Igor Adamovich: Laser Diagnostics for Measurements of Electric Field and Excited Metastable Species in Nonequilibrium Plasmas and Reacting Flows

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Igor Adamovich
January 18, 2019
12:30PM - 1:30PM
Location
4138 Physics Research Building

Date Range
Add to Calendar 2019-01-18 12:30:00 2019-01-18 13:30:00 Igor Adamovich: Laser Diagnostics for Measurements of Electric Field and Excited Metastable Species in Nonequilibrium Plasmas and Reacting Flows Non-intrusive laser diagnostic measurements of temporal and spatial distributions of electric field and the number densities of excited metastable species in nonequilibrium plasmas are essential for development of engineering applications such as plasma flow control, plasma-assisted combustion, plasma materials processing, and plasma medicine. This talk presents an overview of recent electric field and species measurements in ns pulse discharge plasmas, by ps Four-Wave Mixing (FWM), ps Electric Field Induced Second Harmonic (EFISH) generation, Cavity Ring Down Spectroscopy (CRDS), and Tunable Diode Laser Absorption Spectroscopy(TDLAS) .Picosecond FWM measurements have been done in ns pulse discharges in ambient air, for several electrode geometries. For short voltage rise times of several ns, peak electric field considerably exceed the DC breakdown threshold. Sub-nanosecond time resolution is obtained by monitoring the timing of individual laser shots relative to the voltage pulse, and post-processing four-wave mixing signals saved for each laser shot, placing them in the appropriate “time bins”. The main advantage of EFISH over FWM is that it is considerably more sensitive and species independent, such that it can be used in any high-pressure plasma. Ps EFISH is used to measure electric field in dielectric barrier discharge plasma flow actuators, atmospheric pressure flames enhanced by transient plasmas, and atmospheric pressure plasma jets. In both techniques, electric field vector components are isolated by monitoring signals with different polarizations, and absolute calibration is done by measuring a known Laplacian field. Absolute time-resolved populations of N2(A3Σu+) excited electronic state, which is a major precursor of O atoms and NO in air plasmas, as well as H atoms and other radical species in fuel-air plasmas, are measured in a repetitive ns pulse discharge and the afterglow in nitrogen. Two complementary techniques are used, CRDS (at a relatively low pressure of 11 Torr) and single-pass TDLAS (at an intermediate pressure of 130 Torr). In the afterglow of a low-pressure discharge, N2(A3Σu+,v=0-2) populations exhibit a relatively slow decay on the time scale of about 500 μs. In a medium pressure discharge, N2(A3Σu+,v=0,1) populations decay significantly more rapidly compared to the low-pressure conditions, both between the discharge pulses and in the afterglow, on the time scale of 50-100 μs. In both cases, the decay is likely due to the quenching by the vibrationally excited N2 molecules in the ground electronic state. The results demonstrate considerable potential of laser diagnostic techniques for characterization of high-pressure nonequilibrium plasmas, where they provide quantitative insight into kinetics of ionization, charge transport, molecular energy transfer, energy thermalization rate, and plasma chemical reactions. 4138 Physics Research Building Institute for Optical Science spectroscopy@osu.edu America/New_York public
Description

Non-intrusive laser diagnostic measurements of temporal and spatial distributions of electric field and the number densities of excited metastable species in nonequilibrium plasmas are essential for development of engineering applications such as plasma flow control, plasma-assisted combustion, plasma materials processing, and plasma medicine. This talk presents an overview of recent electric field and species measurements in ns pulse discharge plasmas, by ps Four-Wave Mixing (FWM), ps Electric Field Induced Second Harmonic (EFISH) generation, Cavity Ring Down Spectroscopy (CRDS), and Tunable Diode Laser Absorption Spectroscopy(TDLAS) .

Picosecond FWM measurements have been done in ns pulse discharges in ambient air, for several electrode geometries. For short voltage rise times of several ns, peak electric field considerably exceed the DC breakdown threshold. Sub-nanosecond time resolution is obtained by monitoring the timing of individual laser shots relative to the voltage pulse, and post-processing four-wave mixing signals saved for each laser shot, placing them in the appropriate “time bins”. The main advantage of EFISH over FWM is that it is considerably more sensitive and species independent, such that it can be used in any high-pressure plasma. Ps EFISH is used to measure electric field in dielectric barrier discharge plasma flow actuators, atmospheric pressure flames enhanced by transient plasmas, and atmospheric pressure plasma jets. In both techniques, electric field vector components are isolated by monitoring signals with different polarizations, and absolute calibration is done by measuring a known Laplacian field.
 
Absolute time-resolved populations of N2(A3Σu+) excited electronic state, which is a major precursor of O atoms and NO in air plasmas, as well as H atoms and other radical species in fuel-air plasmas, are measured in a repetitive ns pulse discharge and the afterglow in nitrogen. Two complementary techniques are used, CRDS (at a relatively low pressure of 11 Torr) and single-pass TDLAS (at an intermediate pressure of 130 Torr). In the afterglow of a low-pressure discharge, N2(A3Σu+,v=0-2) populations exhibit a relatively slow decay on the time scale of about 500 μs. In a medium pressure discharge, N2(A3Σu+,v=0,1) populations decay significantly more rapidly compared to the low-pressure conditions, both between the discharge pulses and in the afterglow, on the time scale of 50-100 μs. In both cases, the decay is likely due to the quenching by the vibrationally excited N2 molecules in the ground electronic state.
 
The results demonstrate considerable potential of laser diagnostic techniques for characterization of high-pressure nonequilibrium plasmas, where they provide quantitative insight into kinetics of ionization, charge transport, molecular energy transfer, energy thermalization rate, and plasma chemical reactions.