Temperature measurements in the plume of an inductively coupled plasma torch using CARS

We perform Coherent Anti-Stokes Raman Scattering (CARS) measurements at different axial locations in the plume of an inductively coupled plasma torch with air as the working gas. We use a nano-second broadband multiplex CARS system to record spectra of nitrogen ro-vibrational transitions in the plasma plume. The spectra are least-squares fitted to theoretical results from the Sandia CARSFT code and resulting temperatures are compared to measurements using emission spectroscopy. The high temperatures in the air plasma make such measurements more challenging than, e.g., measurements in a hydrocarbon flame: the gas densities are lower, the population peak is shifted away from the ground state to higher vibrational levels, and the population differences are reduced, significantly lowering the achievable CARS signal and generating strong background emissions. This work represents a first step in the determination of spatially resolved thermodynamic properties and species composition in the plasma plume.