A new facility developed by nuclear engineers at Purdue University will be hitched to an experimental fusion reactor at Princeton University to learn precisely what happens when extremely hot plasmas touch and interact with the inner surface of the reactor.

The work is aimed at understanding plasma-wall interactions to help develop coatings or materials capable of withstanding the grueling conditions inside the fusion reactors, known as tokamaks. The machines house a magnetic field to confine a donut-shaped plasma of deuterium, an isotope of hydrogen.

Fusion powers the stars and could lead to a limitless supply of clean energy. A fusion power plant would produce 10 times more energy than a conventional nuclear fission reactor, and because the deuterium fuel is contained in seawater, a fusion reactor's fuel supply would be virtually inexhaustible.

"One of the biggest challenges for thermonuclear magnetic fusion is understanding how plasma in the fusion reactor modifies the inner wall," said Jean Paul Allain, an associate professor of nuclear engineering. "This is a big unknown because now we can't see what happens in real time to the wall surfaces."

Purdue is working with researchers in the Princeton Plasma Physics Laboratory, which operates the nation's largest spherical tokamak reactor, known as the National Spherical Torus Experiment. This particular machine is ideal for materials testing.

The materials analysis particle probe, or MAPP, will be connected to the underside of the tokamak. Our students custom designed each aspect of the probe assembly to be small enough to fit under the reactor.

"This was an engineering feat, to fit a suite of instruments in a package only a few feet tall," Allain said. "It's a miniature materials characterization facility that will allow for a direct correlation between the plasma behavior and its interaction with an evolving wall material surface."

A major challenge in finding the right coatings to line fusion reactors is that the material changes due to extreme conditions inside the reactors, where temperatures reach millions of degrees. Scientists have historically used "wall conditioning," or applying thin films of materials to induce changes to plasma behavior.

"But it's been primarily an Edisonian approach," Allain said. "We don't know what mechanisms are primarily at work, and we need to if we are going to perfect fusion as an energy technology."

However, observing the surface interactions is daunting because of the extreme conditions inside the reactor vessel.

The probe will help researchers learn how the coating materials evolve under plasma conditions and how the interaction correlates with changes in the plasma itself. Data from the instrument will help researchers develop innovative materials for the reactor vessel lining.

"Currently we don't have the materials needed to sustain these large plasma and thermal fluxes," he said. "Some completely break down and melt. We need to understand how to operate and control the wall itself and the plasma together as they interact."

Researchers now analyze the effects of plasma on surface materials by removing test specimens from the lining after a year of running the reactor. Allain's group has worked with researchers at Purdue's Birck Nanotechnology Center to analyze tiles used in the Princeton tokamak. This approach shows only the cumulative results of hundreds of experiments, whereas researchers would prefer see the fine details associated with individual experiments.

"That's what this new probe can do," he said. "It's a new type of surface-analysis diagnostic system designed to be integrated in a tokamak."

The probe will allow scientists to study how specific materials interact with the plasma and yield data within minutes after completing an experiment. Data from the analyses will be used to validate computational models and guide design of new materials.

"The device is completely remote controlled, in principle from anywhere in the world," Allain said.

Researchers might be able to access the instrument using nanoHUB.com, based at Purdue.

"We will have a remote control GUI software, and people will be able to use it online, working with a partner at Princeton," Allain said.  “-- Therefore someone from overseas will have the opportunity to use MAPP without leaving their home institution.” Allain added.

The project is funded by the U.S. Department of Energy through the DOE's Office of Fusion Energy Sciences.

The lead graduate student in the project is Bryan Heim, who has worked with Prof. Allain since he was a junior in undergraduate research.  Additional students involved in the work are nuclear engineering students: Zhangcan Yang (Ph.D. student), Chase Taylor, (Ph.D. student), senior Sean Gonderman, junior Miguel Gonzalez, senior in electrical engineering Sami Ortoleva and electrical engineering technology senior Eric Collins.

Heim and Gonderman will spend six weeks at Princeton this summer to set up the instrument.  Details of the MAPP system and its capabilities were recently presented at the 24th Symposium on Fusion Engineering held in Chicago, Illinois and co-located with the 38th International Conference on Plasma Science chaired by Purdue’s School of Nuclear Engineering Head, Prof. Ahmed Hassanein.  The work will be published in a special issue of the IEEE Transactions on Plasma Science next year.

Writer:    Emil Venere, 765-494-4709, venere@purdjue.edu

Source:    Jean Paul Allain, 765 496-9718, allain@purdue.edu

Related website:
Jean Paul Allain: https://engineering.purdue.edu/NE/People/ptProfile?id=34246

Summer 2011