Cross-Beam Energy Transfer Saturation by Ion Trapping-Induced Detuning*

The performance of direct-drive inertial confinement fusion implosions relies critically on the coupling of laser energy to the target plasma. Cross-beam energy transfer (CBET), the resonant exchange of energy between intersecting laser beams mediated by ponderomotively driven ion-acoustic waves (IAW's), inhibits this coupling by scattering light into unwanted directions. The variety of beam intersection angles and varying plasma conditions in an implosion results in IAW's with a range of phase velocities. Here we show that CBET saturates through a resonance detuning that depends on the IAW phase velocity and that arises from trapping-induced modifications to the ion distribution function. For smaller phase velocities, the modifications to the distribution function can rapidly thermalize, leading to a blue shift in the resonant frequency. For larger phase velocities, the modifications can persist, leading to a redshift in the resonant frequency. Ultimately, these results may reveal pathways toward CBET mitigation and inform reduced models for radiation-hydrodynamics codes to improve their predictive capability.