Optical Thickness Measurements of Vacuum Ultraviolet Radiation in the X2 Expansion Tube

Experiments were conducted in the X2 expansion tube to measure vacuum ultraviolet radiation from shock layers representative of reentry conditions at flight equivalent velocities of 10.0 and 12.2 km/s. Cylindrical models of varying lengths were used to make measurements of different depths of radiating gases. Spectral emission measurements were made, imaging parallel to the surface of the models along the stagnation streamline. Ratios of integrated spectral bands were compared to quantify increases in radiance with increasing depth of radiating gas. Significant increases in observed radiance were measured with increasing model length. Comparisons were made with simulations that assumed an optically thin shock layer, and it was found that over 98.9% of the total vacuum ultraviolet radiation emitted was self-absorbed. Good agreement was found between the simulations and measurements of total radiance for the different depths of radiating gas. These results highlight the need to accurately quantify the opacity of a shock layer and spectral line broadening because it significantly increases the radiance emitted from the shock layer in the vacuum ultraviolet region.