Preliminary Optimization of fs/ps-CARS Thermometry in the ONR-UTAArc-Jet Facility

This work presents the first application of hybrid femtosecond/picosecond Coherent Anti Stokes Raman Scattering (fs/ps-CARS) to an arc-jet flow for non-equilibrium thermometry. Laser-based arc-jet flow diagnostic techniques offer significant advantages over classical 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, characterization of the chemical and thermodynamic state of the gas is required to accurately measure the flow enthalpy and understand test material response. Fs/ps-CARS enables the measurement of vibrational and rotational temperatures of diatomic species like nitrogen and oxygen; this technique allows for measurement of the thermodynamic state of the gas. In the work fs/ps-CARS was used to probe the rovibrational states of diatomic nitrogen in theplumeofanarc-jetwindtunnelusingtheQ-branchCARSspectrum. Preliminary analysis of the recorded spectra in conjunction with a fs/ps-CARS modeling algorithm yielded a diatomic nitrogen vibrational temperature of 3400 K and rotational temperature of 1390 K. Implementation of this diagnostic technique has been shown to be successful in overcoming the challenges of background radiation and long optical paths inherent to this class of arc-jet facility.