Laboratory investigations of the near surface plasma field and charging at the lunar terminator

2013-07-31The objective of this dissertation is to study the near surface plasma environment and surface charging environment at the lunar terminator through experimental, analytical and numerical investigations. Specifically, this dissertation investigates: 1. the plasma wake expansion process, 2. regolith surface charging, and 3. regolith grain charging. ❧ In the first area, the potential and density of the plume emitted from a gridded ion source is investigated. A capability for efficient two-dimensional measurements of the plume profile is developed. The effects of plume expansion, plume charge-exchange plasma, and facility background plasma on plume characteristics are quantified. It is found that the propellant charge-exchange plasma is the primary factor that terminates the plume expansion process, and thus largely controls the magnitude of the plume potential with respect to the ambient. The plasma plume wake expansion process is then controlled to experimentally simulate low angle of attack lunar plasma flow and surface charging. The control method is validated by analytical and numerical models. ❧ In the second area, conductors, planar dielectrics, and dust dielectric surfaces are charged under mesothermal plasma flux. Charging experiments are conducted in both the main plasma beam and the simulated lunar plasma environment. The effects of high energy ion flux, low energy ion flux, and low electron flux are quantified as a function of angle of attack, material properties, and surface properties. It is found that a dielectric dust surface will charge to a significantly more positive surface potential than a planar dielectric surface of the chemical same composition and thickness, when the ion and electron flux is of the same order. Further, this effect is found to be driven by ion-induced secondary electron emission and ion charge deposition mechanisms. The high ion-driven potential of dust surfaces compared to planar surfaces may be contributing to lunar terminator region charged dust motion and transport. ❧ In the third area, dusty surface layer individual grain charge is compared to the charge of a single, isolated dust grain at the same potential. Dusty surface layer potentials are recorded experimentally, and individual grain charge is derived using a capacitance system model. The results are compared to isolated grain charge that is calculated using the free space capacitance model and the experimental surface potential. It is shown that the charge of dust grains comprising a dusty surface layer is one to two orders of magnitude lower than the charge of an isolated dust grain. This demonstrates the packed dust grain capacitive effect on dusty surface layers such as the lunar regolith, and will lead to significantly different predictions of electrostatic levitation and dust transport dynamics on the lunar surface