  High-gradient and high-efficiency acceleration in plasma-based accelerators
has been demonstrated, showing its potential as the building block for a future
collider operating at the energy frontier of particle physics. However,
generating and accelerating the required spin-polarized beams in such a
collider using plasma-based accelerators has been a long-standing challenge.
Here we show that the passage of a highly relativistic, high-current electron
beam through a single-species (ytterbium) vapor excites a nonlinear plasma wake
by primarily ionizing the two outer 6s electrons. Further photoionization of
the resultant Yb2+ ions by a circularly polarized laser injects the 4f14
electrons into this wake generating a highly spin-polarized beam. Combining
time-dependent Schrodinger equation simulations with particle-in-cell
simulations, we show that a sub-femtosecond, high-current (4 kA) electron beam
with up to 56% net spin polarization can be generated and accelerated to 15 GeV
in just 41 cm. This relatively simple scheme solves the perplexing problem of
producing spin-polarized relativistic electrons in plasma-based accelerators.
