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, to our knowledge. 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 orders 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. 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 of 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.