X-ray Radiative Signatures Influenced by Nonthermal Effects in High-Energy-Density Laboratory Plasmas of Mid-Atomic-Number Z-pinches
Spectroscopy is a powerful utility for noninvasive investigation of high-energy-density (HED) plasma environments with broad application to atomic physics, astrophysics, and HED laboratory physics, including inertial confinement fusion (ICF), making it an inimitable tool for interdisciplinary science. Spectroscopic examination of plasma radiation can yield robust information on plasma conditions such as, temperature, density, size, and elemental composition. Plasma radiation is an atomic fingerprint, providing insight into the diverse microphysics of the complex and intricate plasma system. X-ray radiation, for example, is a signature of ionization kinetics in mid-to-high-Z atoms. X-rays, moreover, are notably valuable for diagnosing HED plasmas, where thermal and nonthermal x-ray features describe, in general, "hot" and "cold" plasmas, respectively. This dissertation focuses on identifying and characterizing nonthermal effects and radiative properties in HED laboratory Z-pinch plasmas. Nonthermal effects describe the interaction of non-Maxwellian electrons or high-energy, external photons with inner-shell electrons in plasma ions, producing distinct radiative signatures which, for example, can diagnose the cooler regions of ICF plasmas as well as plasma accretion around astrophysical compact objects. Z-pinch plasmas are efficient x-ray sources, presenting an attractive platform for laboratory astrophysical studies, such as x-ray-driven emission properties of black hole accretion zones and stellar iron opacity, and for benchmarking spectroscopic codes against plasmas under controlled conditions. In this dissertation, three research projects are presented on two types of pulsed-power Z-pinches of mid-atomic-number material using a suite of experimental, theoretical, and computational techniques. Two projects investigate the role of non-Maxwellian electrons in K-shell Fe, Cr, Ni, and L-shell Mo X-pinch plasmas produced on the Zebra generator at the University of Nevada, Reno. Experimental techniques include analysis of x-ray diode signals (> 3 keV) concurrent with source size evolution to showcase x-ray radiative properties of stainless steel X-pinches of different geometries (in the first project) and measurements of x-ray line polarization, induced from non-Maxwellian electron impact, in L-shell Mo X-pinch spectra (in the second project). Theoretical techniques include spectroscopic modeling of K-shell Fe, Cr, and Ni plasmas to infer plasma parameters as well as analysis of relative line intensity ratios of analogous Fe and Cr line emission to explore plasma opacity. Notable results include: (1) production of hotter, thermal K-shell plasmas with enhanced satellite line emission and an inferred fraction of 0.5% nonthermal electrons for small-angle X-pinches, (2) the first measurements of x-ray line polarization (≤ 15%) in HED Ne-like Mo X-pinch spectra, which are used to characterize nonthermal electron beam energies of 4 - 30 keV (confirmed by diagnostic signals and x-ray spectra). The third study examines nonthermal Fe K-shell fluorescence, driven by a thermal radiation field, in a Magnetized Liner Inertial Fusion (MagLIF) plasma produced on the Sandia National Laboratories' Z-machine. This is performed with a novel Monte Carlo Radiation Transport post-processing code, which employs a computational screened-hydrogenic atomic data package to self-consistently calculate atomic processes underpinning radiative transfer. Numerical computation of atomic data and radiation transport are performed to interrogate the spatial origins of the Fe fluorescence, revealing a broad spatial distribution (~107 m) of Fe fluorescence production in a MagLIF liner plasma shell, centered ~264 m from the pinch axis.
astrophysics, atomic physics, high energy density plasma, radiation transport, x-ray line polarization, X-ray spectroscopy
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