Full Project Title: Near-Surface Cusp Confinement of Micro-Scale Plasma
Funding: AFOSR Young Investigator Program
Magnetic cusp confinement of plasma at conducting surface involves interactions between
- a highly divergent magnetic field,
- plasma species (i.e., ions, electrons, neutral particles),
- and pre-sheath and sheath conditions that develop along the surface boundary.
Large plasma devices have benefitted greatly by using permanent magnet cusps for bulk plasma confinement for both terrestrial and space applications; however, the magnetic cusp confinement mechanisms and the associated plasma dynamics very near the conducting surface are poorly understood. This lack of understanding has prevented researchers of micro-scale discharges from fully realizing the benefits of cusp confinement and control at smaller scales, and has also prevented the optimization of larger discharges.
The behavior of plasma near the cusp is especially important for small discharges (less than 10 cm in diameter) since the cusp field very near the conducting anode wall represents a large percentage of the overall discharge volume. Additionally, a significant benefit for miniature plasma applications comes from the inherent property that magnetic field strength increases significantly with decreasing distance between magnets B ~ 1/r3). Small discharges can easily have relatively large magnetic fields throughout the discharge due to the small dimensions of the discharge. If this inherent advantage is exploited properly, extremely efficient micro-scale discharges may be created if sufficient knowledge is obtained about the near field confinement capabilities of permanent magnets.
The first miniature noble gas discharge was developed by Wirz and showed promising performance and efficiency, leading to the development of the 3 cm diameter Miniature Xenon Ion (MiXI) thruster. Analysis of the MiXI discharge showed that the plasma within a magnetically confined miniature discharge behaves differently than conventionally sized plasma due to the interaction of the plasma with the strong magnetic fields required for efficient operation. The overall intent of this research is to investigate the confinement mechanisms of permanent magnet electron and plasma discharges as they apply to micro-scale discharges, on the order of only ~1 cm in diameter. This research employs a combination of both experimental and computational analysis to ensure an in-depth understanding of the dominant mechanisms at play in these discharges. The knowledge and tools derived from this combined experimental and computational analysis will be used to develop efficient micro-scale discharge devices using permanent magnets. This research will pursue the following sequence of specific objectives:
- Determine confinement properties of a single permanent magnet cusp as they relate and differ from existing theory for electromagnetic cusps.
- Investigate the confinement behavior of multiple discrete magnets arranged in a multi-magnet geometry of like polarity. Compare with results from a single magnet of similar orientation.
- Investigate the confinement between two cusps of opposite polarity. Determine the importance of parallel and perpendicular diffusion of plasma at increasing distance from the cusps.
- Use analysis from the above objectives to develop an efficient micro-scale discharge.
Overall Project Approach
The overall approach of the investigation is to combine experimental, computational, and analytical techniques to develop a thorough understanding of the dynamics and structure of cusp-confined plasma near a conducting surface. The experiment utilizes well-defined simple domains and boundary conditions, which allows direct comparisons of the results with computational and analytical models. The complexity of the plasma is increased progressively to ensure the full understanding of the important effects (e.g., collisions between different species, space charge, drift, sheath) at each stage of the effort. The ultimate goal of the project is to develop highly efficient micro-scale plasma devices on the order of only 1 cm in diameter and smaller with the knowledge of micro-scale cusp confinement of plasma gained through the analysis.
Transition of Project Results
This project will provide a significant improvement to the understanding of near-surface cusp confinement and control of plasma. Micro plasma devices developed using the results of this research will provide highly efficient and well-characterized plasma for critical applications. Such devices will enable transformative advancements in several fields, including: plasma processing, plasma-enhanced combustion, spacecraft propulsion, plasma aerodynamics, and other micro plasma applications.
Publications, Presentations & Reports
Wirz R. E., "Near-Surface Cusp Confinement of Micro-Scale Plasma," AFOSR Program Review, Cleveland, OH, Nov 30, 2011.
Wirz R. E., Araki S. J., Dankongkakul B., "Near-Surface Cusp Confinement for Weakly Ionized Plasma," 48th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, Atlanta, GA, July 29-August 1, 2012.
Wirz, R., "Plasma Propulsion and Renewable Energy Activities in UCLA's Wirz Research Group", UCLA MAE Research and Technology Review, April 27, 2012