Tat Loon Chng

New diagnostic methods for laser plasma- and microwave-enhanced combustion

The study of pulsed laser- and microwave-induced plasma interactions with atmospheric and higher pressure combusting gases requires rapid diagnostic methods that are capable of determining the mechanisms by which these interactions are taking place. New rapid diagnostics are presented here extending the capabilities of Rayleigh and Thomson scattering and resonance-enhanced multi-photon ionization (REMPI) detection and introducing femtosecond laser-induced velocity and temperature profile imaging. Spectrally filtered Rayleigh scattering provides a method for the planar imaging of temperature fields for constant pressure interactions and line imaging of velocity, temperature and density profiles. Depolarization of Rayleigh scattering provides a measure of the dissociation fraction, and multi-wavelength line imaging enables the separation of Thomson scattering from Rayleigh scattering. Radar REMPI takes advantage of high-frequency microwave scattering from the region of laser-selected species ionization to extend REMPI to atmospheric pressures and implement it as a stand-off detection method for atomic and molecular species in combusting environments. Femtosecond laser electronic excitation tagging (FLEET) generates highly excited molecular species and dissociation through the focal zone of the laser. The prompt fluorescence from excited molecular species yields temperature profiles, and the delayed fluorescence from recombining atomic fragments yields velocity profiles.

Absolute concentration measurements of atomic oxygen in a flame using radar REMPI

This paper presents an extension of previous work aimed at developing radar REMPI as a combustion diagnostic. The absolute atomic oxygen concentration in a methane/air flame is measured via the use of a noble gas, xenon as a vehicle for calibration. The atomic oxygen concentration is varied by changing the equivalence ratio of the flame. Insight gained from a previous study has established xenon as a viable candidate for calibration. Xenon possesses a two-photon intermediate resonance which lies in close proximity to that of atomic oxygen and furthermore, good signal linearity with concentration in a flame has also been demonstrated. Implementation of this calibration scheme finds decent agreement with the expected equilibrium values under stoichiometric conditions but exceeds these numbers in the fuel rich and fuel lean regimes when the atomic oxygen concentration is an order of magnitude lower. Laser photolysis is examined as a possible reason for this disagreement and initial experiments suggest that molecular oxygen and carbon dioxide are two main photolytic precursors.

Towards Quantitative Flame Species Concentration Measurements Using Radar REMPI

This paper details recent efforts aimed at the development of the Radar REMPI technique as a combustion diagnostic. Radar REMPI, which combines the benefits of coherent microwave scattering and the selective ionization capabilities of resonance enhanced multiphoton ionization (REMPI), has been largely successful when used to measure stable species concentrations under room temperature conditions. As an initial step to extending this diagnostic to a combustion system, a well studied methane/air flame over a Hencken burner under atmospheric conditions is seeded with varying trace amounts of an inert gas, Xenon. Our results show that under calibrated conditions, decent signal linearity with target species concentration down to 310 ppm is achievable. To account for the presence of multiple species expected in a flame, complementary experiments are conducted in a cell at room temperature to establish and isolate the effect of different buffer gases on the signal. The cell experiments suggest the existence of electron loss processes which take place on timescales that are not adequately captured by our detection system. These processes occur even at buffer species concentrations which are typical of that found in flames, and if neglected, can impact the accuracy of the measurement. Overall, these experiments augur well for the development of Radar REMPI as a combustion tool, and in particular the use of an inert gas as a calibration approach. However, significant efforts should continue to be directed at both identifying key flame species that may have a large influence on the electron loss rates as well as characterizing and improving the resolution of the detection system.

Towards remote magnetic anomaly detection using Radar REMPI

We demonstrate remote trace detection of Xe in air using microwave scattering off plasma induced by resonant laser ionization. For the purpose of magnetic detection we propose using isotopic Xe rotationally polarized via double resonant three photon pumping.

