Positron Generation and Acceleration in a Self-Organized Photon Collider Enabled by an Ultraintense Laser Pulse

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.

From linear to nonlinear Breit-Wheeler pair production in laser-solid interactions

During the ultraintense laser interaction with solids (overdense plasmas), the competition between two possible quantum electrodynamics (QED) mechanisms responsible for e^{±} pair production, i.e., linear and nonlinear Breit-Wheeler (BW) processes, remains to be studied. Here, we have implemented the linear BW process via a Monte Carlo algorithm into the QED particle-in-cell (PIC) code yunic, enabling us to self-consistently investigate both pair production mechanisms in the plasma environment. By a series of two-dimensional QED-PIC simulations, the transition from the linear to the nonlinear BW process is observed with the increase of laser intensities in the typical configuration of a linearly polarized laser interaction with solid targets. A critical normalized laser amplitude about a_{0}∼400-500 is found under a large range of preplasma scale lengths, below which the linear BW process dominates over the nonlinear BW process. This work provides a practicable technique to model linear QED processes via integrated QED-PIC simulations. Moreover, it calls for more attention to be paid to linear BW pair production in near future 10-PW-class laser-solid interactions.