Diode-Laser Absorption Spectroscopy of Rubidium in Hypersonic Flows

Kurzfassung

This report describes the development of a tunable diode laser system for performing absorption spectroscopy on a high-enthalpy shock tunnel. A prototype probe was built and placed in the shock tunnel to sample only a small section of the nozzle flow and thus make localised absorption measurements. Rudidium atoms were seeded into the flow and a diode laser operating at 780nm scanned across the Rb absorption line repetitively at 20kHz. Tests were performed with air and nitrogen test gases. The air tests proved unseccessful because Rb atoms were not remaining in the freestream in large enough quantities until the flow had developed. Instead, strong signals were obtained from the boundary layers formed on the model. This phenomena can be explained by the process of oxidation of the Rb atoms and this explanation is supported by the good results obtained from the nitrogen runs. Velocities were obtained throughout the nozzle start-up process and the velocities obtained during the test windw agree with CFD calculations.