  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.
