  Plasma current filamentation of an ultrarelativistic electron beam impinging
on an overdense plasma is investigated, with emphasis on radiation-induced
electron polarization. Particle-in-cell simulations provide the classification
and in-depth analysis of three different regimes of the current filaments,
namely, the normal filament, abnormal filament, and quenching regimes. We show
that electron radiative polarization emerges during the instability along the
azimuthal direction in the momentum space, which significantly varies across
the regimes. We put forward an intuitive Hamiltonian model to trace the origin
of the electron polarization dynamics. In particular, we discern the role of
nonlinear transverse motion of plasma filaments, which induces asymmetry in
radiative spin flips, yielding an accumulation of electron polarization. Our
results break the conventional perception that quasi-symmetric fields are
inefficient for generating radiative spin-polarized beams, suggesting the
potential of electron polarization as a source of new information on laboratory
and astrophysical plasma instabilities.
