Research

LIBS is a chemical analysis technique whereby a high intensity laser pulse is used to vaporize, ionize and excite the elementary components of a material sample which may be in gas, liquid or solid phase. Our lab is developing LIBS as a diagnostic for small particles and droplets between 1 and 100 microns in size. Metal particles in this size range are found in the air as a byproduct of machining and laser drilling processes and may be hazardous if inhaled. Droplets in this size range are produced by atomization of liquid fuels sprays in diesel engines, turbine engines, rocket engines and liquid fueled scramjets. We are developing a novel approach to measuring the laser-particle interaction by suspending the particle or droplet in an electrodynamic balance (EDB). This approach was first reported by Anderson [1] and provides well-controlled experimental conditions. A double-ring EDB has been designed, built and tested for this purpose, as shown in the figure at right. Shadowgraph and Mie scattering images of a levitated droplet are shown as insets.

The reentry of spacecraft into the atmospheres of earth, Mars and other planetary bodies is characterized by hypersonic flows with real gas effects and tremendous heat loading. These conditions are simulated on the ground by arc jet facilities, such as the NASA Ames Interaction Heating Facility (IHF) arc jet. However, the flow produced in such ground test facilities is substantially different than at high altitudes, as a consequence of the arc plasma heating and non-equilibrium expansion process. We are developing a non-intrusive, laser-based diagnostic technique to determine temperature, velocity and Mach number in the free-stream flow. The technique is based on thermal acoustic wave (TAW) generation by laser heating deposition. An example of the acoustic wave generation process is exhibited in the figure at right, which shows the gas density profile around a region of heating generated by a focused femtosecond laser. Our research effort is focused on understanding the plasma-, fluid- and thermo- dynamics of this process and overcoming practical barriers to application. The successful completion of this project will result in an improved understanding of the non-equilibrium flow field, heat transfer and model uncertainties, ultimately improving the testing and design of advanced thermal protection systems developed by NASA and private industry.

Increasingly, researchers have been investigating plasmas as a solution to a variety of Aerospace engineering challenges, from flow control to combustion to chemical processing. In these devices, the electrons and ions that constitute the plasma generate body forces (thrust), heating and chemical reactions. The nature of these effects can change depending on the plasma properties, namely the temperature and density of electrons. In many cases, the temperature of electrons may be different than that of the ions, neutrals and internal degrees of freedom of molecules (e.g. vibrations and rotations). Having a capability to non-intrusively measure the electron density and temperature is therefore desirable for assessing the plasma state. For a number of applications, we are applying Thomson scattering (electromagnetic wave scattering from free electrons) as a diagnostic for both low and high pressure plasmas. Especially in the latter case (e.g. arcs, laser plasmas) conventional diagnostics such as Langmuir probes are not suitable. In our lab we are developing specialized filtering techniques, multiple-wavelength probing schemes, and new instruments to measure Thomson spectra in both coherent and incoherent scattering regimes.

Figure: Rayleigh and Thomson scattering from a laser generated plasma in air. Thomson scattering occurs within the luminous plasma region.

In hypersonic flight and reentry, the strength of shock waves can be so intense that the nitrogen and oxygen molecules in the air dissociate into their atomic constituents: N and O atoms. Dissociation may also occur in arc discharges and other plasma devices used to generate or control flow-fields. Because of the reactivity of N and especially O atoms, determining the degree of dissociation is important for predicting the chemical reactions in plasmas and near surfaces. We are developing a non-intrusive, laser-based diagnostic technique for measuring molecular dissociation based on the polarization state of Rayleigh and Rotational Raman scattering. This technique is being validated in high temperature air and oxygen for future applications in the Interaction Heating Facility (IHF) arc jet at NASA Ames.

Figure: Profile of polarized (parallel) and depolarized (perpendicular) scattering intensity, showing the location of an argon jet in air.