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Three-dimensional ray tracing and advanced applications of multi-monochromatic x-ray imaging to high-energy-density implosion core plasmas
AdvisorMancini, Roberto C.
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X-ray spectroscopy has been used and continues to be used for studying inertial confinement fusion (ICF) implosions. Through the use of the multi-monochromatic x-ray imager (MMI), the plasma parameters extracted using spectroscopy have advanced from spatially averaged to spatially resolved ones---giving unique insight into the implosion core's spatial structure. The MMI records large arrays of gated spectrally-resolved images that are rich in information. Indeed, processing the MMI data permits the extraction of quasi-monochromatic images centered in x-ray line emissions and broad-band images, as well as spatially resolved and integrated spectra. This dissertation continues to advance our understanding of ICF implosions through the progression of the MMI instrument. This is accomplished with the creation of a novel method for extracting the spatial profiles of electron temperature, density and mix between fuel and shell material using spatially resolved spectra, and by furthering the applicability of continuum x-ray spectroscopy to dopant-free (i.e. no spectroscopic tracer) ICF experiments. To apply these methods properly, we need to first improve our understanding and the characteristics of the MMI including its limitations. In the past, the MMI's characteristics were approximated using a paraxial approximation and not taking the thickness of the pinhole array substrate into account. We create a new three-dimensional (3D) x-ray tracing model to alleviate these approximations, derive new analytic approximations of the instrument's characteristics, and show that a new effect is found which causes image brightness variation within and across image arrays. Additionally, the spatial resolution is found to be image location dependent. By introducing an in-line radiation transport algorithm into the 3D ray tracing code, the highest fidelity synthetic MMI data to date is produced. This new high fidelity synthetic data has provided explanations to features observed in the data that were previously unexplainable and has permitted a thorough understanding of the instrument's spectral resolution. The MMI was originally designed to record x-ray line-emission spectra from a tracer added to an ICF target. However, the tracer degrades the neutron yield and cannot be included in cryogenic implosions relevant for ignition. To overcome these issues, we have derived a tracer-free analysis based on MMI narrow-band continuum images which enables the extraction of maps of the core's electron temperature and optical depth of the compressed-shell confining the core. We find that the new image brightness variation determined from the 3D ray tracing plays a critical role in the accuracy of the analysis method. The application to implosion data has revealed a correlation between an asymmetry in the temperature spatial distribution and the target's stalk. Lastly, we propose a novel method for extracting the 1D spatial distribution of the electron temperature, density, and the mix between the fuel and shell materials---caused by the Rayleigh-Taylor and Richtmyer-Meshkov hydrodynamic instabilities---directly from spatially resolved emission spectra based on the interplay between Stark and radiation transport line broadening. Several synthetic data test cases are used to demonstrate the new method's viability and reliability.