M.J. Johnson^1,2, D.R. Boris², C.L. Enloe², Tz. B. Petrova², S.G. Walton^2
1Syntek Technologies, Fairfax, VA; ² U.S. Naval Research Laboratory, Washington, DC

Atmospheric pressure plasma jets (APPJs) have become a valuable tool for the modification of surfaces. One of the large benefits of APPJs is their ability to generate a chemically-rich environment in open air, allowing for the modification of a board range of surfaces including metals, polymers, ceramics, and biological materials. However, when an APPJ interacts with a surface, the surface will influence the structure of the plasma jet and thereby alter the chemistry of the jet. This is particularly vital because different chemical species important for surface modification will form in different quantities depending on the surface. Because of this, two different surfaces treated by the same plasma jet will undergo exposure to slightly different conditions. In this work, time-resolved measurements of the optical emission of a pulsed-DC plasma jet impinging on different surface is measured to investigate how the structure and chemistry of the plasma on the surface evolve in time. Initially, the plasma source emits a streamer which propagates out from the jet nozzle into the open air and eventually collides the surface. With a metal surface, a ‘secondary stroke’ forms on the surface and extends back towards the jet outlet. The formation, extension, and duration of the stroke are functions of the pulse width and frequency of the voltage waveform used to generate the plasma jet. The metal surface allows for the formation of a long-lived, surface plasma that exists for the duration of the pulse. If a dielectric surface is impinged with the APPJ, the streamer will strike the surface and produces an ionization wave that extends along the surface. The ionization wave is short-lived and not significantly affected by the length of the pulse. As the streamer approaches the metal substrate, it induces a certain amount of displacement current before it arrives. Through modeling the system, this displacement current can be used to approximate the position and velocity of streamer over time.