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The substantial angular divergence of electron beams produced by direct laseracceleration is often considered as an inherent negative feature of themechanism. The divergence however arises primarily because the standardapproach relies on transverse electron oscillations and their interplay withthe transverse electric fields of the laser pulse. We propose a conceptuallydifferent approach to direct laser acceleration that leverages longitudinallaser electric fields that are present in a tightly focused laser beam. Astructured hollow-core target is used to enhance the longitudinal fields andmaintain them over a distance much longer than the Rayleigh length by guidingthe laser pulse. Electrons are injected by the transverse laser electric fieldinto the channel and then they are accelerated forward by the pulse, creatingan electron current. The forces from electric and magnetic fields of thiselectron population compensate each other, creating a favorable configurationwithout a strong restoring force. We use two-dimensional particle-in-cellsimulations to demonstrate that a low divergence energetic electron beam withan 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 laserelectric field and, given a sufficient acceleration distance,super-ponderomotive energies can be realized without sacrificing thecollimation.

Highly collimated electron acceleration by longitudinal laser fields in a hollow-core target