Current state-of-the-art plasma thrusters are limited in power density and thrust density by power losses to plasma-facing walls and electrodes. In the case of Hall effect thrusters, power deposition to the discharge channel walls and anode negatively impact the efficiency of the thruster and limit the attainable power density and thrust density. The current work aims to recreate thruster-relevant wall-interaction physics in a quiescent plasma and investigate them using electrostatic probes, in order to inform the development of the next generation of high-power-density / high-thrust-density propulsion devices.
Thruster plasma-wall interactions are complicated by the occurrence of the plasma sheath, a thin boundary layer that forms between a plasma and its bounding wall where electrostatic forces dominate. Sheaths have been recognized since the seminal work of Langmuir in the early 1900’s, and the theory of sheaths has been greatly developed to the present day. The theories are scalable across a wide range of plasma parameters, but due to the difficulty of obtaining experimental measurements of plasma properties in the sheath region, there is little experimental data available to directly support the theoretical development.
Sheaths are difficult to measure in situ in thrusters due to the small physical length scale of the sheath (order of micrometers in thruster plasmas) and the harsh plasma environment of the thruster. Any sufficiently small probe will melt, and available optical plasma diagnostics do not have the sensitivity and/or spatial resolution to resolve the sheath region.
The goal of the current work is to experimentally characterize plasma sheaths
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in a low-density plasma that yields centimeter-thick sheath layers. By generating thick sheaths, spatially-resolved data can obtained using electrostatic probes. The investigation focuses on the effects of electron emission from the wall and several factors that influence it, including wall material, wall temperature, wall surface roughness and topology, as well as the scaling of sheaths from the low-density plasma environment towards thruster conditions.
The effects of electron emission and wall material are found to agree with classical fluid and kinetic theory extended from literature. In conditions of very strong emission from the wall, evidence is found for a full transition in sheath polarities rather than a non-monotonic structure. Wall temperature is observed to have no effect on the sheath over boron nitride walls independent of outgassing on initial heat-up, for sub-thermionic temperatures. Wall roughness is observed to postpone the effects of electron emission to higher plasma temperatures, indicating that the rough wall impairs the wall’s overall capacity to emit electrons. Reductions in electron yield are not inconsistent with a diffuse-emission geometric trapping model. Collectively, the experimental data provide an improved grounding for thruster modeling and design.Current state-of-the-art plasma thrusters are limited in power density and thrust density by power losses to plasma-facing walls and electrodes. In the case of Hall effect thrusters, power deposition to the discharge channel walls and anode negatively impact the efficiency of the thruster and limit the attainable power density and thrust density. The current work aims to recreate thruster-relevant wall-interaction physics in a quiescent plasma and investigate them using electrostatic probes, in order to inform the development of the next generation of high-power-density / high-thrust-density propulsion devices.
Thruster plasma-wall interactions are complicated by the occurrence of the plasma sheath, a thin boundary layer that forms between a plasma and its bounding wall where electrostatic forces dominate. Sheaths have been recognized since the seminal work of Langmuir in the early 1900’s, and the theory of sheaths has been greatly developed to the present day. The theories are scalable across a wide range of plasma parameters, but due to the difficulty of obtaining experimental measurements of plasma properties in the sheath region, there is little experimental data available to directly support the theoretical development.
Sheaths are difficult to measure in situ in thrusters due to the small physical length scale of the sheath (order of micrometers in thruster plasmas) and the harsh plasma environment of the thruster. Any sufficiently small probe will melt, and available optical plasma diagnostics do not have the sensitivity and/or spatial resolution to resolve the sheath region.
The goal of the current work is to experimentally characterize plasma sheaths
xxvi
in a low-density plasma that yields centimeter-thick sheath layers. By generating thick sheaths, spatially-resolved data can obtained using electrostatic probes. The investigation focuses on the effects of electron emission from the wall and several factors that influence it, including wall material, wall temperature, wall surface roughness and topology, as well as the scaling of sheaths from the low-density plasma environment towards thruster conditions.
The effects of electron emission and wall material are found to agree with classical fluid and kinetic theory extended from literature. In conditions of very strong emission from the wall, evidence is found for a full transition in sheath polarities rather than a non-monotonic structure. Wall temperature is observed to have no effect on the sheath over boron nitride walls independent of outgassing on initial heat-up, for sub-thermionic temperatures. Wall roughness is observed to postpone the effects of electron emission to higher plasma temperatures, indicating that the rough wall impairs the wall’s overall capacity to emit electrons. Reductions in electron yield are not inconsistent with a diffuse-emission geometric trapping model. Collectively, the experimental data provide an improved grounding for thruster modeling and design.
| Identifer | oai:union.ndltd.org:GATECH/oai:smartech.gatech.edu:1853/54383 |
| Date | 07 January 2016 |
| Creators | Langendorf, Samuel J. |
| Contributors | Walker, Mitchell L. R. |
| Publisher | Georgia Institute of Technology |
| Source Sets | Georgia Tech Electronic Thesis and Dissertation Archive |
| Language | en_US |
| Detected Language | English |
| Type | Dissertation |
| Format | application/pdf |
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