Polychromatic tomography of high energy density plasmas
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We have developed a new type of tomography, i.e. polychromatic tomography, to study the spatial structure of high-energy density plasmas (HEDP). Unlike the tomography used in medicine, polychromatic tomography relies on a small number of directions of observation (i.e. views) but uses the information encoded in multiple wavelengths along each direction of observation. As a case of illustration, we applied this technique to extract the electron temperature and density distribution of an inertial confinement fusion implosion core. The target was a warm plastic shell filled with deuterium gas and a tracer amount of argon for diagnostic purposes, and the experiments were performed at the OMEGA laser facility. The implosion core was observed simultaneously along three quasi-orthogonal lines-of-sight (LOS) using three identical gated, multi-monochromatic x-ray imagers (MMI) fabricated at the Physics Department machine shop and fielded in the OMEGA target chamber. Each MMI recorded a large collection of spectrally-resolved x-ray images of the plasma source (i.e. implosion core). The MMI data are rich in information, and have resolution in time, space, and photon energy. By processing the MMI data we obtained different types of processed data such as broad- and narrow-band images within a given photon energy range, and space-integrated and space-resolved spectra from each LOS. The shape and size of the implosion core volume were estimated from broad-band images simultaneously recorded along the three LOS. The space-resolved argon line emission spectra, also characteristic of the three LOS, were analyzed self-consistently and simultaneously using a geometry adaptable radiation transport model and multi-objective data analysis. The forward search and reconstruction in parameter space was driven by a parallel Pareto genetic algorithm followed up by a Levenberg-Marquardt non-linear least-squares minimization method. A collection of synthetic data test cases were investigated as a proof of principle, and to understand the requirements and limitations of the method. The application of polychromatic tomography to the implosion core data recorded at OMEGA yielded the first time-resolved, three-dimensional temperature and density spatial structure of the core at the collapse of the implosion extracted from data analysis. The idea of polychromatic tomography is general and can be applied to study the spatial structure of other HEDP sources.