  A novel scheme is proposed for generating a polarized positron beam via
multiphoton Breit-Wheeler process during the collision of a 10 GeV, pC seeding
electron beam with the other 1 GeV, nC driving electron beam. The driving beam
provides the strong self-generated field, and a suitable transverse deviation
distance between two beams enables the field experienced by the seeding beam to
be unipolar, which is crucial for realizing the positron polarization. We
employ the particle simulation with a Monte-Carlo method to calculate the spin-
and polarization-resolved photon emission and electron-positron pair production
in the local constant field approximation. Our simulation results show that a
highly polarized positron beam with polarization above $40\%$ can be generated
in several femtoseconds, which is robust with respect to parameters of two
electron beams. Based on an analysis of the influence of $\gamma$-photon
polarization on the polarized pair production, we find that a polarized seeding
beam of the proper initial polarization can further improve the positron
polarization to $60\%$.
