Direct Laser Acceleration of both Positrons and Electrons in a Colliding Laser Configuration*

Recent simulations have demonstrated that two colliding PW-class laser beams of experimentally achievable intensity can facilitate the production of electron-positron pairs via photon-photon collisions [Y. He, et al, Commun Phys 4, 139 (2021)]. In this work, we investigate the direct laser acceleration of these generated positrons and plasma electrons after the laser collision occurs inside a structured target. The target features a cylindrical channel pre-filled with foam, whose mass density can be varied. We found that altering the foam density changes the structure of the azimuthal plasma magnetic field generated by the two lasers. Specifically, the sign of the field can be flipped, enabling the efficient acceleration of both positrons and electrons by the same high-intensity laser pulse, with the magnetic field aiding in energy gain. This regime necessarily requires two laser pulses.