Direct laser acceleration of Bethe-Heitler positrons in laser-channel interactions

Positron creation and acceleration is one of the major challenges for constructing future lepton colliders. On the one hand, conventional technology can provide a solution, but at a prohibitive cost and scale. On the other hand, alternative, reduced-scale ideas for positron beam generation could bring this dream closer to reality. Here we propose a plasma-based positron acceleration method using a powerful laser propagating through a dense and narrow plasma channel. A large amount of electrons are injected within the channel during laser propagation. This electron loading creates static fields in the plasma, enabling positrons to be guided transversely while they directly gain energy from the laser field itself. Within this context, we present a theoretical model to describe how the laser injects the electrons and estimate the beam-loaded effective electron density. We validate our theoretical predictions through quasi-3D Particle-In-Cell (PIC) simulations and demonstrate the robustness of this guiding and direct laser acceleration process for positrons. Our approach could pave the way for testing this positron acceleration scheme at ELI Beamlines, showcasing an unprecedentedly high average energy gain rate of a few GeV/mm. The fireball jet produced contains GeV-level electrons, positrons, and x-rays and thus brings unique opportunities for applications for laboratory astrophysics such as mimicking the propagation of fireball jets from gamma-ray bursts and seeding pair cascades taking place in pulsars with different ratios of electrons and positrons.