J. Rich

Kinetic Modeling of the High‐Power Carbon Monoxide Laser

A model for the kinetics of the cooled direct‐discharge‐excited carbon monoxide laser is presented. The kinetic mechanism responsible for creating the observed population inversions cannot be explained by simple one‐step resonance transfer between an excited metastable and the CO molecule, in view of the many vibrational bands which lase in this system. The present paper analyzes a kinetic model of the CO laser which includes the following processes: (a) Vibration‐to‐vibration (V‐V) energy exchange among the anharmonic vibrational states occurring in CO–CO collisions. (b) Resonance electron impact excitation of the lower CO vibrational states. (c) Radiative decay of the CO vibrational states. (d) Collisional quenching of vibrational excitation in CO–He collisions. Using a Morse anharmonic oscillator model of the CO vibrational states, kinetic equations are formulated which govern the individual vibrational state populations, subject to the preceding processes. The resulting set of nonlinear algebraic equa...

Low-Temperature Supersonic Boundary Layer Control Using Repetitively Pulsed MHD Forcing

The paper presents results of magnetohydrodynamic (MHD) supersonic boundary layer control experiments using repetitively pulsed, short pulse duration, high voltage discharges in M=3 flows of nitrogen and air in the presence of a magnetic field of B=1.5 T. We also have conducted boundary layer flow visualization experiments using laser sheet scattering. Flow visualization shows that side wall boundary layers in the supersonic test section are considerably thicker near the center plane of the flow. The results also show that as the Reynolds number increases from Rex=2.7·10 5 to 8.1·10 5 , the boundary layer flow becomes much more chaotic, with the spatial scale of temperature fluctuations decreasing. Combined with density fluctuation spectra measurements using Laser Differential Interferometry (LDI) diagnostics, this behavior suggests that boundary layer transition occurs at stagnation pressures of P0~200-250 torr. Operation of a crossed discharge (pulser + DC sustainer) in M=3 flows of air and nitrogen demonstrated that such a discharge produces a stable, diffuse, and uniform plasma. The time-average DC current achieved in such discharges is up to 1.0 A in nitrogen (conductivity of σ=0.073 mho/m) and up to 0.8 A in air (σ=0.072 mho/m). The electrical conductivity and the Hall parameter in nitrogen and air flows are inferred from the current voltage characteristics of the sustainer discharge. LDI measurements detected MHD effect on the ionized boundary layer density fluctuations at these conditions. Retarding Lorentz force applied to M=3 nitrogen, air, and N2-He flows produces an increase of the density fluctuation intensity by up to 2 dB (about 25%), compared to the accelerating force of the same magnitude. The effect is demonstrated for two possible combinations of the magnetic field and current directions producing the same Lorentz force direction (both for accelerating and retarding force). Comparison with the LDI spectra measured with no MHD force applied showed that the effect on the density fluctuations is produced only by the retarding Lorentz force, while the Joule heat effect appears insignificant.

Overtone bands lasing at 2.7–3.1 μm in electrically excited CO

Lasing on the first overtone vibrational‐rotational transitions of carbon monoxide is reported. The laser is a supersonic flow laser, which uses an electric‐discharge‐excited CO/He/O2 mixture. 20 W cw power at 0.6% electrical efficiency is obtained.

Kinetics of Nitric Oxide Formation Behind Shock Waves

The infrared radiation of nitric oxide (NO) behind a shock wave in O2-N2 mixtures has been calculated by two different techniques, and compared with recent shock-tube experiments. The first technique (model I) utilizes the Park model. This model incorporates the vibrational relaxation of O2 and N2 and assumes a Boltzmann distribution of vibrational energy during the relaxation process. Model II uses a master equation solution, employing recently published state-to-state vibration-translation and vibration-vibration transition probabilities. Vibration-chemistry coupling is provided through the MacheretFridman-Rich model (MFR). The calculations are compared with experimental results for shock waves in the range of 3-4 km/s. Results of the two model calculations are compared at speeds up to 9 km/s, for both normal shocks and bow shocks. The two models predict nearly the same NO production rates behind all of the normal shocks, and show the prominent effect of N2 vibrational coupling in the reaction N2 + O —> NO + N. For high-altitude bow shocks, where extreme vibrational nonequilibrium is present, there are large differences in the results calculated by the Park and MFR coupling techniques.

An Electrically Excited Gas‐Dynamic Carbon Monoxide Laser

A carbon monoxide laser is reported which utilizes a glow discharge in the plenum of a supersonic nozzle. The discharge selectively excites the CO vibrational mode, while the gas translational temperature remains relatively cold. Continuous output is obtained from optical cavities established transverse to the flow at two nozzle area ratios. Maximum laser power obtained to date is 6.8 W corresponding to an efficiency of 0.6%, based on electrical power input.

