Laser Propagation Experiments in a Raman-Amplifier–Relevant Plasma*

Plasma-based Raman amplification could be the enabling technology for a new generation of ultrahigh-power laser systems. However, experimentally reported gains continue to be significantly lower than those theoretically predicted. We investigated propagation of an unseeded pump pulse through a gas jet at Raman-amplifier–relevant conditions on the Multi-Terawatt Laser at the Laboratory for Laser Energetics. Backscattered energies, backscattered spectra, transmitted pump energies, and transmitted beam profiles were measured over a range of plasma densities and pump-pulse parameters. Filamentation, ionization-induced refraction, and unseeded stimulated Raman scattering appear to scatter a significant amount of the pump energy and inhibit the formation of a uniform plasma channel, which may result in lower gain. Preliminary results will be presented.