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Abstract

Generation of ultrarelativistic polarized positrons during interaction of an
ultrarelativistic electron beam with a counterpropagating two-color petawatt
laser pulse is investigated theoretically. Our Monte Carlo simulation based on
a semi-classical model, incorporates photon emissions and pair productions,
using spin-resolved quantum probabilities in the local constant field
approximation, and describes the polarization of electrons and positrons for
the pair production and photon emission processes, as well as the classical
spin precession in-between. The main reason of the polarization is shown to be
the spin-asymmetry of the pair production process in strong external fields,
combined with the asymmetry of the two-color laser field. Employing a feasible
scenario, we show that highly polarized positron beams, with a polarization
degree of $\zeta\approx 60\%$, can be produced in a femtosecond time scale,
with a small angular divergence, $\sim 74$ mrad, and high density $\sim
10^{14}$ cm$^{-3}$. The laser-driven positron source, along with laser
wakefield acceleration, may pave the way to small scale facilities for high
energy physics studies.