No abstract

Backward Raman compression in plasma enables pulse compression to intensities not available using material gratings. Mediating the compression with higher density plasma generally produces shorter and therefore more intense output pulses. However, very high density plasma, even if sufficiently tenuous to be transparent to the laser, also produces group velocity dispersion of the amplified pulse, deleteriously affecting the interaction. What is shown here is that, by chirping the seed pulse, the group velocity dispersion may in fact be used advantageously, achieving the maximum intensities over the shortest distances while minimizing unwanted effects.

In plasma-based backward Raman amplifiers, the output pulse intensity increases with the input pump pulse intensity, as long as the Langmuir wave mediating energy transfer from the pump to the seed pulse remains intact. However, at high pump intensity, the Langmuir wave breaks, at which point the amplification efficiency may no longer increase with the pump intensity. Numerical simulations presented here, employing a 1D Vlasov-Maxwell code, show that, although the amplification efficiency remains high when the pump only mildly exceeds the wavebreaking threshold, the efficiency drops precipitously at larger pump intensities

We compare previous analytic predictions, Vlasov-Maxwell simulations, and particle-in-cell results with a new set of comprehensive one and two dimensional particle-in-cell simulations in an effort to clarify apparent discrepancies between the predictions of different models for the efficiency of Raman amplification in the wavebreaking regime. We find reasonable agreement between our particle-in-cell simulations and previous results from Vlasov-Maxwell simulations and analytic work, suggesting a monotonic decrease in conversion efficiency for increased pump intensities past the wavebreaking threshold.