Multibeam Raman amplification of a finite-duration seed in a short distance

A new scheme of multibeam Raman amplification (MRA) is proposed in virtue of the collective mode by sharing a common scattered light. Multiple laser beams can provide a higher growth rate, but the overlapping region limits the amplification length. We suggest to use a finite-duration seed to facilitate MRA in a short distance. Through two-dimensional particle-in-cell simulations, we find that two-beam Raman amplification has a much higher growth rate than that of single-beam one. This growth rate depends on the initial seed amplitude, electron temperature, and seed duration. An empirical criterion, γ ₀ τ _c = 1, where γ ₀ is the theoretical growth rate of MRA, is used to choose a proper duration for a higher growth rate. After a total amplification length of 320 μm, the two-beam Raman amplification shows nonlinear features of pulse compression and a bow-shape wave front, indicating that the amplification has finally entered the self-similar regime. (paper)