Cavity Ring-Down Spectroscopy Measurements of NH₂ in a Pulsed Nanosecond Ammonia Plasma Reactor

Plasma-reforming of ammonia represents a possible strategy to leverage non-equilibrium plasma kinetics to achieve chemical mixtures that are favorable for combustion applications. Numeric modeling has shown that the amidogen radical (NH2) is an important radical species in the overall ammonia decomposition kinetics important in both reforming and combustion applications, yet quantitative measurements of NH2 radical remain elusive. The present contribution reports on the development of a compact optically-accessible ammonia plasma reactor driven by nanosecond high-voltage pulses. We also report on the development and initial measurement results of NH2 by the highly sensitive cavity ring-down spectroscopy (CRDS) absorption diagnostic. The cylindrical shaped plasma reactor is operated with high voltage pulses (5.9 kV) of duration 15 ns delivered at frequencies of 10 kHz. Fast high-voltage probes are used to measure current/voltage traces. The CRDS system is based on a pulsed dye-laser system operating at 566.9-570.9 nm and probing the π (0,10,0)-(0,0,0) vibronic band of the A ̃2 A1 (←) X ̃2 B1 transition. We provide survey spectra showing several NH2 rotational lines from the R-branch. Specific transitions have been identified and wavelength-integrated absorptions have been determined. Results will benefit numerical modeling efforts focused on NH3-plasma systems with applications related to both fuel reforming and plasma assisted combustion.