Nitrogen Thermometry in an Inductively Coupled Plasma Torch using Broadband Nanosecond Coherent Anti-Stokes Raman Scattering

The development of atmospheric hypersonic flight and re-entry capabilities requires the characterization of the thermo-chemical state of representative test environments. This study demonstrates the usage of multiplex nanosecond N₂ Coherent Anti-Stokes Raman Scattering (CARS) to measure temperatures in an atmospheric, high-temperature (>6000 K), air plasma plume, generated by an inductively coupled plasma torch. These are some of the highest temperatures ever accessed via gas-phase CARS. Temperatures of N₂ in the equilibrium plasma plume are determined via theoretical fits to measured CARS spectra. We discuss the practical implementation of CARS at very high temperatures, including the scaling of the N₂ CARS signal strength from 300 to 6700 K, where the expected peak signal from the high temperature plasma torch gases is two order-of-magnitude less than commonly encountered in combustion environments. An intensified CCD camera enables single-laser-shot detection at temperatures as high as 6200 K, by increasing sensitivity and providing a time gate against intense background luminosity. We also discuss the impacts of unwanted two-beam CARS contributions from outside the nominal three-beam measurement volume and population transfer due to stimulated Raman pumping. We present mean axial and radial temperature profiles, as well as time-series data derived from both single-laser-shot and accumulated CARS spectra. The single-laser-shot precision is 1.7-2.6% at temperatures 3500 to 6200 K. The presented results pave the way for the use of CARS at very high temperatures and the measurement of spatially resolved interface processes in high-enthalpy flows.