  Highly polarized, multi-kiloampere-current electron bunches from compact
laser-plasma accelerators are desired for numerous applications. Current
proposals to produce these beams suffer from intrinsic limitations to the
reproducibility, charge, beam shape and final polarization degree. In this
Letter, we propose colliding pulse injection as a technique for the generation
of highly polarized electron bunches from pre-polarized plasma sources. Using
particle-in-cell simulations, we show that colliding pulse injection enables
trapping and precise control over electron spin evolution, resulting in the
generation of high-current (multi-kA) electron bunches with high degrees of
polarization (up to 95 % for > 2 kA). Bayesian optimization is employed to
optimize the multidimensional parameter space associated with colliding pulse
injection to obtain percent-level energy spread, sub-micron normalized
emittance electron bunches with 90 % polarization using 100-TW class laser
systems.
