Spectroscopic investigations of atmospheric pressure microwave torch nitrogen plasma jet

Abstract

Microwave (MW) torches are typically used to produce equilibrium plasmas for various industrial applications. We present spectroscopic investigations of atmospheric pressure afterglow plasmas generated by a Litmas Red MW torch (2.45 GHz, 3 kW) in nitrogen.We employ optical diagnostics: emission spectroscopy and digital photography to characterise the plasma jet. Contrary to standard MW torch geometries (where the gas flows upstream the cavity perpendicular to the MW wave guide), we use special nozzles where the gas is inserted via two tangential inlets on top of the MW cavity. Plasma produced into the torch swirls up tangentially through the inlet. We tested several nozzles with different gas inlet angles and diameters. We present a systematic study of the nitrogen afterglow plasma jet parameters. Varying MW power and gas flow rates results in varying plasma jet sizes and properties. The more energy is inserted in the flowing gas, the greater emission and higher plasma temperatures are obtained. Plasma temperatures are determined from the CN violet system by comparing experimental and simulated (LIFBASE) spectra. Axial profiles of the emission intensity and photo-documentation characterise the evolution of the jet in space. A small admixture of oxygen results in different excited states chemistry, which was apparent from the emission spectra.