Coherent Anti-Stokes Raman Scattering in a Supersonic Inductively Coupled Plasma Torch

Supersonic, high-enthalpy ground-based test facilities are critical for studying hypersonic flight and atmospheric reentry phenomena and for characterizing the response of materials used for Thermal Protection Systems (TPS). To address this need, the 50 kW Inductively Coupled Plasma (ICP) Torch at The University of Texas at Austin has been retrofitted with a Mach 2 nozzle and vacuum chamber to generate a supersonic air plasma jet. The Mach 2 air plasma jet is operated in an approximately ideally expanded condition without shocks in the measurement region, and its thermodynamic state is characterized using two-temperature coherent anti-Stokes Raman scattering (CARS). Spatially resolved CARS profiles of molecular nitrogen yield rotational and vibrational temperatures, revealing a near-nozzle region with weak axial gradients, a hot core with the vibrational temperature greater than the rotational temperature surrounded by cooler outer regions, and sustained thermochemical non-equilibrium across the measurement domain. These measurements provide a baseline thermodynamic characterization of the Mach 2 air jet, which can be used to design TPS material response experiments and to support model validation in the supersonic ICP facility at UT Austin.