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.

High-speed, compressible flow of Argon through a converging-diverging micronozzle is examined using the Direct Simulation Monte Carlo (DSMC) method in this paper. The upstream and downstream conditions are chosen so as to yield Knudsen numbers (either based on the constant transverse dimension of the throat or on the local transverse dimension of the micronozzle) in the transition regime. The structure of a shock wave in the transition regime in a micronozzle is examined for a total pressure P o~1 Torr, total temperature T o =298 K, and exit pressure P exit =0.2 Torr, for the two limiting cases of diffuse walls (no-slip) and specular walls (maximum slip). DSMC results indicate that for these conditions, there is a discernible shock in the diverging portion of the micronozzle in the case of specular walls, reminiscent of shocks in continuum internal flows, while a spread-out shock occupies much of the diverging portion in the case of diffuse walls. The fact that wall-interactions can influence the flow-field and shock structure in such a dramatic fashion presents an opportunity to tailor the performance of micronozzles either through the use of novel coatings or by choice of micronozzle material, for a given gas.

We develop the theory and application of small tip angle NMR techniques that can be used to measure couplings between nuclear spins and multiple neighboring spins. We employ the techniques to measure indirect ͑electron-mediated͒ nuclear spin-spin couplings between neighboring 63,65 Cu and 17 O nuclear spins in the high-temperature superconductor YBa 2 Cu 3 O 7 . Predictions for the values of these couplings can be obtained from the existing phenomenological models of electronic spin susceptibility (q,) and hyperfine couplings that have been used in attempts to understand the wide body of YBa 2 Cu 3 O 7 NMR data gathered to date. We find that the measured couplings are incompatible with these models.

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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: