Control of electron recombination rate and electron density in optically-pumped non-equilibrium plasmas

A two-dimensional time-dependent model of non-self-sustained dc and RF discharges is used for the analysis of recent experiments in optically-pumped non-equilibrium plasmas. The analysis shows that non-self-sustained dc and RF discharges can be successfully used for measurements of the electron production rate, electron recombination rate, and electron density in these plasmas. The inferred rate of electron production per unit volume by the associative ionization mechanism is S 0 = 1.0 × 10 15 cm -3 s -1 and S 0 = 2.2 × 10 14 cm -3 s -1 in CO/Ar = 2/100 and in CO/N 2 = 2/100 plasmas, respectively. The inferred electron-ion recombination rate coefficients are β>6.0 × 10 -6 cm 3 s -1 and β = (7.5±1.5) × 10 -6 cm 3 s -1 in CO/Ar = 2/100 mixtures at P = 100 Torr and 20 Torr, respectively. In CO/Ar = 2/100 mixtures with a 0.05-0.1 Torr admixture of O 2 at P = 100 Torr and 20 Torr, the inferred recombination rate coefficients are β = (1.5±0.3) × 10 -8 cm 3 s -1 and β = (5.1±2.9) × 10 -8 cm 3 s -1 , respectively. Finally, the inferred electron density in optically-pumped CO/Ar/O 2 plasmas at the laser beam axis is n e = (1.7±0.2) × 10 11 cm -3 at P = 100 Torr and n e = (6.2±0.8) × 10 10 cm -3 at P = 20 Torr. These results demonstrate that the electron density in optically-pumped CO/Ar plasmas can be controlled and significantly increased by adding small amounts (up to ~0.1%) of species such as O 2 and NO, which result in the reduction of the electron-ion recombination rate.