Elke Ploenjes

Shock Wave Control by Nonequilibrium Plasmas in Cold Supersonic Gas Flows

Experimental studies of shock modification in weakly ionized supersonic gas flows are discussed. In these experiments, a supersonic nonequilibrium plasma wind tunnel, which produces a highly nonequilibrium plasma flow with the low gas kinetic temperature at M = 2, is used. Supersonic flow is maintained at complete steady state. The flow is ionized by a high-pressure aerodynamically stabilized dc discharge in the tunnel plenum and by a transverse rf discharge in the supersonic test section. The dc discharge is primarily used for the supersonic flow visualization, whereas the rf discharge provides high electron density in the supersonic test section. High-pressure flow visualization produced by the plasma makes all features of the supersonic flow, including shocks, boundary layers, expansion waves, and wakes, clearly visible. Attached oblique shock structure on the nose of a 35-deg wedge with and without rf ionization in a M = 2 flow is studied in various nitrogen-helium mixtures. It is found that the use of the rf discharge increases the shock angle by 14 deg, from 99 to 113 deg, which corresponds to a Mach number reduction from M = 2.0 to 1.8. Time-dependent measurements of the oblique shock angle show that the time for the shock weakening by the rf plasma, as well as the shock recovery time after the plasma is turned off, is of the order of seconds. Because the flow residence time in the test section is of the order of 10 μs, this result suggests a purely thermal mechanism of shock weakening due to heating of the boundary layers and the nozzle walls by the rf discharge. Gas flow temperature measurements in the test section using infrared emission spectroscopy, with carbon monoxide as a thermometric element, are consistent with the observed shock angle change. This shows that shock weakening by the plasma is a purely thermal effect. The results demonstrate the feasibility of both sustaining uniform ionization in cold supersonic nitrogen and airflows and the use of nonequilibrium plasmas for supersonic flow control. This opens a possibility for the use of transverse stable rf discharges for magnetohydrodynamic energy extraction and/or acceleration of supersonic airflows.

Isotope Separation in Optically Pumped Thomson Discharges1

Powerful carbon monoxide (CO) gas lasers are used to prepare mixtures of vibrational-mode-excited CO gas in optical absorption cells, by resonance absorption of the laser radiation. The CO in such cells has high vibrational mode energy content, O[0.1] eV per molecule, while the translational-rotational mode energies remain low. In isotopic mixtures containing 13CO and 12CO, it has been shown that the heavier-weight component contains considerably more vibrational energy. Data and theory for this effect are reviewed. The vibrationally excited CO gas ionizes by an associative mechanism, to yield positive molecular ions consisting of species represented by the general chemical form of (CO)2Cn+, where n can be a small integer, 0<n<15.Ad.c. electric potential is applied to plate electrodes positioned on either side of the optically pumped gas. It is found that copious amounts of the positive ions are collected on the negative electrode within minutes. In a related series of experiments, the vibrational mode of liquid phase mixtures of 13CO and 12CO is excited by the same laser pumping technique. At the low translational mode temperature (88 K) of the liquid mixture, the disequilibrium between the vibrational mode energy content of the two isotopic components is extreme. Measurements show almost all energy going into the heavy component. The kinetic mechanisms for these isotopically selective effects are analyzed and discussed. We discuss the potential of both the gas and liquid phase processes for the physical separation of species enriched in the stable rare isotopes, notably 13C, I7O, and 18Q

CHARACTERIZATION OF ELECTRON-MEDIATED VIBRATION- ELECTRONIC (V-E) ENERGY TRANSFER IN OPTICALLY PUMPED PLASMAS USING LANGMUIR PROBE MEASUREMENTS

The paper discusses the results of experiments in CO laser pumped CO-Ar and CO-N2 plasmas, with and without O2 and NO additives. Ionization in these strongly nonequilibrium plasmas occurs by an associative ionization mechanism, in collisions of two highly vibrationally excited CO molecules. The experiments show that removal of the electrons from the optically pumped plasmas using a saturated Thomson discharge results in considerable reduction of the UV/visible radiation from the plasma (CO 4 positive bands, NO γ bands, CN violet bands, and C2 Swan bands). At some conditions, the removal of electrons results in a nearly complete extinguishing of the UV/visible glow of the plasma. This effect occurs even though electron removal results in an increase of the high vibrational level populations of the ground electronic state CO(XΣ, v~15-35). On the other hand, deliberate electron density increase by adding small amounts of O2 or NO to the optically pumped CO-Ar plasmas produced substantial increase of the UV/visible radiation intensity, which strongly correlates with the electron density. The results of the present experiments suggest that V-E energy transfer processes in optically pumped plasmas may be mediated by electrons, which are created in the absence of an electric field, with low initial energies. Most importantly, this effect occurs at ionization fractions as low as ne/N~10 -10. Electron density measurements using microwave absorption and Langmuir probe diagnostics are compared and the electron temperatures in the optically pumped plasmas are determined for the first time.

High-power CO lasers and some recent applications

The electric-discharge-excited carbon monoxide laser is one of the most efficient laser sources known that is scalable to very high continuous wave powers. We review work at Ohio State where such lasers are used to excite flowing molecular gas plasmas, in mixtures of CO and other diatomic gases, including air. These plasmas are stable, diffuse, and can be operated at high gas pressures and low gas kinetic temperature. They are being employed for various plasma chemistry applications. Recent results are presented in which such an optically pumped plasma reactor is used to synthesize single-walled carbon nanotubes, which show surprising order and alignment.

Electron-beam sustained plasmas in optically pumped atmospheric pressure air

Summary form only given, as follows. Stable, low-power-budget, atmospheric pressure air plasmas are sustained using a combination of an electron beam as an efficient volume ionization source. Simultaneous optically pumped vibrational nonequilibrium of the diatomic air species is maintained to reduce the electron removal rate. The electron production rate in the plasmas is measured using a Thomson discharge probe and electron densities are determined from conductivity measurements as well as using microwave attenuation. Energy is transferred into the vibrational modes of the diatomic air species using resonant absorption of CO laser radiation by small amounts (1-5%) of carbon monoxide seeded into the air with subsequent vibration-vibration energy transfer between carbon monoxide and the other diatomics. Perpendicular to the laser beam axis a 0-80 keV, 0-20 mA electron beam enters the plasma cell through an aluminum foil window. The e-beam is operated in pulsed mode with a low duty cycle to reduce thermal stress on the foil window. At the intersection of the laser and the electron beam a plasma region with a volume of approximately 1 ccm is formed. We present measurements of the dependence of the electron removal rate on the level of vibrational excitation within the plasma. The power budgets required for the electron beam ionization and for maintaining a vibrational nonequilibrium by optical pumping as well as the total power budget required to sustain the cold atmospheric pressure air plasmas will be presented.