Atomic Oxygen Production and Exploration of Reaction Mechanisms in a He‐O₂ Atmospheric Pressure Glow Discharge Torch

Abstract

Summary: This article reports on the performance of a miniature APGD‐t conceived for the production of reactive species participating in bio‐applications. Two operating parameters were varied: the plasma‐forming gas flow rate (0.5–1.5 slm He) and the flow rate of O2 (0–50 sccm), which was injected downstream from the plasma‐forming zone, and which was used as a source of reactive species. The production of reactive species (O) was optimized, and the air entrainment in the plasma jet was minimized for a He gas flow rate of 1–1.5 slm and an O2/He volumetric ratio of 0.3 vol.‐%. A survey of the possible reaction pathways in the afterglow plasma jet suggests that the reactive species present in the plasma afterglow, and possibly reaching a remote substrate, are metastable He(23S), ground state O, OH, O2 (a1Δg), O2 (b1Σg+), N, N2, N 2+ and O3.

Main Species and Physicochemical Processes in Cold Atmospheric‐pressure He + O2 Plasmas

“…Collisions (second term in the right hand side of equation 2) consume more energy than the boundaries (last term in the right hand of equation 2), indicating that more power is coupled to the electrons in the bulk plasma than to ions in the sheaths. This is in agreement with other modeling works of atmospheric pressure plasmas 47. It is interesting to note, however, that, while losses via elastic collisions dominate at low oxygen concentration, losses via inelastic collisions become dominant at high oxygen concentration.…”

“…In many practical scenarios, however, the plasmas are often operated in open air ([O 2 ] air ≈21%), and therefore the ozone generation may be strongly favored in the afterglow. The density of excited atomic oxygen O* is ≈2 orders of magnitude lower than the density of ground state O and it peaks at an oxygen concentration of ≈0.5%, in qualitative agreement with experimental observations 47, 48. On the other hand, the density of excited molecular oxygen O 2 * is comparable to that of O.…”

“…Nonetheless, in pulsed discharges their flux of negative ions should be larger and then O − , O, O and O should also be taken into account. Experimentally, the emission at 777 nm from O( 5 P) and 845 nm from O( 3 P) are usually used to infer the atomic oxygen content in the discharge 47, 48, 51. However, it is noted that in atmospheric pressure He + O 2 plasmas the density evolution of O * and O may not be the same.…”

Inactivation of Staphylococcus aureus in Water by a Cold, He/O2 Atmospheric Pressure Plasma Microjet

“…Emissions at 337.1, 353.7, and 357.7 nm from N 2 second positive system (C 3 Π u –B 3 Π g ) were also clearly observed. This is due to the entrainment of air into the stream and excited by the long lived helium metastables (19.8 eV) via penning excitation 25. Weak ${\rm N}_{{\rm 2}}^{{\rm + }} $ emission at 391 nm was also observed (data not shown).…”

Uncertainty and error in complex plasma chemistry models

“…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.…”