Investigating Plasma Interactions with Multiphase Surfaces using Computational Models
Creators: name: Meyer, Mackenzie
Publisher: name: My University
Year: 2022
Subjects: subject: low-temperature plasmas
subject: plasma physics
subject: electric propulsion
subject: plasma chemistry
subject: atmospheric pressure plasmas
subject: Electrical Engineering
subject: Physics
subject: Engineering
subject: Science

description: Low-temperature plasmas affect multiphase surfaces in contact with the plasma, and the surfaces can in turn affect the plasma. These effects can be destructive (i.e., erosion) or productive (i.e., increased reactive species generation). However, many of these effects are not well understood. In this dissertation, computational modeling of low-temperature plasmas and their interactions with multiphase surfaces is performed to increase understanding and improve systems. One such model, MEOWS, was developed by the author to model erosion of a wire by ion-impact sputtering in the plasma plume of a Hall thruster. The eroded wire profiles from MEOWS were validated against experimental measurements of wire erosion. The sputtering yield models that best fit the measurements were identified. Following validation, distributions of model parameters were generated using a Bayesian approach due to the variation in sputtering yield measurements. MEOWS sampled from the distribution to generate median predictions with credible intervals for the eroded wire profiles. The over-erosion uncertainty in the maximum predicted erosion was demonstrated to be up to 190% of the median maximum predicted erosion, showing the necessity for uncertainty consideration in lifetime estimates. Liquid droplets immersed in the plasma form another multiphase surface examined in this dissertation. The interactions between the atmospheric pressure plasma and the droplet were examined using nonPDPSIM, a 2D plasma dynamics model, and GlobalKin, a 0D plasma chemistry model. Using the results of nonPDPSIM, the sheath that forms around a dielectric droplet immersed in the radio frequency plasma was shown to be asymmetric. The effect of the polarization of the droplet, the sheath electric field, and the bulk electric field led to increased electric field on one equator of the droplet and a decreased electric field on the opposing equator. The charge on the droplet was positive on the poles and negative on the equator. The degradation of the organic compound HCOO-aq was examined using GlobalKin. When HCOO-aq was abundant, OHaq was consumed in reaction with HCOO-aq. If HCOO-aq was substantially reduced before the power turned off, OHaq could decrease due to reactions with HO2-aq and OH-aq, formed as byproducts from HCOO-aq consumption. Pulsed dielectric barrier discharges at atmospheric pressure can generate large quantities of reactive species. O3 production is a common use of these systems. The results of nonPDPSIM showed the maximum in O and O3 followed the electron density maximums. Roughness of the dielectric surface locally enhanced O and O3 density without significant changes in O3 production efficiency. A general surface mechanism was developed and implemented in GlobalKin. The O3 production was maximized at 0.05% N2 in O2 due to adsorbed N occupying surface sites otherwise contributing to O3 destruction. Another application of pulsed dielectric barrier discharges is discharge photoionization detectors, relying on the VUV photons produced by a He discharge. The results of nonPDPSIM showed the lifetime of radiating state He(3P) was shorter than He(21P). He2* was long-lived relative to the pulse length. Different approaches to increase the photon flux to the analyte gas include increasing the capacitance of the surrounding dielectric, moving the electrodes closer to the analyte gas inlet, and adding points to electrodes and additional electrodes. The studies in this dissertation examined low-temperature plasma interactions with a variety of multiphase surfaces using computational models. Better understanding of these interactions was demonstrated and will lead to improvements in real-world systems.