Excitation of O$_{2}(^{1}\Delta )$ in Pulsed Radio Frequency Flowing Plasmas for Chemical Oxygen Iodine Lasers*

In chemical oxygen-iodine lasers (COIL), oscillation at 1.315 $\mu $m ($^{2}P_{1/2} \quad \to \quad ^{2}P_{3/2})$ in atomic iodine is produced by collisional excitation transfer of O$_{2}(^{1}\Delta )$ to I$_{2}$ and I. Plasma production of O$_{2}(^{1}\Delta )$ [eCOIL] is interesting to eliminate liquid phase generators. For the flowing plasmas used for eCOILs (He/O$_{2}$, a few to 10s Torr) self sustaining electron temperatures are 2-3 eV whereas excitation of O$_{2}(^{1}\Delta )$ optimizes with T$_{e}$ = 1-1.5 eV. Lowering T$_{e}$ is of interest to increase system efficiency. One method is the spiker-sustainer (S-S). A high power pulse (spiker) is followed by a lower power quasi-dc period (sustainer). Excess ionization produced by the spiker enables the sustainer to operate with a lower T$_{e}$. Results from global kinetics modeling suggest that S-S can raise yields of O$_{2}(^{1}\Delta )$ to over 30{\%}. In this paper, results from a computational investigation of radio frequency (13, 27, 56 MHz) excited flowing He/O$_{2}$ plasmas will be discussed with emphases on S-S techniques. The model is a 2-dimensional plasma hydrodynamics simulation encompassing a solution of Navier Stokes equations for neutral flow dynamics. The efficiency of S-S methods generally increase with increasing frequency by producing a higher electron density, lower T$_{e}$ and, as a consequence, a more efficient production of O$_{2}(^{1}\Delta )$.