CAVITY RING-DOWN SPECTROSCOPY MEASUREMENTS OF NH2 IN A PULSED NANOSECOND AMMONIA PLASMA REACTOR

Abstract

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 the 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. The reactor operated with both pure ammonia and ammonia/nitrogen mixtures (22% NH3/78% N2). Experiments were conducted at ammonia flow rates of 3.87 and 45.10 SLPM, with pressures ranging from 71 to 77 Torr. We also report on the development and initial results of NH2 measurements by the extremely sensitive cavity ring-down spectroscopy (CRDS) laser absorption diagnostic. The cylindrical shaped plasma reactor operates with high voltage pulses (~1.9 to 28 kV) of duration 15 ns delivered at frequencies of ~10-100 kHz. Fast high-voltage probes are used to measure current/voltage traces. The CRDS system is based on a pulsed dye-laser system probing the (_^R)R_0,2 branch of the π vibronic sub-band of the A ̃^2 A_1 (0,9,0) □(←┬ ) X ̃^2 B_1 (0,0,0) transition, and the (_^P)Q_1,7 branch of the  band of the òA₁(0,9,0) ← X̃²B₁(0,0,0) transition. We provide parametric and transient studies of NH2 densities. Results show the transient nature of NH2 and the relationship of relative power to NH2 production. The results will benefit numerical modeling efforts and reactor design focused on NH3-plasma systems with applications related to both fuel reforming and plasma-assisted combustion.