Electron removal rate modification in optically pumped plasmas

Abstract

Electron production rate and electron density in optically pumped argon and nitrogen plasmas seeded with carbon monoxide, and with the addition of small amounts of oxygen or nitric oxide have been measured. A Thomson discharge probe and microwave attenuation were used to measure the electron production rate and electron density, respectively. Nonequilibrium ionization in these plasmas is produced by an associative ionization mechanism in collisions of highly vibrationally excited carbon monoxide molecules. Carbon monoxide is excited by optical pumping using a carbon monoxide laser. High vibrational states (up to v 4 0 ) of CO are subsequently populated by vibration-vibration energy transfer. It is shown that adding small amounts of oxygen or nitric oxide (50-100 mTorr) to the baseline gas mixtures at P=100 Torr results in an increase of the electron density by up to a factor of 20-40. This occurs while the electron production rate slightly decreases or remains nearly constant. It is also shown that the electron-ion recombination rates inferred from these measurements decrease by 2 to 3 orders of magnitude compared to the case with no oxygen or nitric oxide added. The overall electron-ion removal rates in the presence of equal amounts of oxygen or nitric oxide additives are nearly equal, which shows that the effect of electron attachment to oxygen at these conditions is negligible. These results suggest a novel method of electron density control in cold laser-sustained steady-state plasmas and afford a means of sustaining stable, high-pressure, nonequilibrium plasmas at high electron densities with a low plasma power budget.References: