Plasma Excited Chemical-Oxygen-Iodine Lasers: Optimizing Injection and Mixing for Positive Gain

Chemical oxygen-iodine lasers achieve oscillation on the $^{2}$P$_{1/2}\to ^{2}$P$_{3/2}$ transition of atomic iodine at 1.315 $\mu $m by a series of excitation transfers from O$_{2}(^{1}\Delta )$. In electrically plasma excited devices (eCOILs), O$_{2}(^{1}\Delta )$ is produced in a flowing plasma, typically He/O$_{2}$, at a few to tens of Torr. The iodine is injected into the flow as a He/I$_{2}$ mixture immediately upstream (or in) a supersonic nozzle. A small positive gain with I* limited to a narrow boundary layer near the wall indicates slow mixing when the I$_{2}$ is injected from the wall. This results in low utilization of O$_{2}(^{1}\Delta )$. In this paper we discuss results from 1- and 2-dimensional computational investigations of means to optimize gain in eCOILs by using different I$_{2}$ injection strategies. It was found that due to the plasma generated distribution O$_{2}(^{1}\Delta )$, placement of injectors closer to the axis significantly increased gain by facilitating complete O$_{2}(^{1}\Delta )$/I$_{2}$ mixing. This is partly a function of the inlet flow of NO through the discharge which regulates the density of O atoms produced by electron impact dissociation of O$_{2}$. By optimizing the nozzle dimensions, their location, and I$_{2}$ and NO flow rates, the yield of O$_{2}(^{1}\Delta )$ required to achieve positive gain can be minimized.