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

Summary form only given. Chemical oxygen-iodine lasers (COILs) achieve oscillation on the ²P_1/2rarr²P_3/2 transition of atomic iodine at 1.315 mum by a series of excitation transfers from O₂(¹Delta). In electrically excited COILs, (eCOILs) the O₂(¹Delta) is produced in a flowing plasma, typically He/O₂, at a few to tens of Torr. Many svstem issues motivate operating eCOILs at higher pressures to obtain larger absolute densities of O₂(¹Delta) for a given yield and provide higher back pressure for expansion. Obtaining high yields of O₂(¹Delta) will require careful management of the ozone density [a quencher of O₂(¹Delta)], gas temperature and discharge stability. In this paper, we discuss results from a computational investigation of O₂(¹Delta) production in flowing plasmas sustained at moderate pressures (< 50-100 Torr). This investigation was conducted using plug-flow and 2-dimensional models. In this study, we scaled power deposition and flow rates such that if there are not second order effects, yield should be constant and absolute O₂(¹Delta) production should scale linearly with pressure. We found in many cases that absolute O₂(¹Delta) production scaled sub-linearly with pressure. Ground state and vibrationally excited ozone are found to be one of the major quenchers of O₂(¹Delta) and the production of O₃ is a sensitive function of pressure. Gas heating per molecule increases at high pressures due to exothermic 3-body reactions which 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(¹Delta), quenching by these species can become important in the afterglow. The yield of O₂(¹Delta) is also determined by discharge stability. For the geometries we investigated, discharge constriction becomes problematic at higher pressures, thereby reducing yields.