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 (>6000K), 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.