Transient plasma evolution with spin polarization dynamics in radiation reaction dominated magnetic reconnection is investigated using particle-in-cell simulations. We identify a condensation of plasmoids accumulated into multiple tiny islands within the reconnection layer, where electrons are strongly polarized while emitting energetic $\gamma$-ray photons to undergo radiative spin flips. Nonlinear analyses elucidate that the condensation is caused by a spiral attractor appearing in the electron’s phase space due to radiation reaction. The spiral rotation and contraction of the attractor leads to the electrons’ polarization being almost instantaneously parallel with respect to the magnetic field, which results in a $\gamma$-ray emission with an anomalous linear polarization perpendicular to the electron’s moving plane. Our studies with around ${10}^{10}\mathrm{G}$ magnetic fields demonstrate that spin-polarized condensed plasmoids may be realized in extreme power laser facilities and intrinsically exist in extreme astrophysical reconnection scenarios, potentially explaining atypical polarization features in observed high-energy cosmic radiation.

• Beam polarization
• Magnetic reconnection
• Plasma instabilities
• Laboratory plasma
• Relativistic plasmas
• Space & astrophysical plasma
• Particle-in-cell methods