Electric field measurements in a near atmospheric pressure nanosecond pulse discharge with picosecond electric field induced second harmonic generation

We present an optical electric field measurement method for use in high pressure plasma discharges. The method is based upon the field induced second harmonic generation technique and can be used for localized electric field measurements with sub-nanosecond resolution in any gaseous species. When an external electric field is present, a dipole is induced in the typically centrosymmetric medium, allowing for second harmonic generation with signal intensities which scale by the square of the electric field. Calibrations have been carried out in 100 Torr room air, and a minimum sensitivity of 450 V/cm is demonstrated. Measurements were performed with nanosecond or faster temporal resolution in a 100 Torr room air environment both with and without a plasma present. It was shown that with no plasma present, the field follows the applied voltage to gap ratio, as measured using the back current shunt method. When the electric field is strong enough to exceed the breakdown threshold, the measured field was shown ...

Nanosecond time-resolved 2 + 2 Radar REMPI measurements performed in molecular nitrogen

Many molecular interactions associated with combustion, high speed gas physics and plasma processes involve dynamics associated with molecular non-equilibrium phenomena. Radar REMPI (Resonance-Enhanced Multi-Photon Ionization) is a potential method for measuring local, time accurate non-equilibrium conditions. It uses multi-photon ionization to generate electrons and ions from selected atomic or molecular states. The ionization is resonantly enhanced when the multi-photon progression passes through an intermediate state. Thus, REMPI is state and species specific. Radar REMPI detects the ionization by microwave scattering and provides a remote, non-intrusive means of probing a physical system. This work is focused upon using Radar REMPI to measure population distributions in molecular nitrogen. Current measurements, made under equilibrium conditions, agree well with model predictions. Future work will focus on optimizing the accuracy of the diagnostic and probing a rotational and/or vibrational non-equilibrium environment.

Remote Sensing in Atmospheric Air using Nonlinear Optics

We study nonlinear optical effects using ultrafast pulses in atmospheric air, and show how they lead to backwards air lasing, femtosecond air tagging via multiphoton dissociation, and electric field measurements using second harmonic generation.

Hydrogen Backward Lasing in Atmospheric Air for Remote Detection

We present atomic hydrogen backwards lasing in ambient air. The 656nm stimulated emission follows two-photon excitation of atomic hydrogen obtained from dissociation of water or other H-containing molecule, allowing for standoff optical gas detection.

Methods for Enhancing Radar REMPI Sensitivity

Radar REMPI is a promising technique and alternative to other laser diagnostics such as laser-induced fluorescence, Raman scattering or coherent anti-Stokes Raman scattering. While measurements in nitric oxide illustrated the ability of the technique to capture trace species, the sensitivity ultimately depends upon the experimental configuration and background interference from other species with overlapping transitions. 2+2 radar REMPI measurements performed in molecular nitrogen were limited to densities at or above 10 cm−3. This paper looks at methods for increasing the sensitivity of the technique. Near-field enhancement of the scattered signal was observed to produce a factor of 20 signal enhancement. However, this enhancement was found to be very sensitive to alignment and was difficult to achieve consistently. Alternatively, a double pass configuration was used and found to increase the signal level. Therefore, a multi-pass configuration has the potential for offering an increase in the sensitivity of the technique. This approach may be useful for measurements of trace species in an atmospheric flame, where remote detection is not required.

Towards species concentration measurements for a pulsed subcritical microwave-enhanced flame

Pulsed microwaves are coupled to methane/air laminar ame fronts in a stagnation ame con guration. The energy deposited in the ame front is measured using forward and re ected microwave power measurements. In order to examine the e ect of this pulsed energy addition to the ame front which results in large temperature increases locally, we utilize radar REMPI to measure relative changes in the postame nitric oxide concentration. For microwave coupling e ciency of greater than 50 percent, the nitric oxide concentration can as much as double over the non-microwave enhanced ame. This result has important implications for the application of pulsed microwaves for lean ame enhancement and sustaining de agrations at very lean conditions. In addition, it provides insight on the mechanism by which the microwaves interact with the ame front. Progress on measuring atomic oxygen concentrations with radar REMPI in adiabatic laminar ames is also presented.