  In situ generation of a high-energy, high-current, spin-polarized electron
beam is an outstanding scientific challenge to the development of plasma-based
accelerators for high-energy colliders. In this Letter we show how such a
spin-polarized relativistic beam can be produced by ionization injection of
electrons of certain atoms with a circularly polarized laser field into a
beam-driven plasma wakefield accelerator, providing a much desired one-step
solution to this challenge. Using time-dependent Schr\"odinger equation (TDSE)
simulations, we show the propensity rule of spin-dependent ionization of xenon
atoms can be reversed in the strong-field multi-photon regime compared with the
non-adiabatic tunneling regime, leading to high total spin-polarization.
Furthermore, three-dimensional particle-in-cell (PIC) simulations are
incorporated with TDSE simulations, providing start-to-end simulations of
spin-dependent strong-field ionization of xenon atoms and subsequent trapping,
acceleration, and preservation of electron spin-polarization in lithium plasma.
We show the generation of a high-current (0.8 kA),
ultra-low-normalized-emittance (~37 nm), and high-energy (2.7 GeV) electron
beam within just 11 cm distance, with up to ~31% net spin polarization. Higher
current, energy, and net spin-polarization beams are possible by optimizing
this concept, thus solving a long-standing problem facing the development of
plasma accelerators.
