Wonchul Lee

Optical pumping studies of vibrational energy transfer in high-pressure diatomic gases

Spontaneous Raman scattering is used to experimentally determine the vibrational distribution functions of diatomic species in N2/CO and N2/CO/O2 gas mixtures optically pumped by a CO laser in the pressure range 410–760 torr. In N2/CO mixtures, as many as 38 vibrational levels of CO are observed, in addition to six levels of N2. The CO vibrational distribution function is highly non-Boltzmann, exhibiting the well-known Treanor plateau. In N2/CO/O2 mixtures, up to 13 vibrational levels of O2 are observed, which also exhibit a highly non-Boltzmann distribution. Experimental data are compared to predictions of a master equation kinetic model, which incorporates absorption of the laser radiation, species, and quantum state-specific vibration–vibration and vibration–translation energy exchange, as well as diffusion of vibrationally excited species out of the laser-excited volume. It is shown for the first time that modest power continuous wave lasers can be used to establish highly excited steady-state vibrati...

Vibrational energy storage in high pressure mixtures of diatomic molecules

Abstract CO/N 2 , CO/Ar/O 2 , and CO/N 2 /O 2 gas mixtures are optically pumped using a continuous wave CO laser. Carbon monoxide molecules absorb the laser radiation and transfer energy to nitrogen and oxygen by vibration–vibration energy exchange. Infrared emission and spontaneous Raman spectroscopy are used for diagnostics of optically pumped gases. The experiments demonstrate that strong vibrational disequilibrium can be sustained in diatomic gas mixtures at pressures up to 1 atm, with only a few Watts laser power available. At these conditions, measured first level vibrational temperatures of diatomic species are in the range T V =1900–2300 K for N 2 , T V =2600–3800 K for CO, and T V =2200–2800 K for O 2 . The translational–rotational temperature of the gases does not exceed T =700 K. Line-of-sight averaged CO vibrational level populations up to v =40 are inferred from infrared emission spectra. Vibrational level populations of CO ( v =0–8), N 2 ( v =0–4), and O 2 ( v =0–8) near the axis of the focused CO laser beam are inferred from the Raman spectra of these species. The results demonstrate a possibility of sustaining stable nonequilibrium plasmas in atmospheric pressure air seeded with a few percent of carbon monoxide. The obtained experimental data are compared with modeling calculations that incorporate both major processes of molecular energy transfer and diffusion of vibrationally excited species across the spatially nonuniform excitation region, showing reasonably good agreement.

Enhancement of spectral purity of injection-seeded titanium:sapphire laser by cavity locking and stimulated Brillouin scattering

We report improvements to and better characterization of the spectral purity of a diode laser injection-seeded, cavity-locked titanium sapphire laser that serves as the source for a previously reported rubidium vapor spectrally filtered Thomson scattering apparatus at 780.24 nm. In a detailed set of measurements the spectral purity P of the laser, defined as the ratio ofthe narrowband component of the laser output to the total output, has been studied as a function of frequency mismatch between the seed laser frequency and the central frequency of the unseeded cavity. It is found that spectral purity exceeding 0.999 can be obtained for a seed-cavity mismatch as high as +/- 0.25 nm, corresponding to approximately 950 cavity longitudinal-mode spacings and as high as approximately 0.9999 for a cavity-seed mismatch in the range +/- 0.10 nm (380 mode spacings). It is also shown that the addition of an external-cavity stimulated Brillouin-scattering phase-conjugate mirror increases both the spectral purity, to a minimum of 0.99999, and the cavity-seed mismatch range, to +/- 0.25 nm, for which this maximum effective purity is obtained.

Spectrally Filtered Raman/Thomson Scattering Using a Rubidium Vapor Filter

A new spectrally e ltered light scattering apparatus is presented based on a diodelaser injected seeded titanium: sapphire laser and rubidium vapor e lter at 780.24 nm. It is shown that the realizable line center attenuation of quasi elastically scattered light, limited by a residual broad spectral linewidth, unseeded, component to the laser output, isashigh as 1 :67£10 5 using the laser system alone. Preliminary measurementsindicatethat incorporation of a set of dispersing prisms and a stimulated Brillouin scattering phase conjugate mirror external to the laser provides an additional approximate factor of 10 extinction. The utility of the system for measurement of electron density and temperature by Thomson scattering is demonstrated in a 3950-Pa (30-torr) argon dc discharge. At 100-mA current, an electron density of 3 :7£1013 cmi3 is measured on the discharge centerline with a 2 ae value of statistical uncertainty equal to 8 :0£10 11 cm i3 . The corresponding electron temperature is 0 :63§ 0:025 eV.

