Generation of Ultrabrilliant Positron Beam via Superponderomotive Injection in Laser Wakefield Acceleration
Plasma-based acceleration of positrons attracts extensive interest owing to the ultrahigh accelerating gradient and ultrashort duration, while generating wakefield positron beam by the inherent injection is still a great challenge. Here, we put forward a superponderomotive injection method of positrons in the blowout regime of laser wakefield acceleration. The dephasing-rate integral equation reveals a twofold mechanism: the longitudinal laser field delays phase-locking, guiding positrons into the paraxial focusing region, while the transverse laser Lorentz force suppresses the dephasing rate below unity, trapping them into the laser-modulated wakefield. Particle-in-cell (PIC) simulations demonstrate this via a donut-wake--pair-jet collision, generating low-emittance ($\sim$0.05~mm~mrad) multicycle positron beams. Start-to-end simulations for post-acceleration in the second-stage donut wakefield confirm high-throughput injection-to-acceleration coupling, yielding quasi-monoenergetic beam with six-dimensional brightness $\sim 10^{15}~\rm{A/m^2}/0.1\%$. This plasma-based injection-acceleration scheme opens a novel compact route to ultrabrilliant positron sources for ultrafast material diagnostics, laboratory astrophysics, and next-generation electron--positron colliders.