Investigations of Raman laser amplification in preformed and ionizing plasmas

The recently proposed backward Raman laser amplification scheme [V.M.Malkin, et al., Phys. Rev. Lett., 82:4448 (1999)] utilizes the stimulated Raman backscattering in plasma of a long pumping laser pulse to amplify a short, frequency down-shifted seed pulse. Theoretically, focused output intensities of ∼1025 W/cm2 and pulse lengths of ∼10 fs are accessible by this technique for 1μm lasers—an improvement of 104–5 over current techniques. However, premature Raman scattering of the pump from thermal fluctuations in the plasma prior to its intended interaction with the seed pulse could degrade or entirely disrupt this amplification process. Two techniques for assuring the stability of the pumping pulse and general robustness of the amplification scheme are investigated, namely, introducing a density gradient to detune the instability of the pump without disrupting the amplification and using the intense seed pulse to photoionize the plasma from a precursor gas simultaneous with its amplification so that the pump cannot be subject to plasma-based instabilities.; For amplification in preformed plasmas, a region in the plasma density and temperature plane is identified where premature scattering is tolerable and amplification of the seed is therefore possible. Modifications due to the effects of weak density gradients are also determined. Using particle-in-cell (PIC) simulations, the optimal density for amplification in the presence of a finite plasma temperature is studied and found to be intermediate between the wave-breaking limit for a cold plasma and the limit from warm plasma theory. PIC simulations are also used to verify the saturation of amplification by forward Raman scattering and modulational instabilities at unfocused intensities of ∼1017 W/cm2.; For the ionizing Raman amplification scheme, the pump and seed pulse intensities and gas densities suitable for amplification are determined, and both hydrogen and helium gases are shown to be usable. The amplification effect is verified by 3-wave and PIC simulations, and output intensities comparable to Raman amplification in preformed plasmas are found to be feasible. The integrity of the ionization front in the face of a possible transverse modulational instability is also confirmed.