| Arc heated wind tunnels are the most reliable high temperature facilities to test thermal protection systems and heat shields. At NASA Ames Research Center, a 60 MW rated interactive heating facility (IHF) with a constricted arc heater and a expansion test cabin has been used to develop thermal protection systems for space vehicles. The work presented here involves the development and application of tunable diode laser absorption sensors to a large-scale arc-heated-plasma wind tunnel. Two different flow regions are studied: the arc-heater and the test cabin. For the heater, vertical cavity surface emitting lasers (VCSELs) are tuned across individual transitions of atomic oxygen near 777.2 nm and atomic nitrogen near 856.8 nm enabling measurement of concentration and temperature. Due to hyperfine structure and Zeeman splitting, it is not feasible to infer the translational temperature in the heater from the Doppler-broadened linewidth. The population of an excited state and the assumption of local thermal equilibrium are used to determine the population temperature. Simultaneously the copper concentration is monitored with a transition near 793.3 nm, and therefore the rate of electrode erosion is inferred. In the heater, the gas temperature was measured to be near 7,000 K. For high flow conditions, the measured copper concentration ranged up to 13 ppm. In the test cabin, metastable states of argon and atomic oxygen are monitored with VCSELs near 772.4 and 777.2 nm, and the upper limit of these metastable states is determined. Also a translational temperature of 900 K is extracted from the lineshape analysis of a transition near 769.9 nm of atomic potassium seeded upstream. The number density of metastable states is much lower than estimated using a frozen chemistry approximation. This suggests that in the post-expansion region, most atomic species are in the ground state. |
