Changes in Optical Transmission of Fused Silica Windows in a Supersonic Combustion Facility

The development of hypersonic air breathing propulsion technology depends on wind tunnel testing and a large portion of this testing requires the use of optical measurement techniques. Unfortunately, a change in transmission of fused silica windows is possible in supersonic combustion facilities with long run times. This change can affect the utility of optical diagnostics as this transmission change occurs in the light wavelength band of 350 to 850 nm. This can prevent or disrupt Optical Emission Spectroscopy, Particle Image Velocimetry, Planar Laser Induced Fluorescence, and other techniques. In this study it was determined that the change in transmission was the result of devitrification of the window, which changed the silica from the amorphous form to cristobalite crystals. Cristobalite was detected through X-ray diffraction experiments and the amount was quantified for a used facility window. This is the first time window devitrification has been reported for a hypersonic ground testing facility. A sample of new fused silica was tested in a controlled oven environment to isolate high temperature as the main factor causing crystal formation. Identification of devitrification, and its cause, in a hypersonic ground testing facility, will aid in the application and interpretation of future optical diagnostics and presents facility operators with potential options for preventing it in the future.