Spin Filter for Polarized Electron Acceleration in Plasma Wakefields

We propose a filter method to generate electron beams of high polarization from bubble and blow-out wakefield accelerators. The mechanism is based on the idea of identifying all electron-beam subsets with low polarization and filtering them out with an X-shaped slit placed immediately behind the plasma accelerator. To find these subsets we investigate the dependence between the initial azimuthal angle and the spin of single electrons during the trapping process. This dependence shows that transverse electron spins preserve their orientation during injection if they are initially aligned parallel or antiparallel to the local magnetic field. We derive a precise correlation of the local beam polarization as a function of the coordinate and the electron phase angle. Three-dimensional particle-in-cell simulations, incorporating classical spin dynamics, show that the beam polarization can be increased from 35% to about 80% after spin filtering. The injected flux is strongly restricted to preserve the beam polarization; for example, less than 1 kA in Wen et al. [Phys. Rev. Lett. 122, 214801 (2019)]. This limitation is removed by use of the proposed filter mechanism. The robustness of the method is discussed in terms of drive-beam fluctuations, jitters, the thickness of the filter, and the initial temperature. This idea marks an efficient and simple strategy to generate energetic polarized electron beams on the basis of wakefield acceleration.