A radiative model for determining plasma dissociation using vacuum ultraviolet self-absorption spectroscopy



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This manuscript documents the first five years of collective knowledge gained from the Texas Tech University research program to study the emission and subsequent re-absorption of vacuum ultraviolet radiation present during the initiation of nano-second plasma discharges generated at atmospheric pressure. The initial experimental study resulted in direct observation of vacuum ultraviolet radiation produced by an open transient plasma at atmospheric pressure for the first time in the available literature. Upgrades to the spectral instrumentation enabled more efficient measurement of vacuum ultraviolet radiation for the wavelength range 115 - 135 nm, allowing for enhanced resolution in the recording of emission line profiles. A direct consequence of this effort was the conception of a passive optical diagnostic for measuring the absolute number density of atoms in the discharge plasma: the vacuum ultraviolet self-absorption spectroscopy (VUV-SAS) technique. An integral piece of this technique is a radiative transfer simulation for calculating the radiation trapping physics inside the plasma channel volume, executed in the MATLAB® environment and accelerated via GPU resources using the NVIDIA® CUDA architecture. The combination of experimental and modeling approaches resulted in successful demonstration of the VUV-SAS diagnostic for N2/H2 plasmas at atmospheric pressure, where spatially resolved N and H atom densities on the order 10^17 cm^-3 were observed without an invasive laser absorption diagnostic. Through approximation of the quasi-contiguous Stark broadening of H atoms in the discharge plasma, spatially resolved electron densities on the order 10^18 cm^-3 were observed along the plasma channel. All measured quantities are validated through detailed discussion of the relevant physics concerning the fast ionization shockwave characteristic to the inception of spark discharge plasmas. Finally, the extended VUV-SAS technique was successful in demonstrating the measurement of N and O atom densities present in air discharge plasmas, thereby finding practical application for a variety of future pulsed power laboratory experiments.