  We generate a tabletop pulsed relativistic electron beam at 100 Hz repetition
rate from vacuum laser acceleration (VLA) by tightly focusing a radially
polarized beam into a low-density gas. We demonstrate that strong longitudinal
electric fields at the focus can accelerate electrons up to 1.43 MeV by using
only 98 GW of peak laser power. The electron energy is measured as a function
of laser intensity and gas species, revealing a strong dependence on the atomic
ionization dynamics. These experimental results are supported by numerical
simulations of particle dynamics in a tightly focused configuration that take
ionization into consideration. For the range of intensities considered, it is
demonstrated that atoms with higher atomic numbers like krypton can optimally
inject electrons at the peak of the laser field, resulting in higher energies
and an efficient acceleration mechanism that reaches a significant fraction of
the theoretical energy gain limit.
