Absorption of Laser Light by Coupling to Incoherent Plasma Waves at Quarter-Critical Density

Inertial confinement fusion (ICF) relies critically on the absorption of laser light in high-density plasma. Laser–plasma instabilities, such as two-plasmon decay (TPD), can interfere with this absorption by scattering or prematurely depleting the laser light at lower plasma densities. Here we identify a novel mechanism for laser absorption that occurs as laser light passes through quarter-critical density and interacts with a pre-existing, incoherent ensemble of TPD-driven plasma waves. The interaction, mediated by a newly scattered, but stable, plasma wave, produces an imaginary refractive index proportional to the energy density of the incoherent plasma waves. This additional absorption occurs independent of the intensity and can modify the interplay of TPD, cross-beam energy transfer, and irradiation uniformity in direct-drive ICF implosions. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.