The recently proposed backward Raman laser amplification scheme
utilizes the stimulated Raman backscattering in plasma of a long pumping
laser pulse to amplify a short, and frequency downshifted seed pulse. The
output intensity for this scheme is limited by the development of forward
Raman scattering (FRS) or modulational instabilities of the highly
amplified seed. Theoretically, focused output intensities as high as
1025 W/cm2, and pulse lengths of less than 100
fs, could be accessible by this technique for 1 μm lasers—an
improvement of 104–105 in focused intensity
over current techniques. Simulations with the particle-in-cell (PIC) code
Zohar are presented, which investigate the effects of FRS and modulational
instabilities, and of Langmuir wave breaking on the output intensity for
Raman amplification. Using the intense seed pulse to photoionize the
plasma simultaneous with its amplification (and hence avoid plasmas-based
instabilities of the pump) is also investigated by PIC simulations. It is
shown that both approaches can access focused intensities in the
1025 W/cm2 range.