Experimental evidence of stimulated Raman re-scattering in laser-plasma interaction

We present the first experimental evidence of stimulated Raman re-scattering of a laser in plasma: The scattered light produced by the Raman instability is intense enough to scatter again through the same instability. Although never observed, re-scattering processes have been studied theoretically and numerically for many years in the context of inertial confinement fusion (ICF), since the plasma waves they generate could bootstrap thermal electrons to high energies [Phys. Rev. Lett. \textbf{110}, 165001 (2013)], preheating the fuel and degrading ignition conditions. Our experimental results are obtained with a spatially smoothed laser beam consisting of many speckles, with an average intensity around $10^{14}$ W/cm$^2$ and close to $10^{15}$ W/cm$^2$ in the speckles, such as those usually used in direct-drive ICF. Kinetic and hydrodynamic simulations show good agreement with the observations.