Nanosecond CARS Measurements of Temperature and Relative CO Concentration in the Boundary Layer of a Graphite Ablator in an Inductively Coupled Plasma Torch

The development of heat shields for hypersonic flight and atmospheric re-entry requires experimental data obtained in relevant high-enthalpy flow environments. Measurements in such environments are challenging, due to the high temperatures, high background luminosity, and complex chemistry. In the present work, we use nanosecond Coherent Anti-Stokes Raman scattering with an adjustable broadband dye laser as the Stokes source, simultaneously probing CO and N2 molecules in 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 and temperatures in the plasma are around 5000-6000 K. The ~100 micrometer diameter CARS measurement volume is situated 0.1-3 mm away from the graphite surface. CARS spectra are fitted to obtain temporally and spatially resolved profiles of the rotational-vibrational equilibrium temperature and the concentration of CO relative to N2. Results suggest that CO can reach up to 60% of the N2 concentration and temperatures change from ~6000 K in the freestream to ~170 K at the graphite surface over a distance of ~3~mm.