Advanced optical diagnostic techniques relevant to propulsion were investigated. The techniques studied were based on laser spectroscopy, with emphasis on spectrally-resolved absorption and laser-induced fluorescence (LIF). Laser sources included tunable cw near-infrared diode lasers and tunable (or fixed-wavelength) pulsed lasers operated at ultraviolet (UV) or infrared (IR) wavelengths. The cw lasers were spectrally narrow, allowing study of innovative diagnostics based on spectral lineshapes, while the pulsed lasers provided intense bursts of photons needed for techniques based on LIF. Accomplishments of note included: (1) development of a new imaging diagnostic based on infrared planar laser-induced fluorescence (IR PLIF), (2) investigations of quantitative ultraviolet (UV) PLIF of NO and CO2 in high-pressure combustion environments, (3) the development of a new temperature diagnostic using UV absorption of CO2 for high-temperature combustion environments, (4) development of advanced wavelength-multiplexed diode laser absorption sensing of non-uniform temperature distributions, gas temperature in scramjet flows, and tunable mid-IR-based fuel sensing, and (5) further development of quantitative tracers to image fuel distribution using ketones and the aromatic toluene. The full spectrum of results was published in thirty-eight papers in the AIAA and peer reviewed literature, seven PhD theses, and forty-three presentations and invited lectures.

Significant milestones for quantitative laser-based diagnostics of reacting flows were reached on two separate tasks: (1) the extension of laser-based diagnostics to shorter vacuum ultraviolet (vuv) wavelengths and (2) the development of amplified spontaneous emission (ASE) diagnostics for light atoms. For the first task, phase-matching was applied to produce significant increases in vuv laser intensities, producing over 5 mu J at Lyman alpha (121.6 nm) in a mixture of krypton and argon. Factors affecting the long-term stability of vuv powers were studied. The diagnostic potential of two-photon excited ASE of atomic hydrogen and oxygen was explored in a variety of low-pressure flames. Direct ASE gain measurements gave oxygen concentrations. A model of the ASE signal was developed, and a new understanding of both ASE and laser-induced fluorescence of two-photon excited atoms emerged from this model.

The goal of this research is to develop advanced laser-based techniques for non-intrusive measurements relevant to air-breathing combustion. In general, the program emphasizes spectrally-resolved absorption using tunable laser sources and planar laser-induced fluorescence (PLIF), conducted using either near-infrared or ultraviolet laser sources. Detailed below is progress on the exploration of IR PLIF as a diagnostic for imaging IR-active gases and on the suitability of pentanone as an alternative flow tracer to acetone for PLIF imaging. Furthermore, a novel absorption sensor for NO2 at temperatures up to 1900 K has been developed, and continuing work is reported on the spectroscopy of high-pressure gases.

This work has produced extensive sets of new data on low-temperature plasma-assisted fuel oxidation in hydrogen-oxygen-argon and hydrocarbon-oxygen-argon mixtures. The measurements have been made in two different plasma flow reactors, at an initial temperature of 500 K and pressures ranging from 300 Torr to 700 Torr. In both reactors, the plasma is generated by a high peak voltage, ns pulse discharge, operated at high pulse repetition rates (up to 20 kHz). Metastable Ar atom number density distributions in the discharge afterglow are measured by Tunable Diode Laser Absorption Spectroscopy (TDLAS), and used to characterize plasma uniformity. Temperature in the discharge-excited reacting flow is measured by Rayleigh scattering. Two-photon Absorption Laser Induced Fluorescence (TALIF) is used to measured absolute H and O atom number densities. The results are compared with predictions of a kinetic model analyzing reaction kinetics of excited species and radicals generated by the plasma at low temperatures and high pressures. The modeling predictions show good agreement with the data, with the exception of fuel-limited mixtures, when nearly all fuel available in the mixture of reactants is oxidized in the discharge. Kinetic modeling analysis identified dominant processes of generation and decay of atomic and radical species in the discharge and in the afterglow. At the present low-temperature conditions, the effect of chain branching reactions on plasma-assisted fuel oxidation kinetics is insignificant.