Stimulated Raman backscattering in plasma has been suggested as a way to amplify short laser pulses to intensities not limited by damage thresholds as in chirped pulse amplification using conventional media. Energy is transferred between two transverse electromagnetic waves, pump and probe, through the parametric interaction with a longitudinal Langmuir wave that is ponderomotively excited by their beat wave. The increase of the plasma temperature due to collisional absorption of the pump wave modifies the dispersion of the Langmuir wave: firstly, its resonance frequency rises (Bohm-Gross shift), and secondly, Landau damping sets in. The frequency shift acts in a similar way to a chirp of the pump frequency, or a density ramp: different spectral components of the probe satisfy the resonance condition at different times. This limits their growth, while increasing the bandwidth of the amplifier, thus leading to superradiant amplification. Landau damping may shorten the probe pulse, but reduces the amplification efficiency. We investigate these effects analytically and using numerical simulations in order to assess their importance in experimental demonstrations, and the possibility of applications.
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