Chemical kinetics and reactive species in atmospheric pressure helium–oxygen plasmas with humid-air impurities

Abstract

In most applications helium-based plasma jets operate in an open-air environment. The presence of humid air in the plasma jet will influence the plasma chemistry and can lead to the production of a broader range of reactive species. We explore the influence of humid air on the reactive species in radio frequency (rf)-driven atmospheric-pressure helium-oxygen mixture plasmas (He-O 2 , helium with 5000 ppm admixture of oxygen) for wide air impurity levels of 0-500 ppm with relative humidities of from 0% to 100% using a zero-dimensional, time-dependent global model. Comparisons are made with experimental measurements in an rf-driven micro-scale atmospheric pressure plasma jet and with one-dimensional semi-kinetic simulations of the same plasma jet. These suggest that the plausible air impurity level is not more than hundreds of ppm in such systems. The evolution of species concentration is described for reactive oxygen species, metastable species, radical species and positively and negatively charged ions (and their clusters). Effects of the air impurity containing water humidity on electronegativity and overall plasma reactivity are clarified with particular emphasis on reactive oxygen species.

Uncertainty and error in complex plasma chemistry models

“…and in recent work it is commonly assumed that the products include electronically excited states with appreciable branching fractions [71,95]. However, the primary sources give no support for this supposition [141,10,7,119], suggesting instead that the predominant products are molecules in vibrationally excited states.…”

“…Such models may involve dozens of chemical species and thousands of reactions [117,41,95]. Each reaction is characterized by a rate constant, and these are are never known with certainty.…”

“…Recent interest in helium-oxygen chemistry has been stimulated by applications in chemical lasers (where excitation of the O 2 (a 1 ∆ u ) state is the primary concern) and biomedicine [132,139,77,81,100,142,50,95]. As we have already noted, plasma chemistry models are typically derived by complicated paths, and we will not try to follow these historical developments in detail.…”

Graphical Classification in Multi-Centrality-Index Diagrams for Complex Chemical Networks

“…In low-temperature reactive plasma, high-energy electrons trigger a number of simultaneous dissociations of mother molecules, and its chemistry is more complex than other chemical systems in artificial environments for chemical plants [10][11][12]. After definitions of a node (for one species) and an edge (for each reaction) for a display in a graph, centrality indices of nodes derived from the graph work as representatives of chemical roles in the system, such as agents, intermediates, and products.…”

“…Another example of graphical network approaches for medium-sized chemical complexity was on numerical calculations of rate equations in plasma-enhanced chemical reactions, and the calculated results were visualized in reaction pathways in a graph to summarize complicated time evolutions of densities of species [15]. In comparison with the previous achievements based on numerical finite-difference methods [10][11][12]15], direct visualization of reactions based on graph theory is applied here, and we focus on graphical classification of nodes or species on statistical aspects that are missing in our previous studies [8,9].…”