High-Speed Imaging of Atomic Oxygen Emission in the HyMETS Arcjet Tunnel

Bandpass-filtered imaging of electronically excited atomic oxygen emission at 777 nm was performed at a frame rate of 300 kHz in the NASA Langley Hypersonic Materials Environmental Test System (HyMETS) arc-heated wind tunnel. The facility was operated with a fixed working-gas composition of 95% argon and 5% oxygen. A continuous tunnel run was partitioned into test cases corresponding to arc-currents of 150, 175, and 200 A - each with and without the insertion of a cylindrical silicon-carbide (SiC) test sample oriented with the axis parallel to the streamwise flow direction. Image sequences were analyzed to extract qualitative information from time-averaged spatial distributions, temporally resolved quantitative spatial metrics such as standoff distances, and frequency content of both freestream and shock-layer emission. Emission intensity increased with arc-current in both flow regions, with shock-layer intensities approximately an order of magnitude higher than those of the freestream. The freestream emission exhibited a parabolic radial profile, with a core diameter expanding from 55 to 65 mm over an axial distance of 21.5 mm within the camera's field of view. Shock standoff distances were consistently measured at 11+/-1 mm upstream of the cylindrical sample across all arc-current conditions, with temporal fluctuations dominated by frequencies near 60, 180, and 360 Hz. The region of highest emission was located 2.0--2.4 mm from the front face of the cylindrical sample, decreasing with increasing arc-current, and exhibited the same frequency-specific oscillations as the shock front. These distinct frequency signatures were also observed in the freestream emission and appear to be independent of arc-current.