A practical approach to the LTE to non-LTE transition in radiation-hydrodynamic simulations

In radiation-hydrodynamic simulations of laser-driven inertial confinement fusion hohlraums, models to predict the absorption and emission opacities of the material are employed. Local thermodynamic equilibrium (LTE) models are tabulated to return these opacities as functions of density and temperature. However, the LTE assumption is often violated especially in the underdense corona where the laser intensity drives the electrons out of equilibrium with the radiation field. In those regions, simulations use in-line non-LTE atomic models, which are more computationally expensive and have less atomic detail. For this reason, a choice must be made about which model to apply to a given zone, and this is traditionally done by selecting an electron temperature at which to transition. Here, we propose a formula based on a two-level atom description that can be used to determine which model to use. We further apply this model to a hohlraum simulation and demonstrate that it can indeed predict when to transition between models. Finally, we apply the insights observed to hohlraum simulations, and we find that a better LTE to non-LTE transition combined with previously reported insights can largely resolve the M-band discrepancy between simulations and measurement on a few representative ICF experiments.