Generation of relativistic polarized electron beams via collective beam-target interactions

arXiv:2408.05768v1 Announcement Type: new Abstract: Relativistic polarized electron beams can find applications in broad areas of fundamental physics. Here, we propose for the first time that electron spin polarization can be efficiently realized via collective beam-target interactions. When an unpolarized relativistic electron beam is incident on the surface of a solid target at a grazing angle, strong magnetic fields are induced on the target surface due to the formation of a high backflow of target electrons. This results in a violent self-focusing of the beam and a corresponding increase in the beam density via magnetic pinching. The dense pinched beam in turn further enhances the magnetic fields up to the level of a few Giga-Gauss, which is high enough to trigger strong ultrarelativistic electron synchrotron radiation. During the interaction, the electron spin polarization develops along the magnetic field direction, which is achieved via radiative spin flips in the quantum radiation-dominated regime. As a result, the incident electron beam can be efficiently polarized via the spin-dependent radiation reaction, e.g., the average electron polarization of energies below 2 GeV can reach more than 50% for an initial beam of 5 GeV. This offers a robust method for the development of polarized electron sources.