OH number densities and plasma jet behavior in atmospheric microwave plasma jets operating with different plasma gases (Ar, Ar/N2, and Ar/O2)

Abstract

OH radical number density in multiple atmospheric pressure microwave plasma jets is measured using UV cavity ringdown spectroscopy of the OH (A–X) (0–0) band at 308 nm. The plasma cavity was excited by a 2.45 GHz microwave plasma source and plasma jets of 2–12 mm long were generated by using three different plasma gases, argon (Ar), Ar/N2, and Ar/O2. Comparative characterization of the plasma jets in terms of plasma shape, stability, gas temperature, emission intensities of OH, NO, and N2, and absolute number density of the OH radical was carried out under different plasma gas flow rates and powers at various locations along the plasma jet axis. With three different operating gases, the presence of OH radicals in all of the plasma jets extended to the far downstream. As compared to the argon plasma jets, the plasma jets formed with Ar/N2 and Ar/O2 are more diffuse and less stable. Plasma gas temperatures along the jet axis were measured to be in the range of 470–800 K for all of the jets formed in the different gas mixtures. In each plasma jet, OH number density decreases along the jet axis from the highest OH density in the vicinity of the jet tip to the lowest in the far downstream. OH density ranges from 1.3 × 1012 to 1.1 × 1016, 4.1 × 1013 to 3.9 × 1015, and 7.0 × 1012 to 4.6 × 1016 molecule/cm3 in the Ar, Ar/N2, and Ar/O2 plasma jets, respectively. The OH density dependence on plasma power and gas flow rate in the three plasma jets is also investigated.