ROCSTAR: an LTE Opacity Code for High-Energy-Density Plasmas*

We introduce ROCSTAR, a newly developed opacity code targeting high-energy-density (HED) plasma applications. Its distinguishing feature is the coupling of detailed atomic structure together with an average-atom (AA) description of the plasma medium. The atomic structures are calculated in the presence of an AA plasma screening potential, rather than for isolated atoms. This removes any need for external models of density effects like continuum lowering and polarization shift. These become problematic at HED conditions, limiting the utility of traditional opacity models based on isolated-atom electronic structure. The AA model also provides the free electron density, which is used as a constraint to determine the charge state distribution. ROCSTAR leverages the Flexible Atomic Code to provide high-quality atomic structure and cross-sections, which serve as the base layer for subsequent models of the populations and line profiles needed for monochromatic and mean opacities. We present LTE opacity calculations for a variety of plasma conditions of experimental interest to HED science and compare to both traditional isolated-atom approaches as well as modern DFT-based ab initio predictions.