Spatio-temporal characterization of a pulsed DC atmospheric pressure plasma jet interacting with substrates

Atmospheric pressure plasmas generate a variety of chemically active species in open air, thus providing the unique ability to treat a variety of materials that do not require or are not compatible with vacuum systems. Producing the plasma-surface interaction that leads to a desired change in the substrate is complicated by the codependency between the plasma and the substrate: while the plasma will modify the surface, the surface will also influence the plasma properties. In this work, a pulsed-DC plasma jet produced in helium and impinging upon glass and metal substrates is studied over a range of applied voltage pulse widths extending from 1 to 10 µs. Current-voltage measurements, high speed images, and time-resolved optical emission from three important He and nitrogen excited species are used to examine the evolution of the plasma and its interaction with the surface. At ignition, a streamer is ejected into the open air from the jet exit and eventually collides with the substrate. For a glass substrate, the streamer will hit the surface and form a short-lived plasma across it. This surface plasma is almost completely unaffected by changes in the voltage pulse width. In contrast, when the streamer hits a metal substrate, a surface discharge will form that will last the entirety of the voltage pulse. If the pulse is long enough, a 'reflected discharge' will slowly develop that extends from the substrate back towards the outlet of the plasma jet. The emission intensity of the surface discharge closely matches that of the initial streamer, but not the reflected discharge, which suggests different electron kinetics between the two features. The addition of capacitors or resistors between the metal substrate and ground show how differences in substrate electrical properties can account for some of these behaviors. Emission line ratios are used to examine the evolution of electron temperature and the relative importance of Penning processes during the different plasma phases.