  Compact laser-driven accelerators are versatile and powerful tools of
unarguable relevance on societal grounds for the diverse purposes of science,
health, security, and technology because they bring enormous practicality to
state-of-the-art achievements of conventional radio-frequency accelerators.
Current benchmarking laser-based technologies rely on a medium to assist the
light-matter interaction, which impose material limitations or strongly
inhomogeneous fields. The advent of few cycle ultra-intense radially polarized
lasers has materialized an extensively studied novel accelerator that adopts
the simplest form of laser acceleration and is unique in requiring no medium to
achieve strong longitudinal energy transfer directly from laser to particle.
Here we present the first observation of direct longitudinal laser acceleration
of non-relativistic electrons that undergo highly-directional multi-GeV/m
accelerating gradients. This demonstration opens a new frontier for direct
laser-driven particle acceleration capable of creating well collimated and
relativistic attosecond electron bunches and x-ray pulses.
