Airless bodies in space are exposed to a variety of charging environments in which a balance of currents due to plasma processes determines the surface charge. In the inner solar system, photoelectron emission is the dominant charging process on sunlit surfaces due to the intense solar UV radiation. This results in a positive surface potential with a photoelectron sheath above the surface. Conversely, the unlit side of the body will charge negatively due the collection of the fast-moving solar wind electrons. The interaction of charged dust grains with these positively and negatively charged surfaces, and with the photoelectron and plasma sheaths, may explain the occurrence of dust lofting, levitation and transport above the lunar surface and on other airless bodies. This dust has been recognized as a potentially great hazard to future exploration of dusty planetary surfaces, due to its abrasive and adhesive nature.
An initial investigation explores the mechanisms that control adhesion of dust grains to insulating and conducting surfaces. Unfortunately, there is little known about the mechanisms of adhesion on widely varying surface types, but van der Waals and electrostatic forces are the dominant forces that are taken into consideration in this study, which measures the adhesive forces between ≤ 25 μm JSC-1 lunar simulant grains and various surfaces vacuum using a centrifugal force detachment method. UV irradiation effects on surface adhesion were also examined.
In order to better understand the plasma processes at work on sunlit surfaces, we have performed laboratory experiments to study the physics of photoelectron sheaths above both conducting and insulating surfaces in vacuum. The first set of experiments determines the characteristics of photoelectron sheaths generated over a conducting Zr surface that is large in comparison to the Debye length of the sheath. These characteristics are derived from cylindrical Langmuir probe measurements, and are compared with the results from a 1D PIC-code simulation to gain a greater understanding of the sheath physics. To study the photoelectron sheath above an insulating material, a portion of this conducting surface is covered with insulating material. CeO 2 is used both in powdered and solid disk form, and un-sieved JSC-1 is used to represent planetary surfaces. Electron densities and temperatures of the photoelectron plasma are measured with a single-sided planar Langmuir probe. The measurements taken above the CeO2 are compared with those taken above the Zr to observe the differences in photoemission, and to determine how the insulating surface modifies the structure of the photoelectron sheath. The densities above the surfaces are only found to have a modest dependence on the surrounding surface bias, and the plasma potentials measured above the insulating surfaces are significantly different than those above the Zr, due to the fact that the insulating materials float to an equilibrium potential independent of the surrounding surface bias. These measurements indicate that plasma probes above a planetary body can accurately determine potentials and densities above the surfaces, valuable information for understanding the charging environment of spacecraft and other objects.
|Commitee:||Bagenal, Frances, Brain, David, Burns, Jack O., Horanyi, Mihaly, Robertson, Scott|
|School:||University of Colorado at Boulder|
|Department:||Astrophysical and Planetary Sciences|
|School Location:||United States -- Colorado|
|Source:||DAI-B 74/02(E), Dissertation Abstracts International|
|Subjects:||Planetology, Astrophysics, Plasma physics|
|Keywords:||Dust, Langmuir probe, Lunar photoelectron, Moon, Photoelectron sheath, Plasma|
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