SPATIAL & TEMPORAL EVOLUTION OF ELECTRIC FIELDS DURING PLASMA FORMATION Current effort is underway to measure spatial and temporal evolution of electric fields in (near) atmospheric pressure plasma discharges. Developing picosecond pump-probe diagnostic to scan across electric field induced Stark splitting states of helium. Observed Stark Splitting 31S->81P (Allowed) 31S->81D (Forbidden) Ratio (81D / 81P) Axial Ratio 81P 81D 81S Anode Cathode Sheath forming (Imaged area: 15 mm wide x 3 mm high) Cathode Measured Electric field induced splitting of 81L state of helium Measuring of splitting as an ionization wave makes contact with cathode in 200 Torr Helium. DOE Plasma Science Center Control of Plasma Kinetics HIGHLIGHT April 2019 1

DOE Plasma Science Center Control of Plasma Kinetics RADIAL PROFILES OF ABEL INVERTED OPTICAL EMISSION IN AN ATM PLASMA JET WITH SHIELDING GAS Shielding gas in atm. pressure plasma jets isolates the plasma plume from ambient air. Line-integrated optical emission was Abel inverted to obtain radial intensities at plasma plume tip for a He APPJ with N2 shield gas. OES intensity for O 777 nm (proportional to O+O2 density) and Ar 811.5 nm from in- diffusing air were normalized by He 706 nm intensities. Without shielding, the Ar/He emission ratio, related to relative air intake, decreases 2-fold on axis, indicating a surprising air penetration. The Ar/He ratio reduces by only 2.5-fold with shielding gas, likely a consequences of eddies or other turbulence. HIGHLIGHT DOE Plasma Science Center Control of Plasma Kinetics April 2019 2