First Results from a Laboratory Study of Hydrogen Dominated X-Ray Photoionized Plasmas

Photoionized plasmas are abundant in the universe and can be found in many systems such as x-ray binaries and active galactic nuclei, among others. These systems are driven by the intense broadband radiation flux, typically from a compact object, and the properties are controlled primarily by photon processes such as radiation heating and cooling, photoexcitation, and photoionization. Many studies of these systems use only spectroscopic measurements and rely on sophisticated modeling for the interpretation of results. These models are best-effort theory and thus need verification against laboratory data. Existing laboratory data has proven to be valuable, but they are all limited in one key aspect, the composition. Astronomical systems are hydrogen and helium dominated with small quantities of higher Z “metals”, whereas laboratory experiments focus on one or two key elements alone. Does this factor limit the application of laboratory data to astrophysical questions? Or more specifically, what effect does metallicity have on the properties of photoionized plasmas? To address these questions, we are conducting laboratory experiments which generate hydrogen dominated x-ray photoionized plasmas at parameters equivalent to high energy astrophysical systems (ξ ≫ 1). The experiments are performed on the Z Machine at Sandia National Laboratories where a cm-scale cell filled with neon-hydrogen gas is driven the x-ray flux from a wire array z-pinch implosion. Using absorption spectroscopy of the neon K-shell features, we measure the neon charge state distribution and electron temperature of the plasma independent of atomic kinetics modeling. We then compare results obtained from experiments conducted with neon-hydrogen to those with pure neon with equivalent initial conditions. The results indicate that a small amount of neon in the mixture quickly dominates the plasma energetics while adding substantial quantities of hydrogen does not significantly influence the measured electron temperature but does influence the neon charge state distribution. This work is sponsored in part by US DOE NNSA Grant No. DE-NA0004038, the Wootton Center for Astrophysical Plasma Properties under US DOE Cooperative Agreement No. DE-NA0003843, and the Z Facility Fundamental Science Program of Sandia National Laboratories. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.