Electron density and recombination rate measurements in CO-seeded optically pumped plasmas

Abstract

Electron production rate and electron density in cold optically pumped CO–Ar and CO–N2 plasmas in the presence of small amounts of O2 and NO have been measured using a Thomson discharge probe and microwave attenuation. Nonequilibrium ionization in the plasmas is produced by an associative ionization mechanism in collisions of highly vibrationally excited CO molecules. It is shown that adding small amounts of O2 or NO (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 (from ne<1010 cm−3 to ne=(1.5–3.0)×1011 cm−3). This occurs while the electron production rate either decreases (as in the presence of O2) or remains nearly constant within a factor of 2 (as in the presence of NO). It is also shown that the electron–ion recombination rates inferred from these measurements decrease by two to three orders of magnitude compared to their baseline values (with no additives in the cell), down to β≅1.5×10−8 cm3/s with 50–100 mTorr of oxygen or nitric oxide added to the baseline CO–Ar mixture, and β≅(2 to 3)×10−7 cm3/s with 75–100 mTorr of O2 or NO added to the baseline CO–N2 mixture. The overall electron–ion removal rates in the presence of equal amounts of O2 or NO additives turn out to be very close, 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 open a possibility of sustaining stable high-pressure nonequilibrium plasmas at high electron densities and low plasma power budget.

Effect of electron density on shock wave propagation in optically pumped plasmas

“…Our previous studies of ionization in CO-Ar optically pumped plasmas 31,32 show that electron densities up to n e ϭ3ϫ10 11 cm Ϫ3 can be sustained at Pϭ100 Torr and CO laser power of 10-15 W.…”

“…Ionization of highly excited CO molecules in the cell occurs by the associative ionization mechanism, in collisions of two highly vibrationally excited molecules when the sum of their vibrational energies exceeds the ionization potential, 29,[31][32][33] CO͑v ͒ϩCO͑ w ͒→͑ CO͒ 2 ϩ ϩe Ϫ ,…”

“…The reason for adding oxygen to the CO-Ar optically pumped plasma is that our previous experiments demonstrated that small amounts of O 2 additive (ϳ0.1%) strongly increase the electron density in the plasma ͑by a factor of 20-50͒. 32 At the same time, previous temperature measurements in the optically pumped plasmas 33 showed that adding such small amounts of O 2 and NO did not result in significant gas temperature changes.…”

“…29,30 The gas pressure and temperature in these plasmas ͑Pϳ100 Torr, T ϳ400-700 K͒, 30 as well as the electron density and electron temperature ͑n e ϳ10 10 -10 11 cm Ϫ3 , T e ϳ0.5 eV͒ 30,31 are similar to those achieved in glow discharges. In addition, our previous experiments 32,33 suggest that the electron density and the gas temperature in such plasmas can be varied independently. This makes optically pumped plasmas an ideal environment for studies of possible effects of ionization on the shock propagation.…”

Supersonic Plasma Flow Control Experiments

“…This is the same Equation used by Blevins et al 69 , Frederick et al, 70 and Funaki et al 71 Another data reduction Equation used for obtaining electron number density values from the amount of microwave energy absorbed is 62,72 : Plots of Eqn. (11) and Eqn.…”