Multi-stage scheme for non-linear Breit-Wheeler pair-production utilising ultra-intense laser-solid interactions

Multi-petawatt (PW) lasers enable intensities exceeding 10^23 Wcm^-2, at which point quantum electrodynamics (QED) processes, such as electron-positron pair-production via the non-linear Breit-Wheeler process, will play a significant role in laser-plasma interactions. Using 2D QED-particle-in-cell simulations, we present a two-stage scheme in which non-linear pair-production is induced via an ultra-intense laser-solid interaction. The first stage is the generation of a gamma-ray beam, through the interaction of an ultra-intense laser pulse with a thick target, whose features are found to be strongly dependent on collective plasma effects. This compact, high energy gamma-ray beam (characterised by a divergence angle of approximately 10 degrees and average photon energy approximately 10 MeV) then interacts with two counter-propagating laser pulses. By varying the laser polarisation and angle of incidence, we show that in the case of two circularly polarised laser pulses propagating at an angle equal to the divergence angle of the gamma-ray beam, the produced positron distribution is highly anisotropic compared to the case of a standard head-on collision.