Investigation of Atmospheric Pressure Plasma Jet Properties and Operations in Ambient Air

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Plasma is gas ionized by energizing electrons through electric fields, leading to various collisional reactions that result in a number of reactive species useful in many applications. A main characteristic of plasmas is the creation of luminous excited species that relieve their energy through emissions. The capturing of these emissions: optical emission spectroscopy (OES) is a major method for plasma diagnostic. Atmospheric pressure plasma jet (APPJ) is a type of plasma that is distinctive for the creation of a small plasma jet extending from the plasma reactor (often a quartz tube equipped with electrodes), making the APPJ a prime candidate for plasma medicine applications. This study aims to investigate the fundamental properties of APPJs that are relevant to applications through the use of OES. Firstly, the excitation mechanisms of plasma species in a He APPJ was revealed through spatially and temporally resolved OES, showing the major role of He metastables in excitations of various air species. Secondly, the gas temperature and electron density of Ar/He APPJs were derived by fitting the emission line shapes of N2 and H, respectively. The gas temperature increases (400 K – 600 K for Ar, 650 K – 750 K for He) with increasing applied power (9 W – 18 W for Ar, 19 W – 24 W for He). Electron density of Ar plasma increases (2.5x1013 cm-3 to 5x1013 cm-3) with a grounded copper sheet behind a quartz substrate. Finally, diffusion of air species into a He plasma jet was measured through self-actinometry and controlled by addition of a shielding gas curtain separating the plasma gas from ambient air. Self-actinometry involves adding trace gases (Ar, N2, and O2) into the plasma feed gas and observing changes in Ar, N2, O2, and He emission intensity in order to correlate emission intensity and mole fractions. Air mole fraction in the plasma center increases along the jet axis from none at 1 mm, to 10-3 at 3 mm and 10-2 at 5 mm axial distance from the nozzle. Addition of N2 shielding gas reduces air diffusion by 2 to 3 times.

Plasma, atmospheric pressure plasma, atmospheric pressure plasma jet
Portions of this document appear in: Nguyen, Tam, Eduardo Hernandez, Vincent M. Donnelly, and Demetre J. Economou. "Excitation mechanisms in a nonequilibrium helium plasma jet emerging in ambient air at 1 atm." Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 36, no. 4 (2018): 04F406.