Cavity-Enhanced Absorption Spectroscopy in an Arc-Heated Wind Tunnel

Abstract

D ETAILED characterization of an arc-heatedwind-tunnel flow is essential for the modeling of thermally nonequilibrium plasma, and a number of spectroscopic measurements have been applied to resolve thermal nonequilibrium states. Especially, laser diagnostics have been used to obtain the knowledge of atomic oxygen [1–3]. Laser absorption spectroscopy (LAS) is a powerful tool to resolve an atomic absorption profile and has been applied to the measurement of translational temperature, flow velocity, and number density of absorbing particles in arc-heated oxygen and argon flows [3–5]. However, there has been no successful absorption measurement in arc-heated air flows. A reason could be that a large part of the input energy is consumed in the dissociation of nitrogenmolecules, and the resulting number density of excited state atoms is below the detectable limit of LAS.Another reason is a decrease in the electronic excitation temperature through a Laval nozzle. For absorption measurements in the flow, it is necessary to enhance the sensitivity of LAS. In the field of combustion measurements and trace gas measurements, a large number of highly sensitive absorption spectroscopy have been developed [6–9]. In this study, we applied cavity-enhanced absorption spectroscopy (CEAS) to a 750 kW archeated wind tunnel at the Japan Aerospace Exploration Agency (JAXA) for shock-layer measurements.