Production of O2(Δ1) in flowing plasmas using spiker-sustainer excitation

In chemical oxygen iodine lasers (COILs), oscillation at 1.315μm in atomic iodine (P1∕22→P3∕22) is produced by collisional excitation transfer of O2(Δ1) to I2 and I. Plasma production of O2(Δ1) in electrical COILs (eCOILs) eliminates liquid phase generators. For the flowing plasmas used for eCOILs (He∕O2, a few to tens of torr), self-sustaining electron temperatures, Te, are 2–3eV whereas excitation of O2(Δ1) optimizes with Te=1–1.5eV. One method to increase O2(Δ1) production is by lowering the average value of Te using spiker-sustainer (SS) excitation where a high power pulse (spiker) is followed by a lower power period (sustainer). Excess ionization produced by the spiker enables the sustainer to operate with a lower Te. Previous investigations suggested that SS techniques can significantly raise yields of O2(Δ1). In this paper, we report on the results from a two-dimensional computational investigation of radio frequency (rf) excited flowing He∕O2 plasmas with emphasis on SS excitation. We found that the efficiency of SS methods generally increase with increasing frequency by producing a higher electron density, lower Te, and, as a consequence, a more efficient production of O2(Δ1).