Generation and Microwave Scattering Diagnostics of Small Volume Plasmas

This dissertation focuses on the development of novel generation and microwave scattering diagnostic techniques for small volume plasmas. The small volume plasmas presented in this work fall under the two generalized categories: 1) laser-induced plasmas and 2) non-equilibrium microdischarges.

Chapter I presents the application of microwave scattering theory to laser-induced breakdown in air. The MIE solution to Maxwell’s equations is employed to reveal three distinct phases of the evolution of the laser-induced breakdown in air. Chapter II presents a novel method of quantifying thresholds for laser-induced breakdown. These thresholds are established via total electron number measurement from dielectric calibration of microwave scattering. Chapter III presents high-repetition-rate (HRR) nanosecond laser pulse train scheme for laser ignition. Demonstration of the ignition of combustible gaseous mixtures is shown to have an order-of-magnitude reduction in per-pulse energy using the HRR LI method over traditional laser ignition methods.

Chapter IV presents ion-kinetic measurements of a laser induced plasma in sodium-argon and sodium-air gaseous mixtures. Coherent microwave Rayleigh scattering (Radar) from Resonance Enhanced Multi-Photon Ionization (REMPI) is utilized for the measurement of sodium ion neutral stabilized and cluster dissociative recombination rates. Chapter V presents rotational temperature measurements in a DC microdischarge produced in air. Radar REMPI measurements of O₂ rotational temperature is performed at eight axial locations between pin-to-pin electrodes. Chapter VI presents relative concentration measurements of atomic oxygen in DC and pulsed Discharges. Relative atomic oxygen concentrations were obtained via Radar REMPI. The effects of pressures, gas composition, and discharge voltage were explored for the DC and pulsed discharges. Comparisons between two-photon absorption laser induced fluorescence (TALIF) and Radar REMPI techniques were made for atomic oxygen concentration measurements in a pulsed discharge. Chapter VII presents a method of reducing the breakdown voltage of a DC microdischarge via metal nanoparticle seeding. Reductions in the breakdown voltage were seen to be as high as 25% for a PD scaling of 40 Torr-cm from the seeding of iron and aluminum nanoparticles into the discharge gap.