Current Research Projects

TALIF of Neutral Xenon in a Hall Thruster Plasma

Neutral particle dynamics in the plume of a Hall Effect thruster play an important role in the device’s performance and lifetime. In particular, charge exchange reactions (CEX) between neutral particles and ions lead to sputtering of the thruster walls. CEX occurs more frequently in vacuum facilities where the pressures remain higher than that of space. To account for facility effects during lifetime tests, the density of neutral particles must be known. We are using Two-Photon Absorption Laser Induced Fluorescence (TALIF) to perform these measurements.

Cavity Enhanced Thomson Scattering

Interest in Hall thruster technology has continued to grow with emphasis on the development of high-power long life thrusters and improved understanding of plasma characteristics. Electrons play a key role in these devices; however available electron diagnostics are limited. To aid in the development of physics-based plasma models, improved capabilities for measuring electron number density (ne) and the electron energy distribution function (EEDF) are needed. Our group is developing Cavity Enhanced Thomson Scattering (CETS) to perform these measurements.

Aerosol Characterization with Laser-Induced Breakdown Spectroscopy

The occupational health hazards associated with the inhalation of large airborne particles (20+ μm in diameter) are primarily determined by the chemical constituents of the particle. Elemental composition analysis can be performed in real time with laser-induced breakdown spectroscopy (LIBS), a process that involves sample ablation within a plasma generated by a high power laser pulse and inspection of the resulting emission spectrum.

Pre-Ionization Controlled Laser Plasma Formation

Laser based ionization sources have proven to have many benefits over conventional techniques such as spark plugs and igniters. However, plasma properties and associated ignition characteristics are largely uncontrollable. We are investigating the separation of the two stages of laser plasma formation (multiphoton ionization and electron avalanche ionization) by using two temporally resolved lasers operating at different wavelengths. By separating the ionization processes in this manner, we are able to access a wider range of plasma properties that were previously inaccessible. We then employ several diagnostic techniques such as Rayleigh and Thomson scattering to make 1-D measurements of the electron density and gas temperatures.

Cavity Ring-Down Spectroscopy of a LaB6 Hollow Cathode

We are performing in situ Cavity Ring-Down Spectroscopy (CRDS) to measure La density in a LaB6 hollow cathode. The measurement includes both La neutrals and ions and will help validate existing plasma models of the cathode. Additionally, quantification of the La density will provide a more fundamental understanding of the erosion product transport and will inform future design and operation of LaB6 cathodes to meet the demanding requirements for future deep space missions.

Mobile Open-Path CRDS for Atmospheric Methane Measurement

The background concentration of atmospheric methane has been steadily rising over the past decade. Anthropogenic sources such as agriculture and the oil and gas industry account for 60% of Methane emissions into Earth’s atmosphere. Our project works on the development of a mobile methane sensor that uses open-path cavity ring-down spectroscopy in order to accurately measure methane plumes caused by anthropogenic sources.

Past Research Projects

Laser Based Sensor for Hall Thruster Erosion Measurements

Hall thrusters experience sputter erosion in the insulator channel, which is the primary factor that limits the lifetime of the thruster. It is crucial to develop accelerated testing capabilities allowing rapid measurements of low sputter erosion rates. Continuous wave cavity ring-down spectroscopy (CW-CRDS) is an ultra-sensitive laser absorption technique which is well suited to make measurements of low density sputtered boron nitride, the material of interest for Hall thrusters.

Ultraviolet Prism Based Cavity Ring-Down Spectroscopy

Conventional CRDS uses high reflectivity mirrors to enhance the absorption path length of the laser. The sensitivity of this measurement process is strongly dependent upon the mirror quality. However, sufficiently high quality mirrors are not available for UV light that is resonant with the boron-nitride. Our approach used calcium fluoride (CaF2) prisms, based on the principle of total internal reflection, to establish a high quality optical cavity.

Laser Measurement of Combustion Pollutants in Algae-Derived Biodiesel

Fatty acid methyl esters are molecules which comprise a liquid motor fuel commonly referred to as biodiesel. Biodiesel has been implemented on a small scale as a replacement for petroleum-derived fuel for compression ignition engines. What is unclear when comparing biodiesel and petroleum diesel, are the relative rates of production of nitrogen oxide pollutants. Through the use of laser spectroscopy we conduct experiments to compare the pollutants emitted by biodiesel produced from algae to that produced through classical means.

Sensitive and Versatile Tropospheric HCl Instrument

The role of tropospheric hydrogen chloride (HCl) in atmospheric halogen chemistry is largely unknown, however it is speculated to be important in storage mechanisms involving oxides of nitrogen (NOx) dark reactions as well as a source of atomic chlorine in marine pollution. In order to enable field measurements of HCl, which is generally present at ambient concentrations of ~1 ppbv or less, our group is developing a sensitive cavity ring-down spectroscopy (CRDS) laser-based instrument targeted at HCl.

