Resolving Discrepancies in Bang-Time Predictions for Indirect-Drive ICF Experiments on the NIF: Insights from the Build-A-Hohlraum Campaign

Accurate modeling of x-ray drive in indirect-drive inertial confinement fusion (ID-ICF) hohlraums is critical for predicting capsule implosion dynamics and achieving reliable ignition at the National Ignition Facility (NIF). However, a persistent "drive deficit"—where experimentally measured capsule bang-times are systematically 400–700 ps later than predicted by state-of-the-art radiation-hydrodynamic simulations—limits the predictive power and efficiency of experimental design. The Build-A-Hohlraum (BAH) campaign was developed to systematically diagnose the origins of this discrepancy by incrementally varying hohlraum complexity, including laser entrance hole (LEH) hardware, capsules, and gas fills.

Our results show that neither the addition of LEH hardware, gas fill, nor capsule introduces significant new discrepancies between measured and simulated x-ray drive. This allows us to exclude several previously proposed explanations, such as errors in local thermodynamic equilibrium (LTE) wall modeling, LEH closure, and unmodeled plasma species mix. Instead, our measurements reveal that simulations consistently overpredict x-ray emission in the 2–4 keV range by approximately 30%, which is directly linked to higher-than-predicted electron temperatures in the gold bubble region. These findings indicate that inaccuracies in non-LTE (NLTE) emission modeling are the dominant source of the drive deficit.

By introducing an empirical opacity multiplier (0.87) for photon energies above 1.8 keV in simulations, we achieve improved agreement with experimental data and reduce the bang-time discrepancy to within measurement uncertainty. This work underscores the urgent need for refined NLTE opacity models to enhance the predictive capability of hohlraum simulations, streamline experimental design, and accelerate progress toward robust ignition.