  Acceleration of electrons in vacuum directly by intense laser fields, often
termed vacuum laser acceleration (VLA), holds great promise for the creation of
compact sources of high-charge, ultrashort, relativistic electron bunches.
However, while the energy gain is expected to be higher with tighter focusing
(i.e. stronger electric field), this does not account for the reduced
acceleration range, which is limited by diffraction. Here, we present the
results of an experimental investigation of VLA, using tungsten nanotips driven
by relativistic-intensity few-cycle laser pulses. We demonstrate the
acceleration of relativistic electron beams with typical charge of 100s pC to
15 MeV energies. Two different focusing geometries (tight and loose, with
f-numbers one and three respectively) produced comparable results, despite a
factor of ten difference in the peak intensities, which is evidence for the
importance of post-injection acceleration mechanisms around the focus. Our
results are in good agreement with the results of full-scale, three-dimensional
particle-in-cell simulations.
