Coherent Raman Measurements of Temperature and CO∕N₂ Concentration During Plasma Torch Graphite Ablation

Measurements in high-enthalpy flows are important to understand hypersonic flight and reentry environments. In this study, we use nanosecond coherent anti-Stokes Raman scattering (CARS) to simultaneously probe CO and N₂ molecules within the reaction layer of a graphite sample exposed to an atmospheric pressure plasma plume. The plasma plume is generated by an inductively coupled plasma torch, with temperatures in the plasma freestream ranging from 5000 to 7000 K. The CARS measurement volume, with a diameter of approximately 100 μm, can be positioned as close as 0.2 mm from the graphite surface. The acquired CARS spectra are fitted to theoretical results to determine rotational-vibrational equilibrium temperatures and relative CO to N₂ mole fractions. We present highly spatially resolved profiles of these quantities in the boundary layer of the graphite ablator, report uncertainties, and discuss the presence of chemical nonequilibrium in the boundary layer.