Progress towards absolute time-resolved opacity measurements at Z

Time-resolved spectroscopy using a novel hCMOS Ultra-fast X-ray Imager (UXI) is transforming stellar interior opacity measurements at the Sandia Z facility. Models for the Sun and stars are uncertain because opacity models are unable to reproduce previous iron opacity measurements at stellar interior conditions [Bailey et al. Nature (2015), Nagayama et al. PRL (2019)]. The unprecedented novel time-resolved data help to resolve this dilemma in three important ways. First, one hypothesis for the opacity model-data discrepancy is that the temporal integration influenced the results. Time-resolved measurements of the backlighter history, sample evolution, together with calculated opacities at each time step allows film-based measurements to be synthesized. These tests show that the sample evolution cannot explain the reported discrepancy, unless opacity calculations are invalid. Second, measurements of the sample temperature and density evolution refine our understanding of the Z opacity platform and enable improved experimental design. Third, Sandia’s UXI technology enables measurements of iron opacities at multiple conditions from a single experiment. This increases the rate of learning, since more information is obtained to test the model predictions for trends in how opacity changes with plasma conditions. In this presentation, I will summarize the results on sample evolution in Fe experiments as well as progress towards the first extraction of absolute time-resolved opacity and remaining challenges to obtain that goal.