Observation and Characterization of Trends in the Ionization of the Warm Absorber Photoionized Plasma Experiment at Z
Many types of astronomical systems containing extremely bright and energetic radiation sources create photoionized plasmas from the material in their vicinity. This class of plasma is notable for high degrees of ionization while having relatively low temperatures, wherein photon-driven atomic processes dominate over collisional processes in the atomic kinetics. The systems in which these exist are complex, and astrophysicists endeavoring to understand them utilize modeling software created to simulate the physics in such plasma environments. Due to the extreme nature of the conditions involved, it has only recently become feasable to create adequate radiation sources in the laboratory capable of driving photoionized plasmas in an effort to test the physics models employed by astrophysicists.We are engaged in one such experiment utilizing the Z Machine at Sandia National Laboratories, which exploits the intense, broadband, x-ray radiation generated by the collapse of a z-pinch to drive neon gas, creating a photoionized plasma to study in the laboratory. The Warm Absorber Photoionized Plasma Experiment uses a cm-scale gas cell placed several centimeters from the z-pinch to contain neon gas of atom number densities 10$^{17}$ -- 10$^{18}$ cm$^{-3}$. Windows along the line of sight to a spectrometer allow (1) the radiation from the z-pinch to pass through the cell and drive the system and (2) a spectral measurement of K-shell line absorption by neon using the z-pinch as the backlighter. The TREX elliptical crystal spectrometer collects time-integrated or time-gated spectral data allowing characterization of the plasma conditions by spectral analysis. This experiment is notable in its flexibility to vary the x-ray flux $F$ and electron density $n_e$, together or separately. This allows a systematic study of their effect on plasma conditions covering, for the first time with one experimental platform, an order of magnitude in the ionization parameter $\xi \propto F/n_e$. This parameter measures the relative importance between photoionization and recombination atomic processes.Analysis of the experimental data yields ion fractions, charge state distribution, and electron temperature of the photoionized plasma. The measurements show trends in ionization and charge state distribution with changes to the experimental parameters, revealing the net result of the competition between photon-driven and electron-driven atomic processes in a photoionized plasma. We found no trend in temperature within the range of parameter variation. Additionally, time-gated data provide information on the time-evolution of the plasma. This reveals more about the behavior of photoionized plasmas due to the competition between photoionization and recombination. It has also shown evidence for transient effects in the plasma due to the short, i.e.~$<10$ ns, x-ray drive pulse.Simulation and modeling of this experiment by laboratory-oriented plasma codes have been a crucial source of insight into the physics that is important in this experimental platform. It has predicted hydrodynamic effects in the system due to expansion of the windows and shown that the atomic kinetics and x-ray heating of the system are intimately linked. Comparison of these simulation results with experimental measurements has shown reasonable agreement overall, but with significant quantitative differences in the charge state distribution. With knowledge of the comparison between these simulations and experiment, one can then use this physics model to make a fair comparison with astrophysical codes in steady state calculations.
Neon, Photoionized plasma, X-ray Spectroscopy
Networked Digital Library of Theses and Dissertations