The interaction of ultra-intense lasers with high-Z targets can generate quasi-monoenergetic positron beams, which are of great significance for laboratory astrophysics and advanced particle-source research. To address the unclear mechanism of positron transport and acceleration at the target rear, as well as the lack of a concise theoretical model, this work systematically investigates the sheath-field acceleration of laser-driven positron beams. Starting from the laser parameters and combined with the Pukhov scaling, the conversion from laser conditions to the temperature and number of front-side hot electrons is established. Coupled with Geant4 simulations, the relationship between front-side electrons and the key electron and positron parameters at the target rear is then established. On this basis, a one-dimensional transport model at the target rear is developed, incorporating field propagation and reflux-electron shielding effects, to describe the emission time, energy distribution, and space-charge-field evolution of electrons and positrons. The model reveals the physical mechanism by which positrons undergo spectral modulation and form a quasi-monoenergetic peak under the action of the electron sheath field. Furthermore, two-dimensional PIC simulations are employed to determine the effective acceleration time of positrons under different laser pulse durations. Finally, the model is applied to several representative experiments. The results show good agreement with experimental observations and can reasonably predict the positron spectral structure and the energy of the monoenergetic peak. This study establishes a theoretical framework for the sheath-field acceleration of laser-driven positron beams at the target rear, and provides a useful method for experimental analysis and parameter estimation.
 

Laser plasma interaction,Positron,Ultra-short ultra-intense laser pulse,Beam properties