Suppressing parametric instabilities for direct-drive high-energy-density physics and inertial-confinement-fusion plasmas using broadband laser light

Effective suppression of laser-plasma instabilities would allow HEDP experiments at higher ablation pressures and reduce target preheat by hot electrons. None of the solid- state or gas lasers used in contemporary ICF/HEDP experiments, though, has sufficient bandwidth to directly suppress any of the three predominant instabilities that presently impair direct-drive implosions, namely, cross-beam energy transfer, two-plasmon decay and stimulated Raman scattering. In this talk, we present selected numerical simulations of these instabilities and explore the efficacy of large laser bandwidths for their mitigation. We also discuss two experimental approaches currently under development at the U.S. Naval Research Laboratory to increase the bandwidth of present and next-generation ICF/HEDP drivers. The first of these is the argon fluoride (ArF) laser. In addition to possessing a large native bandwidth of approximately 10 THz, the ArF laser also has an ultra-short 193-nm wavelength that helps to suppress deleterious laser-plasma instabilities even further and also improves laser-target coupling. Another approach is based on stimulated, rotational Raman scattering, which might provide a viable means of significantly broadening the bandwidth of green laser light for use in experiments on the National Ignition Facility.