Characterization of a Large-Scale Arcjet Facility Using Tunable Diode Laser Absorption Spectroscopy

A laser-based sensor was developed and deployed for in situ monitoring of the gas temperature in the plenum after the arc heater and before the expansion nozzle in the 60 MW interaction heating facility at NASA Ames Research Center. Early measurements suggested the cold air added to the arc-heated gas to control enthalpy was not well mixed, and modifications were made to add a mixing volume in the plenum between adding air and the expansion. Measurements of the gas temperature conducted before and after this modification are presented. An external cavity diode laser was wavelength scanned to monitor the population of electronically excited (3s) oxygen atoms via absorption by the P-5(3)-S-5(2 degrees) transition near 777.2 nm. Assuming thermochemical equilibrium, the population of electronically excited 3s atoms provided a sensitive measurement of the gas temperature in the 4000-8000 K range expected in the arc-heater plenum. Time-resolved measurements were obtained on multiple lines of sight, and differences between the inferred temperatures for the different lines of sight revealed important details regarding the degree of flowfield uniformity in the arcjet plenum.