Radio frequency energy coupling to high-pressure optically pumped nonequilibrium plasmas

This article presents an experimental demonstration of a high-pressure unconditionally stable nonequilibrium molecular plasma sustained by a combination of a continuous wave CO laser and a sub-breakdown radio frequency (rf) electric field. The plasma is sustained in a CO/N2 mixture containing trace amounts of NO or O2 at pressures of P=0.4–1.2 atm. The initial ionization of the gases is produced by an associative ionization mechanism in collisions of two CO molecules excited to high vibrational levels by resonance absorption of the CO laser radiation with subsequent vibration-vibration (V-V) pumping. Further vibrational excitation of both CO and N2 is produced by free electrons heated by the applied rf field, which in turn produces additional ionization of these species by the associative ionization mechanism. In the present experiments, the reduced electric field, E/N, is sufficiently low to preclude field-induced electron impact ionization. Unconditional stability of the resultant cold molecular plasma ...

Mitigation of electron attachment to oxygen in high pressure air plasmas by vibrational excitation

A series of time resolved microwave attenuation measurements are performed of the electron number density of an electron beam generated, CO laser excited nonequilibrium O2∕N2 plasma. Resonant absorption of infrared radiation from the CO laser produces the nonequilibrium state, in which the heavy species vibrational modes are disproportionately excited, compared to the rotational and translational modes (Tvib≈2000–3000K vs TR∕T≈300K). It is shown that this results in an increase in the plasma free electron lifetime by two orders of magnitude compared to the unexcited cold gas, an effect which is ascribed to complete mitigation of rapid three-body electron attachment to molecular oxygen. A series of heavy species filtered pure rotational Raman scattering measurements are also presented, which exhibit minimal temperature change (+50K), indicating that the observed lifetime increase cannot be due to heavy-species thermal effects. Finally, computational modeling results infer an increase in the rate of O2− det...

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.

RUBIDIUM VAPOR FILTER SPECTRALLY FILTERED THOMSON SCATTERING

Improvements have been made to a spectrally filtered Thomson scattering apparatus which consists of an external cavity diode laser injection-locked titanium:sapphire laser and an optically thick rubidium vapor filter at 780 nm. In a detailed set of measurements the spectral punty, P, of the laser, defined as the ratio of the narrow band component of the laser output to the total output, has been studied as a function of circulating seed power. Incorporation of a feed back loop to “lock” the titaniwn:sapphire laser cavity to the cw seed frequency and addition of a Stimulated Brillouin Scattering Phase Conjugate mirror results in a measured spectral punty in excess 0.99999. These improvements have enabled Thomson scattering measurements to be performed with the vapor filter temperature reduced from 320 “C to 270 “C, corresponding to a decreased filter linewidth by a factor of approximately three. The utility of the improved system for measurement of electron density and temperature by Thomson scattering was demonstrated in an argon dc constricted glow discharge. At 100 mamps current and 30 torr of argon an electron density of 6.75 x 1013 cm-3 was measured on the discharge center line with a 20 value of statistical uncertainty equal to 0.4 x lo’* cms3. The corresponding electron temperature was measured to be 0.27 _+ 0.036 eV .

Determination of O2 and N2 vibrational state distributions in CO laser-sustained plasmas

Spontaneous Raman scattering is used to experimentally determine the Vibrational Distribution Function (VDF) of diatomic species in CO laser-sustained non-equilibrium plasmas in the pressure range 200 740 torr. In NJCO plasmas, as many as 40 vibrational levels of CO’are observed, in addition to 7 levels of N2. The CO VDF is highly non-Boltzmann. exhibiting the well-known Treanor plateau. In N&O/O2 plasmas, 9 vibrational levels of O2 are observed, which also show a non-Boltzmann distribution. Experimental data are compared to predictions from a kinetic master equation model, which incorporates effects of non-uniform pump laser distribution, molecular energy transfer, and species and thermal diffusion.