Nitrogen Vibrational Population Measurements in the Plenum of a Hypersonic Wind Tunnel

Picosecond coherent anti-Stokes Raman scattering is used for measurement of nitrogen vibrational distribution function in the plenum of a highly nonequilibrium Mach 5 wind-tunnel incorporating a high-pressure pulser– sustainer discharge. First-level vibrational temperatures of the order of 2000K are achieved in the 300 torr non-selfsustained plasma discharge generated by a high E=n ( 300 Td) nanosecond-pulsed discharge, which provides ionization in combination with an orthogonal low E=n ( 10 Td) dc sustainer discharge, which efficiently loads the nitrogen vibrational mode. It is also shown that operation with the nanosecond-pulsed plasma alone results in significant vibrational energy loading, withTv N2 of the order of 1100K.Downstream injection ofCO2, NO, andH2 results in vibrational relaxation, demonstrating the ability to further tailor the vibrational energy content of the flow. N2-NO vibration–vibration andN2-H2 vibration–translation rates inferred from these data agreewell with previous literature results to within the uncertainty in rotational-translational temperature.

Vibration‐vibration pumping of carbon monoxide initiated by an optical source

A steady‐state nonequilibrium population distribution of the vibrational states of carbon monoxide has been produced by optical pumping. A supersonic flow CO laser was used to excite a flowing mixture of CO which is pressure broadened by argon. A V‐V pumped nonequilibrium distribution was measured over the first 30 vibrational states of CO.

Shock wave propagation in weakly ionized plasmas

Shock propagation into weakly ionized gases show several features differing markedly from conventional, non-ionized gas, shock structure. Phenomenological analysis of general macroscopic features of the previously observed plasma shock effects allows only two possible interpretations: (i) existence of an energy (momentum) flux toward the wave precursor; (ii) volumetric energy release (exothermic phase transition) in the upstream portion of the wave (precursor) followed by the reverse transition in the downstream portion of the wave. It is shown that known microscopic mechanisms are not capable of producing such a flux or energy release: typical processes involving electrons, ions, and excited species do not couple strongly to neutral atoms and molecules, and there is not enough energy stored in these species because of the low ionization fraction. The theoretical basis for phase transitions in low-density nonequilibrium plasmas is also unknown. Analysis of the steady two-wave system created by either of the two effects raises a question as to whether the observed plasma shocks are stable objects. Another question raised is whether there exists phase transition within the plasma shock. Finally, the contribution of thermal nonhomogeneity of the plasma to the experimentally observed shock wave distortion remains unknown. Answering these three fundamental questions requires additional experimental and theoretical studies of the problem.

Picosecond CARS Measurements of Vibrational Distribution Functions in a Nonequilibrium Mach 5 Flow

The design and implementation of a new picosecond Coherent Anti-Stokes Raman Scattering (CARS) spectroscopy instrument for measurement of nitrogen Vibrational Distribution Function (VDF) in a highly nonequilibrium Mach 5 flow is described. First level vibrational temperatures of the order of 2000 K are achieved in the 300 Torr nonself-sustained plasma wind tunnel plenum, generated by a high E/n (300 Td) nanosecond pulsed discharge, which provides ionization, in combination with an orthogonal low E/n (~10-30 Td) DC sustainer discharge, which efficiently loads the nitrogen vibrational mode. It is also shown that operation with the nanosecond pulsed plasma alone results in significant vibrational energy loading, with Tv of the order of 1100 K. Downstream injection of CO2, NO, and H2 results in vibrational relaxation, demonstrating the ability to further tailor the vibrational energy content of the flow.

MITIGATION OF OXYGEN ATTACHMENT IN HIGH PRESSURE AIR PLASMAS BY VIBRATIONAL EXCITATION

We present a series of measurements of the temporal evolution of electron density in high pressure, room temperature pulsed e-beam generated molecular plasmas. We show that vibrational excitation, to Tvib of order 2,000 – 3000 K, results in nearly complete mitigation of direct O2 attachment, which is the principal free electron loss process under equilibrium conditions. Spatially and temporally resolved temperature measurements, performed using spectrally filtered pure rotational Raman scattering, indicate heavy species rotational/translational temperature is only slightly increased as a result of vibrational excitation, to approximately 350 K. Kinetic modeling of two limiting cases, accelerated detachment and inhibited attachment, suggests that two non-equilibrium mechanisms may be playing a role simultaneously: i), detachment enhancement by collision of O2 - (or other negative) ions with vibrationally excited neutrals, and ii), attachment inhibition due to electron heating by superelastic collisions with vibrationally excited neutrals.