Coherent Microwave Scattering from Laser-Generated Plasma in External Magnetic Field and Weakly Ionized Plasma Environments

Abstract

Radar Resonance-Enhanced Multi-Photon Ionization (Radar REMPI) is a remote diagnostic technique which selectively ionizes a gaseous species of interest using a tunable laser to create a REMPI-generated plasma (the “REMPI” part) and probes this plasma via coherent microwave scattering (the “Radar” part) to obtain time-resolved information about the plasma. This dissertation presents a theoretical and experimental investigation of the physics and robustness of Radar REMPI as the diagnostic is extended to new environments (low pressures, magnetic fields, and background plasma) and capabilities (vector magnetic field measurements and remote atomic physics experiments) relevant to plasma propulsion, plasma materials processing, and magnetometry applications. The strengths of Radar REMPI that are useful for these novel environments include: standoff measurement capability, spatial and temporal resolution (on the order of millimeters and nanoseconds respectively), and species-selectivity.The effects of collisions are demonstrated in higher (near atmospheric) pressures via comparison of 0-dimensional kinetic models with experiments, while in lower pressures (< 1 Torr), faster-than-expected decay rates are explained by considering the loss of coherence (decoherence) of the scattered microwaves due to rapid expansion of the laser-generated plasma. Considering the effects of an applied magnetic field led to the discovery of a novel magneto-optical effect: magnetically induced depolarization of the scattered microwaves from the REMPI plasma due to anisotropy in the transport properties of electrons in a magnetic field. This effect introduces a novel capability to Radar REMPI, namely, the standoff measurement of local vector magnetic fields, which would benefit applications involving limited access environments. The introduction of background plasma is shown to result in a substantial increase in ionization in the long-term signal (> 2 μs) after the addition of laser-generated ionization, which can impact some measurement capabilities of Radar REMPI but can also provide increased sensitivity for trace species detection. Lastly, a REMPI scheme using combined femtosecond and picosecond lasers was shown to vary in ionization yield based on the relative polarizations of the two laser pulses, which provides information about the fine structure populations and spin polarization of the excited state and shows the potential use of Radar REMPI to conduct atomic physics experiments remotely.