Backward Raman amplification of broad band pulses
A reduced fluid model of Raman backscattering is proposed that describes backward Raman amplification (BRA) of pulses with duration t0 comparable to or even smaller than the plasma period 2p/op. At such a small t0, a seed pulse can be amplified even if it has the same frequency as the pump (which is technologically advantageous) as opposed to that satisfying the Raman resonance condition. Using our theoretical model, we numerically calculate the BRA efficiency for such pulses as a function of t0 and show that it remains reasonably high up to t02p/op. We also show that using short seed pulses in BRA makes the amplification less sensitive to quasistatic inhomogeneities of the plasma density. Amplification can persist even when the density perturbations are large enough to violate the commonly known condition of resonant amplification.