Accuracy of the ray-tracing modeling of CBET in ICF hohlraums

Cross Beam Energy Transfer (CBET) is a three-wave coupling involving two laser beams and an acoustic wave. In Inertial Confinement Fusion (ICF) hohlraums, numerous laser beams cross each other, and CBET occurs at each of these crossings. Due to the large size of ICF hohlraums, simulations are performed at the hydrodynamic scale, requiring the implementation of reduced CBET models.

A recent work [1] showed that ray tracing models accurately capture the average transferred power if the beam’s angular spread is considered. Yet, this angular spread, determined by the beam’s f-number, is not always included in CBET inline models. The f-number effect leads to a substantial broadening of the resonance shape for two crossing beams in a homogeneous, weakly Landau-damped plasma. However, in realistic hohlraums, where many beams cross in an inhomogeneous plasma, the impact of the f-number effect is expected to be less significant.

We discuss the effect of the f-number on CBET in ICF hohlraums, with a full-scale model [2] calculating CBET from a hydrodynamic simulation map. We further assess the validity of the strongly damped approximation in weakly Landau-damped plasmas, such as gold.