Comprehensive Study of Plasma-Wall Sheath Transport Phenomena
Sponsored by AFOSR, researchers from Georgia Tech (GT), University of Alabama (UA) and George Washington University (GWU) are conducting a highly integrated, cumulative research program to develop an understanding of the plasma-wall interaction and how both the sheath and the wall material affect the plasma as a whole. The research will study the most fundamental aspects of mass, charge, and energy transport in the plasma sheath, the relationship between plasma properties and wall surface modification as a function of wall material, and finally develop new material design methodologies.
GT High-Power Electric Propulsion Laboratory (HPEPL)
Plasma sheaths are typically extremely thin, with thicknesses commonly in the microns because the electron density is high. To alleviate the difficulties of resolving the plasma sheath structure, initial fundamental studies will be performed in a low-density plasma cell with a very large sheath. This cell will allow diagnostic development and scaling of the data to real sheaths. Efforts will extend to non-intrusive transport property measurement techniques with high spatial resolution.
GT Research Institute (GTRI)
Throughout this five-year program, GTRI will conduct several materials-centric experimental studies to ascertain the fundamental behavior of wall materials in response to surface modifications and other influences. It is a goal of this area of work to provide transformative, basic research in materials design and processing to enable radical alternations to the plasma-sheath behavior, which would be used to reduce wall degradation and control energy transfer from the plasma.
GT Materials Modeling (GTMM)
While a Hall Effect thruster operates, the channel walls erode over time as they are exposed to the energetic plasma. The erosion of the channel walls limits the lifespan of hall effect thrusters. Life testing of Hall Effect thrusters is a major undertaking, involving thousands of hours of vacuum chamber time. Because of this, improving modeling of channel wall erosion can allow reduced life testing, and prediction of performance under varying thrust and Isp conditions over the course of a mission. Current erosion models are empirical curve fits to erosion depth over time, or models reduced from certain numerical experiments. They are usually two dimensional, and do not capture certain physics which become apparent during some long qualification life testing. The focus of the current effort is in understanding the physics behind channel wall erosion, including microstructural effects which may seed larger scale erosion variations.
University of Alabama (UA)
Within the program, the University of Alabama group works with the corresponding team members in providing microstructural characterization of the boride nitride material exposed to the various plasma conditions. The characterization work is aimed at identifying and quantifying critical instabilities within the microstructures. This includes surface and near-surface modifications to the topology and mechanical strength. This information is then fed into the microstructure modeling effort to elucidate dominate mechanisms that led to erosion. The materials are characterized using scanning electron microscopy, transmission electron microscopy, atomic force microscopy and nano-indentation. While the experimental testing apparatus is being brought on-line, the UA team has explored the microscopy preparation techniques and influence of ion implantation into the boride material. Specimens have been implanted with various fluencies of Ga+ and characterized.
George Washington University (GWU)
- Development of the multi-scale plasma simulation
- Sheath simulations in EP environment
- Plasma-material simulations including sputter yield, SEE, current dependence, energy dependence, angle effect, and temporal behavior of the sputtering process
- Model validation by direct comparison with experimental data obtained at Georgia Tech