Ionizing nitrogen and air flows in a superorbital expansion tube

Vehicles on interplanetary missions enter atmospheres at very high (superor bital) velocities. The kinetic energy of the craft causes dissociation of m olecules and ionization of all of the species present in the gas. Ground-ba sed testing of such conditions in nitrogen has been performed in a new supe rorbital expansion tube, X2, using two optical techniques. Emission measure ments identified sources of visible radiation that may influence optical me asurements and can also contribute to radiative heat transfer. As well, the emission spectra from Stark-broadened hydrogen lines were used to measure electron concentrations between the bow shock and the body. Two-wavelength holographic interferometry was used to provide two-dimensional density and electron concentration profiles of the flow. Peak electron levels of around (4.0 +/- 0.6) x 10(16) cm(-3) were observed that agreed well with equilibr ium estimates, A gradual increase in electron population after the shock wa s observed, reaching a maximum of about one-quarter of the distance from th e body to the shock along the stagnation streamline. Thereafter the concent ration decreased because of the influence of the body. Comparisons were als o made between Rows over different sized cylinders and between air and nitr ogen Rows.