Oxygen Atom Measurements in a Nanosecond Pulse Discharge by Two Photon Absorption Laser Induced Fluorescence1

摘要

Two Photon Absorption Laser Induced Fluorescence (TALIF) is used to measure time-resolved absolute atomic oxygen concentrations in nitrogen-oxygen nanosecond pulse plasmas. The plasma is initiated using a 40-50 kV peak voltage, 5 nsec duration pulses generated at a low pulse repetition rate, 10 Hz, in order to study the nascent kinetics of a single pulse discharge event at a low temperature. Relative atomic oxygen concentration data is put on an absolute scale using a calibration method in which the O atom signal intensity is compared to reference TALIF spectra of xenon, obtained using identical laser beam focusing, fluorescence collection, fluorescence spectral filtering, and photomultiplier detector gain. Calibrated TALIF spectra show peak atomic oxygen concentrations generated by a single pulse in N2/O2 mixtures in the range (1-3)TO14 cm3 at total pressures of P=40-60 torr. This corresponds to an O atom mole fraction of (0.5-1.5)-10"4. The peak O atom density in air at P=60 torr, -3-1014 cm"3, assuming 5.2 eV energy cost per dissociate O2 molecule, would require ~1 mJ energy per pulse for uniform dissociation within the entire pulsed discharge volume. The pulse energy coupled to the load, estimated from the high voltage probe measurements, is about 2 mJ/pulse. The O atom decay in air occurs on a time scale of-1-2 msec. The results of kinetic modeling calculations, which are in satisfactory agreement with the experimental data, identify key processes of O atom generation and decay in the nanosecond pulse plasma. Future work will focus on performing O atom and flow temperature measurements operating the plasma generator in the pulse-burst mode, to detect the accumulation of atomic oxygen concentration in the flowing plasma reactor and plasma chemical reactions with hydrocarbon fuel species.