EFFECTS OF IMPURITIES ON SHOCK WAVE STABILITY AND STRUCTURE IN IONIZING MONATOMIC GASES.

The effects of impurites - hydrogen, water vapour and sodium chloride - on shoch wave stability and structure in ionizing-argon and krypton flows were investigated by using a dual-wavelength Mach-Zehnder interferometer in conjunction with the UTIAS 10 cm multiplied by 18 cm (4 double prime multiplied by 7 double prime ) Hypervelocity Shock Tube. At shock Mach number of about 15, when pure argon or krypton is used as a test gas, the resulting translational shock front develops sinusoidal oscillations. The subsequent ionization relaxation region, the electron-cascade front and the quasi-eqilibrium state also exhibit unstable characteristics. The addition of small amounts of hydrogen ( greater than equivalent to 0. 5% by pressure) to the test gas stabilizes the entire flow and at the same time drastically shortens the relaxation length to about 1/3 of the original value in argon and 2/3 in krypton. The addition of about 1% of water vapor (by pressure) to the argon test gfas also stabilizes the flow and shortens the relaxation region. A thin coating of dissolved sodium chloride in water on the shock tube wall, which on evacuation leaves a rough surface of crystals, however, does not seem to have any effect on the flow. Although the small impurities of hydrogen and water vapor have a significant effect in stabilizing the shock, the overall values of density and electron number density in the quasi-equilibrium region remain nearly the same. The present experimental results provided the data for a theoretical study which took into account atom-atom collisions, electron-atom collisions and radiation-energy losses to deduce a precise value for the atom-atom collision excitation cross-section constants for argon and krypton having values of S*//A//r//-//A//r equals 1. 0 multiplied by 10**-**1 9 cm**2/eV, respectively.