Parametric pulse amplification by acoustic quasimodes in electron-positron plasma.

In a recent paper, M. R. Edwards, N. J. Fisch, and J. M. Mikhailova [Phys. Rev. Lett. 116, 015004 (2016)PRLTAO0031-900710.1103/PhysRevLett.116.015004] reported that in electron-positron plasma stimulated Brillouin scattering is drastically enhanced, while stimulated Raman scattering is completely absent. However, when theory was compared to particle-in-cell (PIC) simulations, a discrepancy by at least a factor four appeared. Authors correctly argued that the disparity might be due to the fluid approximation of the low-frequency mode. They noted that a more precise analytic description of the acoustic resonance requires a kinetic approach, which was beyond the scope of the mentioned paper. Here we deliver the so-far-missing kinetic calculation. It shows quite good agreement with the PIC simulations presented in the above-mentioned paper by Edwards et al. The principal result of enhancement of Brillouin scattering and absence of Raman scattering remains valid. The Brillouin enhancement factors depend on electron temperature and background particle density. These dependencies as well as the transition to the well-known behavior of electron-ion plasma are discussed. It is also shown that pulse amplification in electron-positron plasma crosses over to the strong-coupling regime when the pump amplitude becomes large. Then, the fluid approximation becomes acceptable again.