O2(1Δ) production in high pressure flowing He/O2 plasmas: Scaling and quenching

Chemical oxygen-iodine lasers (COILs) oscillate on the 2P1/2→2P3/2 transition of atomic iodine at 1.315 μm by a series of excitation transfers from O2(1Δ). In electrically excited COILs (eCOILs), the O2(1Δ) is produced in a flowing plasma, typically He/O2, at a few to tens of Torr. Many system issues motivate operating eCOILs at higher pressures to obtain larger absolute densities of O2(1Δ) for a given yield and to provide higher back pressure for expansion. In this paper, we discuss results from a computational investigation of O2(1Δ) production in flowing plasmas sustained at moderate pressures (≤50 Torr). Power deposition and flow rates were scaled such that in the absence of second order effects, yield should be constant and absolute O2(1Δ) production should scale linearly with pressure. We found in many cases that absolute O2(1Δ) production scaled sublinearly with pressure. Ozone is found to be one of the major quenchers of O2(1Δ) and its production increases with pressure. Gas heating also increases with increasing pressure due to exothermic three-body reactions. The gas heating reduces O3 production, increases O3 destruction and, for certain conditions, restores yields. With increasing pressure and increasing absolute densities of atomic oxygen and pooling reactions of O2(1Δ), quenching by these species also becomes important, though the influence of O-atom quenching can be controlled by managing the density of O atoms with additives. The yield of O2(1Δ) is also determined by discharge stability which becomes problematic at higher pressure