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 [1]. 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.

[1] G. P. Loisel, J. E. Bailey, T. Nagayama, et al., Phys. Rev. Lett., 134, 9, p. 095101, (2025).