Impact of target density on direct laser acceleration in hollow core tubes.

Conventional direct laser acceleration in plasma utilizes transverse laser electric fields and transverse electron oscillations to produce a beam of ultra-relativistic electrons. However, the beam can have significant divergence as a consequence of the electron oscillations. Structured targets can drastically reduce this divergence by enabling a different regime of direct laser acceleration. In this work, we consider an initially empty tube with a diameter comparable to the laser beam's focal spot. The transverse laser electric field extracts electrons from the tube, but it is the longitudinal electric field that accelerates the electrons as they move forward, essentially without oscillations [1]. The phase velocity of the laser determines how long the electrons can remain in the accelerating phase and thus gain energy. This velocity is necessarily greater than the speed of light, i.e., it is superluminal. We show that by changing the tube’s electron density, one can adjust the relative degree of superluminosity and potentially increase electron energy gain without any adjustments to the laser beam.

[1] Z Gong et al 2019 Plasma Phys. Control. Fusion 61 035012