Ionization measurements in a super-orbital expansion tube

The flow in a new super-orbital expansion tube, X2, has been investigated using two optical techniques. X2 is capable of generating air flows at around 10 km/s simulating re-entry into Earth’s atmosphere by vehicles on interplanetary missions. Emission measurements identified sources of visible radiation which may influence optical measurements and can also contribute to radiative heat transfer. As well, Stark broadened hydrogen lines were used to measure electron concentrations between the bow shock and the body. Peak levels of around (3.5±0.5)xl016 cm^-3 were observed. Two-wavelength holographic interferometry was used to provide two-dimensional density and electron concentration profiles of the flow. The results compared well with the emission measurements. A gradual increase in electron population was observed, reaching a maximum about one quarter of the distance from the body to the shock along the stagnation streamline. Thereafter the concentration decreased due to the influence of the body. Comparisons were also made between flows over different sized cylinders and between air and nitrogen flows.