All-optical generation of dense multi-GeV positron beams with longitudinal polarization

The production of high-yield, high-energy, longitudinally polarized positron beams remains a pivotal challenge in advanced accelerator science. Laser-driven schemes offer an integrated pathway but typically yield only transverse polarization, or require pre-polarized electron beams, and struggle to efficiently accelerate positrons to high energies. Here we show an all-optical scheme that overcomes these limitations by integrating positron generation, acceleration, and spin manipulation into a single, coherent framework. In our design, an ultraintense circularly polarized laser pulse collides head–on with a counterpropagating unpolarized electron beam, initiating a robust Quantum Electrodynamics(QED) cascade. Within this cascade, positrons are generated via the nonlinear Breit–Wheeler(NBW) process directly inside the strong laser field. Although the positrons are born with transverse polarization, their spin orientations are random, resulting in zero net polarization. The same driving laser field accelerates the positrons to multi–GeV energies via direct laser acceleration and, through in-situ spin precession, rotating their initially random transverse polarizations into a common longitudinal direction simultaneously. Our Monte–Carlo simulations confirm that at normalized laser amplitude a 0 = 1000 the scheme simultaneously achieves a high positron yield of 25.8, positrons/electrons ( e + / e − ), a high average longitudinal polarization (47.0%), and multi–GeV energies (up to ~ 9 GeV), for positrons collected within 30 ° .