- My research focuses on the development and application of optical diagnostics to plasma, combustion and hypersonic systems. Below are projects which I have worked on:
- Vacuum UV emission studies of high temperature radiation
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We present absolute intensity measurements of CO 4th positive emission from a CO2/Ar plasma jet produced using an atmospheric pressure plasma torch facility. The centerline temperature of the plasma jet is approximately 6300 K. A VUV emission spectroscopy system was adapted to the plasma torch facility to measure spectrally resolved emission down to 140 nm. The emission from the plasma in the VUV, UV and near IR is consistent with thermochemical equilibrium. Carbon lines in the UV and VUV are found to be optically thick and their amplitude is therefore substan- tially affected by the line broadening parameters used for the calculation. For the CO 4th positive emission, these measurements test the performance of various models for the electronic transition moment function (ETMF, see below). These ETMFs are used to calculate sets of Einstein coefficients that are used by SPECAIR to calculate the spectrally resolved emission of the plasma for comparison with experiments. We find that the EMTF of Kirby and Cooper and Spielfiedel accurately reproduce the measured emission from 150 - 215 nm.
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- Ultraviolet Raman spectroscopy of N2 in a recombining atmospheric pressure plasma
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We report ultraviolet spontaneous Raman scattering measurements of temperature in a recombining nitrogen plasma. The plasma source is an atmospheric pressure RF torch facility. A N2/Ar mixture is injected into the torch and heated to approximately 6800 K at the torch exit. Raman scattering measurements are in agreement with optical emission spectroscopy measurements of temperature at the torch exit. A 15-cm water-cooled tube is then mounted at the torch exit to rapidly cool the gas and produce a recombining plasma. Raman scattering measurements indicate a temperature of approximately 3300 K at the tube exit, significantly lower than previous emission spectroscopy estimates. The Raman measurements were confirmed via separate Rayleigh scattering measurements. The lack of agreement between emission spectroscopy, which only yields information on excited states, and Raman scattering measurements shows that the plasma is far from equilibrium at the exit of the recombining tube. These experiments provide a basis for studying recombining plasmas which, among other applications, are important for atmospheric reentry applications and plasma processing.
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- Infrared Spectroscopy: Carbon Monoxide production in a high temperature ablative boundary layer
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The design of reliable and cost effective heat shield material is an important consideration for future space missions. Our work looks at quantifying ablative species introduced into a high temperature ablative boundary layer. These species, particularly Carbon Monoxide, are predicted to have a significant impact on the heat transfer to the vehicle surface. We use infrared emission and absorption spectroscopy to provide absolute concentration measurements of ground state Carbon Monoxide. These measurements are documented in our 2016 Journal of Physics D: Applied Physics article..
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The Radar REMPI concept was developed by the Applied Physics Research Group at Princeton University and applied to combustion species. Radar Resonance Enhanced Multi-Photon Ionization, or Radar REMPI, uses a laser to resonantly ionize a specific atom or molecule at a point of interest. The amount of ionization produced is related to the concentration of the resonant species. This ionization is viewed remotely using a microwave radar system, providing a measure of the ionization level and, consequently, the resonant species concentration. Such an approach is being evaluated as an alternative to techniques such as Laser Induced Fluorescence. My work used a 2+2 REMPI scheme to target molecular nitrogen. Experiments were performed at different pressures and temperatures to evaluate diagnostic performance. In addition to using the diagnostic for measurements of rotational temperature, we identified new collision-induced spectroscopic features in molecular nitrogen.
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The ionization level depends on the laser wavelength. On resonance, the amount of ionization dramatically increases and is proportional to the resonant species concentration.
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Microwave scattering (homodyne detection) is used to remotely monitor the ionization level, thereby providing a remote measure of species concentration.
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Microwave scattering time traces are collected. The temporal peak in this trace is due to the time-dependent ionization induced by the laser pulse. The amplitude of the peak depends upon the level of ionization which, in turn, is proportional to the resonant species concentration. The negative signal value has to do with the phase of the microwave signal relative to a reference used to demodulate the microwave signal.
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Rotational spectrum involving a two photon resonance between the ground electronic (X) and excited electronic (a) states. Experimental spectra were compared with theoretical spectra to provide a measure of rotational temperature.
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- Laser Ionization Tagged Radar Anemometry (LITRA)
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The LITRA concept was developed by the Applied Physics Research Group at Princeton University. The concept uses a nanosecond laser pulse to non-resonantly ionize air and generate a laser spark at a point of interest in a supersonic airflow. This spark is visible to microwave radar and serves as a seed particle. As the spark moves in the flow, its motion is tracked using the radar system, providing an estimate of the flow velocity. Figure 1 identifies the primary components of the LITRA diagnostic. The radar system utilizes heterodyne detection, permitting the phase of the microwaves scattered from the ionization to be measured. The output power of this radar detection system is on the order of 10’s of milliwatts. Measuring the phase variation with time permits one to determine the velocity of the spark relative to the radar system. The radar system, which consists of a transmitting and receiving horn, is sensitive to the velocity component which bisects the angle (θ) defined by the transmitting horn, laser ignited plasma and receiving horn. The spark is assumed to faithfully follow the flow, just as seed particles in techniques like Particle Imaging Velocimetry (PIV) are assumed to.
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Experimental configuration for implementation of the LITRA concept. A laser is used to remotely generate a laser spark at a point of interest. Microwave scattering is then used to track the motion of the spark within the flow providing a remote estimate of flow velocity.
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The velocity component measured is that component which bisects the angle between microwave source and detector.
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