  The substantial angular divergence of electron beams produced by direct laser
acceleration is often considered as an inherent negative feature of the
mechanism. The divergence however arises primarily because the standard
approach relies on transverse electron oscillations and their interplay with
the transverse electric fields of the laser pulse. We propose a conceptually
different approach to direct laser acceleration that leverages longitudinal
laser electric fields that are present in a tightly focused laser beam. A
structured hollow-core target is used to enhance the longitudinal fields and
maintain them over a distance much longer than the Rayleigh length by guiding
the laser pulse. Electrons are injected by the transverse laser electric field
into the channel and then they are accelerated forward by the pulse, creating
an electron current. The forces from electric and magnetic fields of this
electron population compensate each other, creating a favorable configuration
without a strong restoring force. We use two-dimensional particle-in-cell
simulations to demonstrate that a low divergence energetic electron beam with
an opening angle of less than 5$^\circ$ can be generated in this configuration.
Most of the energy is transferred to the electrons by the longitudinal laser
electric field and, given a sufficient acceleration distance,
super-ponderomotive energies can be realized without sacrificing the
collimation.

Plasma Physics;Accelerator Physics;Applied Physics