Investigation of LTE Condition inside a Microwave Induced Nitrogen Plasma Torch

Abstract

Optical emission spectroscopy is employed to determine the Fe I excitation temperature (T exc ) and NH rotational temperature (T rot ) inside the microwave excitation region of an atmospheric-pressure, microwave-induced plasma (MIP) torch operating in nitrogen. The plasma is operated at 2.45 GHz at an input power of 900 W with 50 W reflected, and inner and annular auxiliary nitrogen flow rates of 1 slpm each, and annular outer gas nitrogen flow rates of 13 and 25 slpm. Iron is introduced to the inner flow at a concentration of 200 ppm in a 1% HNO 3 solution. Measured relative emission intensities of Fe I emission lines in the band between 370.56 and 376.72 nm are used to determine T exc using a fitting method assuming Boltzmann equilibrium. NH radical A( 3 Π)-X( 3 Σ -) spectra are also employed (in the band between approximately 335 and 343 nm) to determine T rot by comparing measured spectra with spectra calculated using the Specair software. The presence of overlapping spectral features from the N 2 (C-B) second positive band near 337 nm are accounted for by activating these transitions in Specair in addition to the NH transitions. Good agreement between the T exc of Fe and T rot of NH is observed, indicating a local thermodynamic equilibrium (LTE) condition. These temperatures range from approximately 5000 K near the inner gas flow injector to approximately 4000 K at a distance of 25 mm downstream of the injector. The temperatures do not change significantly as a function of the outer gas flow rate.