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 point 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.