  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),
inhibits this coupling by scattering light into unwanted directions. The
variety of beam intersection angles and varying plasma conditions in an
implosion results in IAWs 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 results from trapping-induced modifications to the ion
distribution functions. For smaller phase velocities, the modifications to the
distribution functions can rapidly thermalize in the presence of mid-Z ions,
leading to a blueshift 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 towards CBET mitigation and
inform reduced models for radiation hydrodynamics codes to improve their
predictive capability.
