Stand-Off Gas Phase Diagnostics by Microwave Detection of Laser Generated Ionization

Abstract

Laser diagnostics have been critical for making gas-phase measurements in fields ranging from combustion to plasma physics. Measurement parameters of interest include gas velocity, chemical composition and microscopic population distributions. A number of laser diagnostics have been developed to measure these parameters of interest. These include diagnostics such as particle imaging velocimetry (PIV), laser doppler velocimetry (LDV), Rayleigh/Raman scattering, coherent anti-Stokes Raman scattering (CARS), laser induced fluorescence (LIF) and a number of others. This thesis discusses a new class of laser based diagnostic. This approach uses a laser to generate ionization at a point of interest and radar to remotely detect the ionization which, depending upon how and where it is produced, has some diagnostic utility. Two specific diagnostics are addressed in this thesis. The first, known as Laser Ionization Tagged Radar Anemometry (LITRA) is used to estimate the velocity of an airflow. With this approach, a nanosecond laser pulse is used to non-resonantly ionize the air at a point of interest within the air flow. This ionization is visible to microwave radar and its motion is tracked remotely as it moves in the air flow. Heterodyne detection techniques are used to measure the phase of microwaves scattered from the ionization. This phase varies as the ionization moves with the flow and measurements of this phase shift yield information on the flow velocity. Experiments were conducted in various wind tunnel facilities to test this approach and evaluate diagnostic performance. The second approach discussed in this thesis is known as radar REMPI. For this approach, a laser is used to resonantly ionize, through multi-photon absorption, a particular atom/molecule of interest. The number of free electrons produced, measured remotely via microwave scattering detection, is proportional to the resonant species concentration. In addition to being species selective, this approach is also state selective, permitting measurement of population distributions within a given species. Experiments demonstrating the application of radar REMPI to molecular nitrogen were conducted. Specifically, rotational population distributions were measured under atmospheric pressure conditions. Interesting new collision-induced spectroscopic features were also identified using this approach.