Spin-polarized electron beams of relativistic energies find important applications in nuclear physics, high energy physics and in the search for new physics beyond the standard model, e.g., probing nuclear spin structure, producing polarized gamma rays and positrons, and studying symmetry violation. However, current methods for generating such beams require conventional accelerators or ultrastrong laser facilities, which face critical limitations: accelerator-based approaches require large-scale infrastructure, while laser-driven methods suffer from stringent demands for ultrastrong laser pulses or prepolarized plasmas.

Here, we put forward a novel method for constructing a compact efficient ''polarizer'' that achieves direct ultrafast conversion of relativistic dense electron beams into polarized ones, based on a beam ''self-polarization'' mechanism via beam-target interactions. In this scheme, as the electron beam grazes through the polarizer (a double-layer solid target), it ionizes the target and excites an asymmetric plasma field due to the plasma backflows. This field then reacts on the beam itself, triggering spontaneous radiative polarization and reflecting approximately 10% of the beam, thereby yielding a dense polarized electron beam. Moreover, the double-layer target setup induces a plasma bubble that focuses the polarized beam and reshapes its polarization distribution. Our method is robust with respect to the beam and target parameters, and opens a new avenue for relativistic beam polarization with compact accessible devices, which would facilitate their broad applications and the development of related polarized beam experiments.

Beam polarization,Beam-plasma interactions,polarized beam generation