We present the first experimental validation at ignition scale of a linear kinetic ion-acoustic wave model for cross-beam energy transfer (CBET), using data from recent polar direct-drive experiments at the National Ignition Facility. In this configuration, CBET plays a critical role in both laser–target coupling efficiency and irradiation symmetry. While advanced CBET models have been successfully applied at OMEGA scale, their validation under ignition-relevant conditions—where CBET strongly impacts both energy deposition and shock geometry—has remained limited. Here, we use the fully coupled 3D radiation-hydrodynamics platform ASTER/IFRIIT to assess several CBET modeling approaches, including a linear kinetic ion-acoustic wave model, polarization effects, and inverse ray-tracing, without introducing tunable parameters. Through detailed sensitivity analyses, we quantify the impact of CBET, Langdon effects, thermal transport, and equation-of-state variations on laser energy deposition and shock dynamics. Good agreement is obtained between simulations and experimental measurements, both in terms of shock trajectories and angular distribution of scattered light. We show that CBET preferentially transfers energy away from the equatorial region—where beam overlap and gain are largest—leading to oblate shock shapes and spatially non-uniform absorption, with higher deposition at the poles and reduced absorption at the equator.
Viala et al. (Mon,) studied this question.