  Relativistic positron sources with high spin polarization have important
applications in nuclear and particle physics and many frontier fields. However,
it is challenging to produce dense polarized positrons. Here we present a
simple and effective method to achieve such a positron source by directly
impinging a relativistic high-density electron beam on the surface of a solid
target. During the interaction, a strong return current of plasma electrons is
induced and subsequently asymmetric quasistatic magnetic fields as high as
megatesla are generated along the target surface. This gives rise to strong
radiative spin flips and multiphoton processes, thus leading to efficient
generation of copious polarized positrons. With three-dimensional
particle-in-cell simulations, we demonstrate the production of a dense
highly-polarized multi-GeV positron beam with an average spin polarization
above 40% and nC-scale charge per shot. This offers a novel route for the
studies of laserless strong-field quantum electrodynamics physics and for the
development of high-energy polarized positron sources.
