Date of Award
Summer 9-13-2023
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
A new wave of lunar exploration is underway. The Artemis program aims to place humans on the Moon for the first time since the 1972 Apollo 17 mission. The main objective of the Artemis program is to enable sustained presence on the Moon. A significant inhibitor to such a sustained presence on the Moon is lunar dust. The fine, electrostatically-charged dust mobilizes through various anthropogenic and natural mechanisms, all of which are poorly understood, and adheres to nearly every surface with which it comes into contact. Hence, to ensure nominal long-term operation on the lunar surface, extensive measures must be taken to control and mitigate dust. However, our understanding of dust mobilization processes and the design of dust mitigation strategies is limited due to a poor understanding of the size and charge characteristics of dust. This dissertation applies and advances Earth-based aerosol measurement techniques to measure lunar dust characteristics and charging phenomena. In the first part of the dissertation (Chapter 2), the size and charge characteristics of a popular lunar dust simulant (JSC-1A) were examined in detail. Simulant aerosols were generated via atomization and fluidized bed aerosolization, and size distributions were measured using a Scanning Mobility Particle Sizer and an optical particle counter. Two particulate matter sensors were calibrated with dust aerosols to demonstrate portability of optical techniques. Distinct differences between the size distribution and shape of simulant particles were observed when compared to lunar regolith, highlighting the importance of developing accurate simulants. The second part of the dissertation (Chapters 3 – 5) focuses on optical techniques for in-situ measurement of lofted lunar dust size and concentration. In the first section, five commercial off-the-shelf nephelometers were evaluated and their performance parameters established: sensitivity, limit of detection, saturation limit, accuracy, and precision. Despite having identical optical chambers (e.g., focal length, light source, etc.), the different front-end signal processing yielded different performance parameters, and further, could be tuned for the particulate matter environment of interest. In the second section, the focus was on the measurement of highly concentrated dust aerosols generated from lunar anthropogenic activity such as engine plumes. A dust sensor was calibrated at atmospheric pressure with lunar simulants of varying composition and size distribution, then evaluated for dust concentration measurement in vacuum. Dust mass deposition was predicted with excellent accuracy, whereas number deposition exhibited higher error owing to differences in particle size distributions lofted at vacuum pressures. The third section focuses on measurement of single dust particles in the dilute (~1-10 particles per cubic meter) environment of electrostatically lofted lunar dust. The dust sensor from the previous section was also evaluated for its ability to accurately measure single particles. Limitations of the dust sensor were addressed by developing a novel optical sensor that detects scattered light on both sides of the particle. The newly designed optical sensor exhibited excellent size resolution and sized single particles accurately in high vacuum. The third part of the dissertation (Chapters 6 – 9) aims to examine dust charging processes from theoretical, methodological, and simulation approaches. In the first section, ultrafine dust simulant particles were introduced through an atmospheric pressure glow discharge and soft x-ray irradiation and measured charge distributions. Charging of dust exposed to plasma exhibited dependencies on both simulant composition and plasma density. In the second section, a vacuum chamber was designed and tested to charge and estimate charge-mass ratio of dust particles, a characteristic that determines lunar dust transport. Measured charge-mass ratios were predominantly negative for fine tribocharged particles, with increasing charge distribution spread for larger particles. In the third section, the charging of ultrafine particles in the afterglow of a glow discharge plasma for their removal from industrial exhaust streams was studied. By controlling plasma geometry and aerosol flow rates, control of particle charging efficiency and polarity was demonstrated. In the final section, a Monte Carlo particle-in-cell simulation was used to study the charge evolution of a lofted dust particle. Simulated charge evolutions agreed closely with calculations from the conventional theoretical model (orbit motion-limited theory).
Language
English (en)
Chair
Pratim Biswas