Jian-Xing Li

High-energy, high-yield structured spin-polarized positron beams have important applications in nuclear physics, high-energy physics, and information storage. However, their generation remains a significant challenge. Here, we put forward a scheme to generate these beams using a dense relativistic electron beam interacting with a gas-filled cone–channel target. The interaction induces composite focusing fields comprising the electric field from a plasma bubble and skin-layer magnetic fields from the electron beam and displacement currents. These fields compress the seed beam to ultrahigh density, thereby inducing extreme fields that enable efficient γ-photon emission and subsequent pair production, leading to a high positron yield. Furthermore, the azimuthal topology of these fields is imprinted onto the generated positrons, creating unique azimuthal polarization. Our simulations demonstrate the generation of an azimuthally polarized positron beam with a charge of 0.63 nC, a polarization approaching 60%, and an energy conversion efficiency exceeding 3%. Our method provides a highly efficient pathway for generating azimuthally polarized positrons, paving the way for their potential applications.

High‐brilliance circularly polarized γ‐photon beams are of great significance for a wide range of applications. However, their generation through nonlinear Compton scattering must require a high‐density longitudinally‐spin‐polarized electron beam and consequently is still a great challenge. Here, a novel method is proposed to generate such γ‐photon beams via the vacuum dichroism (VD)‐assisted vacuum birefringence (VB) effect, only utilizing a well‐established unpolarized electron beam. A linearly polarized laser pulse is splitted into two subpulses with the first one colliding with a dense unpolarized electron beam to generate a linearly polarized γ‐photon beam (via nonlinear Compton scattering), which then further collides with the second subpulse and is transformed into a circularly polarized one via the VB effect. It is found that by manipulating the relative polarization of two subpulses, one can “purify” the polarization of the γ‐photon beam via the VD effect, thereby significantly enhancing the circular polarization of the γ‐photon beam. Due to the VD assistance, the VB effect reaches optimal when the relative polarization is nearly 30°, not the widely used 45° in the common VB detection methods. The numerical results show that one can obtain a circularly polarized γ‐photon beam with average degree of about 30% (43%) for energies above 500 (1000) MeV and brilliance of about 10 ²⁴ (10 ²³ ) photons/(s · mm ² · mrad ² · 0.1%BW) at 500 (1000) MeV by using a currently feasible laser with a peak intensity of about 10 ²² Wcm ⁻ 2. And, it can be further improved to above 60% (75%) by increasing the laser pulse duration. Moreover, our method is shown to be robust with respect to the laser and electron beam parameters, and can also be used to efficiently confirm the well‐known VB effect itself, which has been predicted a very long time ago but has not been directly observed in experiments yet.

Relativistic structured spin-polarized (SSP) particle beams, characterized by polarization structures, are of critical importance in a wide range of applications, such as material properties investigation, imaging, and information storage. However, generation of relativistic SSP beams faces significant challenges. Here, we put forward a novel method for generating relativistic SSP lepton beams via employing a moderate-intensity terahertz (THz) wave. Building upon our foundational work on velocity-matched spin rotation in dielectric-lined waveguides [Phys. Rev. Lett. 134, 075001 (2025)], we present the first demonstration of spin-polarization mode matching - a novel mechanism that establishes a direct relation between waveguide modes and beam polarization states. This breakthrough enables precise spatial control over spin structures at relativistic energies, generating customizable spin-polarization configurations such as spider-like, azimuthal, and helical structures, etc. Such SSP beams have the potential to generate high-energy structured photon beams and open a new avenue for research on relativistic structured particle beams, especially in nuclear physics, high-energy physics, materials science and atomic physics.

Single-electron acceleration in vacuum by exponentially chirped plane-wave and tightly focused Gaussian laser pulses is investigated. Numerical calculations and particle-in-cell simulations, employing the chirped fields, demonstrate that GeV net energy gain, over sub-millimeter excursion distances, is possible from PW-class femtosecond laser pulses.

Compact spin-polarized positron accelerators play a major role in promoting significant positron application research, which typically require high acceleration gradients and polarization degrees, both of which, however, are still greatly challenging. Here, we put forward a spin-polarized positron acceleration method, which employs an ultrarelativistic high-density electron beam passing through any hole of multilayer microhole array films to excite strong electrostatic and transition radiation fields. Positrons in the polarized electron-positron pair plasma, filled in the front of the multilayer films, can be captured, accelerated, and focused by the electrostatic and transition radiation fields, while maintaining high polarization degrees of above 90% and high acceleration gradients of about TeV/m. A multilayer design allows for capturing more positrons and achieving cascade acceleration. Our method offers promising solutions to the issues of accelerator miniaturization, positron injection, and polarization preservation, and also can be used to accelerate other charged particles.