The stability of dissociating and ionizing shocks in diatomic gases under spontaneous emission of acoustic as well as entropy-vortex waves is investigated. It is shown that spontaneous emission may occur in the range of high-frequency perturbations for whom thermal equilibrium cannot be maintained between the electrons and the heavy particles behind the shock. A two-fluid model is constructed in order to describe the partially ionized plasma behind the shock in that range of frequencies. The ionization frequency is assumed to be much bigger than the frequencies of the perturbations hence the Saha equation may be used at each time. An expression for the sound velocity is derived for that system, that is employed in the Dyakov-Kontorovich stability criterion. The latter is modifeid in order to take into account the thermal nonequilibrium between the electron and the heavy particles. It is shown that dissociation alone cannot destabilize the shock, however, for high enough Mach numbers for which the gas is fully dissociated, the ionization processes give rise to the spontaneous emission instability. The threshold for the occurrence of spontaneous emission in nitrogen at atmospheric pressure is in a range that is relevant for space vehicles atmospheric reentry.
|Title of host publication
|American Physical Society, 41st Annual Meeting of the Division of Plasma Physics, November 15-19, 1999 Seattle, WA
|Published - 1 Nov 1999