Generation of polarized positron beams via collisions of ultrarelativistic electron beams

A scheme is proposed for generating a polarized positron beam via multiphoton Breit-Wheeler process during the collision of a 10-GeV-level, 20-pC seeding electron beam with a second 10-GeV, multi-nC driving electron beam. The high-density driving beam with the length and transverse size less than 1 mu m provides a strong self-generated field. The field experienced by the seeding beam can be unipolar by incorporating a proper impact parameter, which is crucial to realizing the positron polarization. We employ semiclassical Monte Carlo simulations to calculate the spin- and polarization-resolved photon emission and electron-positron pair production in the locally constant-field approximation. The simulation results show that a highly polarized positron beam with a polarization degree above 40% can be generated in several femtoseconds, which is robust against the beam parameters. The positron polarization degree can be further increased to 60% provided the seeding beam is initially polarized.