Speaker
Description
Opacity quantifies the absorption of photons in matter and is essential for understanding energy transport through radiation in high-energy-density (HED) plasmas, such as those found in stars and inertial confinement fusion. While opacity models provide local thermodynamic equilibrium (LTE) opacities for all elements based on temperature and density, they depend on multidisciplinary physics and include untested approximations. Calculating opacities under HED conditions is challenging due to density effects and the overwhelming number of excited states involved. This raises questions about the accuracy of calculated opacities, especially when discrepancies arise between simulations and observations. For instance, solar models have shown inconsistencies with helioseismic observations, raising concerns about the opacities being used. Experimental validation of HED opacity is difficult due to the need for a bright backlight to overcome self-emission from hot, dense plasmas. In 2015, we utilized the Z-machine at Sandia National Laboratories, which is the most energetic X-ray source on Earth, to test iron opacity at solar interior temperatures for the first time. The measurements revealed significant discrepancies between calculated and measured iron opacities, raising significant concerns within the HED and astrophysics communities. We will summarize experimental and theoretical challenges associated with HED opacity, as well as our recent progress in our time-resolved measurements and first oxygen opacity measurements. Systematic experimental investigations into opacity issues will help resolve the solar problem and reduce uncertainty in radiation-hydrodynamic simulations.
SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.