| Summary: | The atomic translational temperatures and velocities of a low pressure, high enthalpy air plasma are measured using Fabry–Perot spectroscopy. The measurements presented here are the first measurements using this system at this enthalpy level. The sub-picometre resolution of the unique system has allowed accurate translational temperature and velocity measurements of the atomic species in the plasma. The detection system allows the Doppler broadening of multiple atomic nitrogen and oxygen lines to be measured simultaneously. Additionally, having two optical paths, one perpendicular to the flow and one at 45 deg. allows the Doppler shift to be measured.
Measurements were taken during three different plasma wind tunnel tests. Mean atomic nitrogen temperatures of $1.08\pm 0.11 \times 10^4$ K and atomic oxygen translational temperatures of $1.23\pm 0.12 \times10^4$ K were measured. The thermal non-equilibrium determined verified earlier measurements of the same phenomena, however, the mechanism behind this has not yet been determined. The mean measured flow velocity was $3350\pm 840~{\rm m~s}^{-1}$ and was consistent between the atomic species.
The translational temperature and velocity contribute approximately 35% of the local enthalpy of the flow. The direct measurement of these parameters, removing previously required assumptions, increases the fidelity of the flow characterisation significantly. This allows high quality testing to be conducted in this flow field.
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