DOE Plasma Science Center Control of Plasma Kinetics ATMOSPHERIC PRESSURE PLASMA JET (APPJ) WITH CO-AXIAL SHIELDING GAS: EFFECTIVENESS OF ISOLATION The plasma chemistry and plasma-surface interactions in APPJs depend on the ambient air entrained by the active plasma jet. To achieve better control of a He APPJ, a co-axial shielding gas (N2 here) was employed. There appears to be a threshold flow rate of the shielding gas to achieve effective shielding. The measured emission intensity of the O-atom 777 nm line (normalized to the He 706 nm line) correlates well with the O2 density in the APPJ predicted by models. The 777 nm line results from dissociative excitation of O2, rather than direct electron impact excitation of O, and so reflects the O2 density. (top) Predicted O2 mole fraction as a function of radius 6 mm from the nozzle for different flow rates of the shielding gas. (bottom) Comparison of simulations with experimental data (6 mm from the nozzle) of contaminant oxygen in a He APPJ for different flow rates of nitrogen shielding gas. DOE Plasma Science Center Control of Plasma Kinetics HIGHLIGHT January 2019 1

DOE Plasma Science Center Control of Plasma Kinetics GAS BREAKDOWN IN MICROGAPS WITH MULTIPLE CONCENTRIC CATHODE PROTRUSIONS The effects of multiple concentric cathode protrusions with mutual shielding on gas breakdown were studied. Electric shielding plays an important role on breakdown characteristics. When protrusion spacing, X, increases and shielding weakens, the electric field becomes larger at both the protrusion tip and the cathode substrate. The weaker shielding lowers the ignition voltages of both long- and short-path breakdowns. a b Electric potential distribution with concentric protrusions. Breakdown voltage vs. gas pressure DOE Plasma Science Center Control of Plasma Kinetics HIGHLIGHT January 2019 2