Fiber Optic Delivered Laser Ignition Systems

By tightly focusing the beam from a high-power pulse laser to form a combustion-initiating spark in flammable mixtures including those used in engines, we accomplish what is called laser ignition. Our research area is primarily in large (megawatt class) stationary gas engines that are typically used for power generation and natural gas compression. We demonstrate the use of fiber optics for laser ignition of natural gas engines.

Cavity Enhanced Instrument for Aerosol Optical Properties

Aerosols play critical roles in the atmosphere influencing visibility, air quality, and radiative forcing. Currently, the imprecision in the data, and the lack of data at different wavelengths and for certain types of aerosols, seriously limits the accuracy and precision of model predictions which in turn limits the ability to make appropriate policy decisions. A cavity enhanced absorption spectroscopy (CEAS) laser-based instrument for measurement of aerosol extinction has been developed and undergone a preliminary characterization.

Quantum Cascade Laser Sensor for Engine Emissions (NO)

Recently, mid-infrared (MIR) Quantum Cascade Lasers (QCLs), which are becoming increasingly commercially available, have been used as light sources for sensors measuring NO, NO2, CO and other species. Our group developed a QCL based laser sensor for NO measurement for gas engine applications. The QCL laser sensors allow species-specific measurements free of interferences and, in comparison to previously developed near-infrared (NIR) sensors, allow stronger signals (improved sensitivities).

LIF Studies of Xe Velocity Distributions in Plasma Sheaths

This research sought to develop a method to characterize the plasma potential distributions near ceramic surfaces representative of those used in the discharge channels of HETs. Laser Induced Fluorescence (LIF) studies have proven useful in measuring sheath potentials by measuring ion velocities at various points within the sheath. By measuring the Doppler shifted frequency at which the moving ions absorb the incident photons, one can obtain velocity distribution function (VDF) data and thus calculate the potential through which the ion was accelerated.

Hollow Core and Photonic Crystal Fibers for High Power Pulse Delivery

A key challenge for practical laser ignition systems is the need for fiber-optic delivery. Our interest is to investigate fiber delivery of high-peak-power megawatt pulsed laser beams in a way that allows spark formation (in the gas phase) after exiting the fiber. This capability would have general application for the ignition of many other combustion devices in which laser ignition may have applicability (turbines and aircraft).

Cavity Ring-Down Spectroscopy for Sputter Measurements and End-Point Detection

Sputtering is the process in which an energetic bombarding particle is incident on a material and causes the ejection of atoms, ions, and/or molecules from a surface. It has many applications including thin-film deposition, etching and analytical techniques. It also plays an important role in spacecraft propulsion, since sputtering can damage the thruster and cause re-deposition on other spacecraft surfaces. We employ the cavity ring-down spectroscopy (CRDS) technique to help study sputtering characteristics

Engine Air-Fuel Ratio from Laser Induced Breakdown Spectroscopy (LIBS)

Lean combustion strategies are commonly employed to reduce NOx in stationary large-bore natural gas engines and turbines; however, overly lean conditions can result in degraded engine performance and higher misfire rates. A feedback scheme based on fast measurements of individual cylinder equivalence ratio would allow the users to operate each cylinder at its optimum equivalence ratio. We demonstrate the first simultaneous use of laser sparks for laser ignition of a single cylinder natural gas engine and in-cylinder cycle-resolved air-fuel ratio measurement.

Quartz Crystal Microbalance Based Sputter Measurements

Through the use of a Quartz Crystal Microbalance (QCM), we measure the differential sputter yield profile of a material over a hemisphere above the target. The QCM allows us to record sputter yield as a function of angle over the target from +90° to -90° with respect to the target normal. These individual data points are then collected and fitted, typically with a modified Zhang Equation, which can then be integrated to find an overall total yield.

Boron Nitride Sputtering Measurements

Ion sputtering is a primary life-limiting mechanism in electric propulsion thrusters used for satellite and space exploration. We are interested in the sputtering of boron nitride (BN) because of its widespread use as an acceleration channel wall material in stationary plasma thrusters (SPTs). Despite the importance of BN erosion there is a lack of basic sputtering data on BN. A high sensitivity quartz crystal microbalance (QCM) based sputter measurements system was used to acquire this data.

Plasma-Enhanced Combustion in an Oil Burning Furnace

Typical well-conditioned oil burners provide 80-90% efficiency in terms of transferring energy stored in the heating oil to usable heat, however, older burners can have efficiencies well below these standards. With the collaboration of Clean Diesel LLC, a start-up clean energy company out of Massachusetts, their design of a Plasma-Enhanced Ignition System is to be tested and developed to increase the efficiency of oil burners.

Laser Bone Drilling for Improved Allografts

Several medical procedures utilize allografts from a donor for orthopedic reconstruction. Unfortunately, the healing time required for the host to completely accept the allograft is lengthy. The donor tissue is at risk of biological rejection/ infection and susceptible to stresses or possible fracturing. To effectively minimize these risks, the allograft’s surface-area to volume ratio should maximized. We studied the advantages of laser drilling as a possible solution to increase the surface-area to volume ratio on bone samples.