  The production of polarized proton beams with multi-GeV energies in
ultra-intense laser interaction with targets is studied with three-dimensional
Particle-In-Cell simulations. A near-critical density plasma target with
pre-polarized proton and tritium ions is considered for the proton
acceleration. The pre-polarized protons are initially accelerated by laser
radiation pressure before injection and further acceleration in a bubble-like
wakefield. The temporal dynamics of proton polarization is tracked via the
T-BMT equation, and it is found that the proton polarization state can be
altered both by the laser field and the magnetic component of the wakefield.
The dependence of the proton acceleration and polarization on the ratio of the
ion species is determined, and it is found that the protons can be efficiently
accelerated as long as their relative fraction is less than 20%, in which case
the bubble size is large enough for the protons to obtain sufficient energy to
overcome the bubble injection threshold.
