Cross-beam energy transfer saturation by ion trapping-induced detuning

TL;DR

This paper investigates how ion trapping modifies ion-acoustic wave phase velocities, causing saturation of cross-beam energy transfer in inertial confinement fusion, and suggests potential mitigation strategies.

Contribution

It demonstrates that ion trapping-induced detuning causes CBET saturation, providing new insights into energy transfer dynamics in fusion plasmas.

Findings

Ion trapping modifies ion distribution functions.

Detuning depends on IAW phase velocity and ion species.

Results suggest pathways for CBET mitigation.

Abstract

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.