Air Force Grant Funds Fundamental Study of Plasma-Wall Interactions

Researchers at the Georgia Institute of Technology and the University of Alabama have received a $2.5 million grant from the U.S. Air Force Office of Scientific Research (AFOSR) to conduct fundamental research into the ways in which plasmas interact with the walls of the structures containing them. The research will also examine potential improvements to materials used for the walls.

The five-year research program could lead to improvements in a broad range of areas, including higher performance satellite thrusters, improved tubes for Department of Defense radar and communications systems, more efficient high-intensity lamps, and new plasma deposition and spray-coating processes.

The researchers will utilize new analysis techniques, including a terahertz-frequency laser for non-intrusively studying the plasma sheath, which is the portion of the plasmas that interacts with the wall. The researchers will use atomic probe technology to study how the plasmas -- a state of matter that contains ionized particles -- interact with and are affected by the walls. Modeling and simulation techniques will also help predict how plasmas may interact with improved wall materials.

"In these systems, the plasma is dumping energy into the wall, and the wall may be giving back some particles or energy that affect the plasma," explained Mitchell Walker, associate professor in the Georgia Tech School of Aerospace Engineering. "There is a dance between the plasma and the wall that needs to be understood so we can improve the materials across a range of applications."

Plasmas are created when electrons are added to or removed from atoms, giving them a charge. The interaction between the resulting ionized gas and wall can be complex, involving the transfer of mass, charge and energy from the plasma to the wall -- and sometimes from the wall back to the plasma. This energetic interaction may damage the wall, eroding the surfaces and leading to device failure.

Existing plasma wall materials have been developed largely by trial-and-error. Developing a fundamental understanding of the plasma-wall interaction will give researchers the information they need to develop better wall materials.

"We need to get at the fundamental issues, then use that knowledge to make the materials better," said Jud Ready, a principal research engineer in the Georgia Tech Research Institute (GTRI). "Before we can produce better materials to make better applications, we need to understand the environment in which the materials have to operate."

A major part of the research will involve the use of a terahertz-frequency laser to study the sheath, a narrow portion of the plasma where the wall interaction takes place. Within that small region, usually just a fraction of millimeter or so wide, plasma particles collide with the wall, transfer electrical charge, and apply energy.

"The sheath has a strong electric field which is either pulling or pushing electrons from it," explained Walker, who is director of Georgia Tech's High-Power Electric Propulsion Laboratory. "By adjusting what the wall material contains, we can change the sheath and watch how the plasma adjusts to the wall."

Traditional probe techniques used for studying such phenomena alter the sheath activity when they penetrate it, so the researchers must develop a technique that does not physically enter the plasma sheath. Their solution will use a very fast terahertz laser that won't affect the plasma as it measures the sheath. To give the laser a larger target for study, Walker will produce plasma sheaths as much as a centimeter wide.

"This will allow us to make measurements that nobody has ever done before," he explained. "Using the data we obtain, we will be able to look at all of the analytical models that people have generated and compare them to real experimental data."

Improving the wall materials will also depend on detailed knowledge of how the plasma affects them. For that information, the researchers will use unique tools available at the University of Alabama that are able to identify individual plasma atoms that may be embedded in the walls. Researchers will also use modeling and simulation techniques to predict, based on the experimental data, how a broad range of materials would interact with the plasmas.

"A plasma places a material under extreme environmental conditions, including high temperature erosion, exposure to ion implantation and field emission from the surface," said Gregory Thompson, associate professor in the Department of Metallurgical and Materials Engineering at the University of Alabama, in Tuscaloosa, Ala. "These conditions will affect the structural integrity of materials, but an understanding of the underlying mechanisms that control the response of the materials' structure is lacking. Working with Georgia Tech, we will systematically characterize how plasmas interact and contribute to the underlying phase and mechanical stability characteristics in the materials."

Finally, Ready and GTRI colleagues will apply their experience with thin film deposition and phosphors to create an additional analytical tool. By embedding certain phosphors in the walls, the research team will be able to tell how much energy is being transferred -- and where that is occurring.

"The more robust the material, the better it will be for military or commercial applications," Ready noted. "We expect that there will be dramatically improved performance."

This material is based upon work conducted under contract FA9550-11-1-0160. Any opinions, findings and conclusions or recommendations expressed are those of the researchers and do not necessarily reflect the views of the Air Force Office of Scientific Research.

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Writer: John Toon

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