DOE Center for Control of Plasma Kinetics Highlight Near - Surface Plasma - Water Interactions

Gas phase non-equilibrium plasmas containing water vapor are of growing interest for many environmental and biomedical applications. Nonetheless the plasma kinetics that is of the utmost importance for these applications remain poorly understood [1]. A He-H2O reaction mechanism has been compiled and validated for a diffuse atmospheric pressure glow discharge [2]. Models of the more complicated plasma kinetics in high electron density filamentary water containing discharges that are ubiquitous in many applications have currently not been investigated in detail. In this work, we present a laser induced fluorescence (LIF) study of the OH density in an atmospheric pressure nanosecond pulsed Ar + 0.26% H2O plasma jet. The plasma jet operates into ambient air but is shielded with a coaxial flow of argon to reduce the diffusion of air into the effluent of the jet. The jet impinges on a metal plate electrode connected to ground through a 50 kΩ resistor to limit the plasma current. The plasma is generated by a 200 ns pulsed voltage with an amplitude of 4.7 kV at a frequency of 5 kHz. A stable plasma filament is formed between the needle electrode in the jet and the metal plate allowing for spatially and time resolved laser diagnostics. The gas temperature (Tg) and electron density are measured by Rayleigh scattering and Stark broadening of hydrogen lines respectively. Electron densities in excess of 10 m have been measured. The gas temperature does not exceed 550 K in the core of the plasma. The strong variation in the time dependent relative OH density as a function of radial distance as obtained by LIF is shown in Fig. 1. This spatial variation in OH kinetics correlates with an increased air and reduced water vapor concentration at a radial position of 1.1. mm compared to 0.4 mm as found by an computational fluid dynamics calculations. The reduced lifetime of OH (~10 μs) at 0.4 mm is due to the large density of H atoms in the core of the discharge. In addition the larger concentration of O2 compared to water at 1.1 mm leads to an OH recycling mechanism as shown in Ref. [3] that can increase the effective lifetime of OH to a few hundred μs. These results illustrate the sensitivity of ambient gas composition in atmospheric pressure plasma jets on the density and kinetics of reactive species that are important for biomedical applications.