PRECURSOR RADIATION IN ELECTROMAGNETIC SHOCK TUBES.
JOHNS HOPKINS UNIV BALTIMORE MD DEPT OF PHYSICS
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The purpose of the investigation was to determine the origin and magnitude of pre-excitation in a shock tube with a cylindrical geometry, and to determine whether a sufficient amount of energy is transferred to the cold gas to account for the observed discrepancies. Pre-excitation of the cold gas ahead of the main luminosity front was examined through spectroscopic analysis of optical precursor radiation and microwave measurements of the electron density. The populations of radiative excited states in a helium filled shock tube were determined from absolute intensity measurements of the precursor radiation. Estimates of the populations of metastable states were obtained from optical absorption measurements, and from measurements of optical radiation arising from a resonant transfer of excitation to an impurity gas. By measruring the spatial decrease of optical precursor light along the observation tube, it was possible to establish a model of pre-excitation in which energy is transferred to the cold gas through the absorption of ultra-violet radiation emitted from the discharge chamber. The production of optical precursor light is found to be consistent with a two-step process consisting of photo-ionization of the cold gas followed by excitation by electron impact. The results of the measurements also provided an upper limit to the populations of excited states. These results showed that only a small fraction of the cold gas is pre-excited. Author
- Plasma Physics and Magnetohydrodynamics