Vibrational temperature measurement of diatomic nitrogen using femtosecond/picosecond coherent anti-Stokes Raman scattering in the ONR-UTA arc-jet facility

Laser-based flow diagnostics offer significant advantages over classical probe-based intrusive instruments in regard to measurement capabilities, spatial and temporal resolution, and measurement uncertainty reduction. Due to the complex thermochemical nature of the gas exiting an arc-jet nozzle, direct experimental measurements of chemical and thermodynamic gas properties are required to accurately measure the flow enthalpy, understand test material response, and verify theoretical predictions. The presented work documents the first application of hybrid femtosecond/picosecond Coherent Anti-Stokes Raman Scattering (fs/ps-CARS) spectroscopy to non-equilibrium thermometry measurements in an arc-jet flow. Vibrational fs/ps-CARS spectroscopy in nitrogen freestream flow enabled thermodynamic measurements by fitting the spectra to a theoretical model of non-equilibrium fs/ps-CARS. Calibration measurements were used to adjust model parameters, capture the nonresonant background effects, and optimize experimental parameters for application in arc-heated Mach 4 nitrogen flow. With a strategy to limit the background radiation and address the challenges posed by the long optical paths, inherent to this class of arc-jet facilities, the measured vibrational and rotational temperatures of diatomic nitrogen were 3690 ± 87 K and 1370 ± 109 K, respectively, successfully capturing the thermochemical non-equilibrium state of the flow, and matching theoretical predictions. This demonstration lays the foundation for broadening the applicability of hybrid CARS to thermometry from other molecular flow constituents (e.g., O2, NO), performing simultaneous rotational and vibrational measurements, and integrating with other ultra-fast optical diagnostics for a more comprehensive flow characterization.