Implosion stability and symmetry analysis of OMEGA direct-drive implosions using spectrally-resolved imaging
AuthorJohns, Heather Marie
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AbstractLine absorption spectroscopy of Ti-doped tracer layers embedded in the shell of inertial confinement fusion targets is a powerful diagnostic to characterize the state of the un-ablated and compressed shell that confines the hot and dense core fuel. In this dissertation we investigate two applications of this diagnostic to warm shell implosion experiments performed at the OMEGA national laser user facility that provide new insights about implosion symmetry, stability and mixing. This was accomplished through two groups of experiments and different types of data processing and analysis. In a first group of experiments, streaked high-spectral resolution but spatially integrated measurements were recorded with a crystal spectrometer to determine the time-history of electron temperature and density, ionization state and areal density for tracer layers initially located at several depths from the shell's inner surface. This analysis included, for the first time, the effect of self-emission of Ti K-shell line transitions. We found that the self-emission is important for tracer layers located close to the core, and has to be taken into account in order to obtain accurate values of temperature and density; but this effect is less important for tracer layers initially placed farther from the core, for which the self-emission may be neglected and analysis of transmission is sufficient to model and interpret the absorption spectrum. This finding is consistent with the idea that regions of the shell close to the core are more significantly heated by thermal transport out of the hot dense core, but more distant regions will remain at lower temperatures because they are less affected by thermal transport. In a second group of experiments, arrays of spectrally-resolved images were recorded with a novel multi-monochromatic x-ray imager: the MMI instrument. The MMI affords simultaneous time-gated (snapshots), spatial- (based on pinholes) and spectral- (multi-layer Bragg mirror dispersive element) resolution. While the spectral resolution of MMI is not as high as that of the crystal spectrometer it is sufficient to extract spatially resolved areal density maps. In this regard, two independent methods were developed to extract these maps, one based on ratios of narrow-band images, and the other based on sets of spatially-resolved spectra. In addition, the observations were made with three identical MMI instruments fielded along three quasi-orthogonal lines-of-sight, hence providing insight into the 3-D nature of the implosion. A Fourier analysis of the areal density maps was performed to unfold their wavelength spectrum and to obtain the power per mode. In turn, this led to the determination of the target breakup fraction and a new method for estimation of the mix width. Results are presented and discussed for experiments that were done using different laser pulse shapes, shell thicknesses and gas filling pressures, and initial location of the Ti-doped tracer layer. Comparison of results from each line of sight that were taken at the same time in the implosion allowed us to determine the relative symmetry and stability of each implosion, in terms of the strength of the confinement the compressed shell was able to provide. We found that implosions driven with lower adiabat drive pulse shapes had better symmetry and stability in the compressed shell than higher adiabat pulse shapes. Additionally, for targets driven with low-adiabat pulses, Ti-doped layers initially placed closer to the fuel-shell interface had higher stability than those that were placed farther away, by the time of peak compression. Even though the low adiabat drives display more evidence of mixing early in time as the Ti-doped layer is placed closer to the core, the final mix width estimate, by the time of peak compression, is less affected. We show that our mix width estimate obtains reasonable values, based on comparison with other studies of mix width from nominally identical implosions, which is an important proof of concept for this new method.