Mass Spectrometric Studies of Ionization Precursors Ahead of Strong Shock Waves

Citation

Robinson, William McKinley, Jr. (1969) Mass Spectrometric Studies of Ionization Precursors Ahead of Strong Shock Waves. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/0QN4-MY03. https://resolver.caltech.edu/CaltechETD:etd-11222005-155153

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. An experimental study was conducted to identify the nature and origin of precursor ions produced by photoionization ahead of strong shock waves in xenon. A magnetic mass spectrometer is mounted at the end of a hydrogen-driven shock tube. Ions produced upstream of a shock wave are sampled by the spectrometer and the collected current provides a continuous time history of a particular ionic species. A mass spectrum is obtained in the precursor region for all impurities found in the shock tube. The incident shock Mach number is varied from 11.9 to 21.3, the initial pressure is varied from 0.050 to 0.500 torr, and the impurity level is altered. In all the conditions studied, the dominant ion present in the precursor is Xe+, although in certain instances, the impurity ion currents are found to be of the same order of magnitude as the xenon ion current. For small impurity levels, photoionization processes in xenon and impurities are apparently independent. Independent double probe measurements determine the total ion density to be about [...] at the shock front, the observable precursor extending about 150 cm from the shock wave. A theoretical model accounting for one-step and multi-step photoionization of xenon and impurities is used to find the ionization level ahead of a shock wave. The calculated ion density profiles agree well with experimental observations at the low pressures, where it appears that one-step photoionization predominates. Lack of agreement at high pressures, where, apparently, multi-step ionization is more efficient than the single-step process, suggests inadequacies in the treatment of photoexcitation and multi-step photoionization. Additional areas for experimental study are suggested. The mass spectrometric data yield a better understanding of the role of radiation in shock structure, of the kinetics of photoionization processes in rare gases, and of the influence of impurities in the experimental facility on the radiation mechanisms.

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