Polarized electron-positron pair plasma generation in QED cascades

One of the most remarkable phenomena predicted in the QED plasma regime is the QED cascade, in which energy from intense electromagnetic fields is efficiently converted into hot, dense electron-positron pairs and gamma-ray photons. Two primary mechanisms can trigger QED cascades. The first, known as the "shower-type" cascade, occurs when a high-energy electron beam collides with an intense laser pulse; here, pair production is limited by the energy of the electron beam. The second mechanism, termed the "avalanche-type" cascade, is initiated by the collision of two intense laser beams on a seeding target, forming a standing wave structure with electromagnetic fields strong enough to accelerate particles and drive further pair production until the field is depleted. While QED cascades have been intensively studied within the unpolarized strong-field QED (SFQED) framework, the spin-resolved dynamics of such cascades have received far less attention. In this work, we present new findings on the generation of longitudinally polarized electron-positron plasma in both shower-type and avalanche-type cascades, utilizing our spin- and polarization-resolved QED module implemented in the PIC code OSIRIS. Our results show that avalanche-type cascades, produced by two counter-propagating, high-intensity circularly polarized (CP) lasers, can generate electron-positron pair plasmas with an average longitudinal polarization exceeding 20%, with peak values over 30%. In the shower-type configuration—where a CP laser interacts with an initially unpolarized electron beam—the resulting electron-positron beam attains an average longitudinal polarization greater than 40%. Remarkably, the highest-energy fraction of the positron beam, accounting for more than 8% of the initial electron beam population, achieves over 70% longitudinal polarization. To elucidate the underlying polarization mechanism, we also develop an intuitive physical model that complements our simulation results. Our work introduces a novel method for generating longitudinally polarized electron-positron plasmas via the QED cascade and provides an efficient, all-optical strategy for producing highly polarized longitudinal positron beams.