-hundreds-of-keV range. In this method, the electrons are accelerated forward in the focal volume by the longitudinal electric field component of the laser pulse. Three-dimensional test-particle and particle-in-cell simulations reveal the feasibility of generating well-collimated electron bunches with an energy spread of 5% and a temporal duration of the order of 1 fs. These results offer a route towards unprecedented time resolution in ultrafast electron diffraction experiments.