A new tool 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 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
“One of the biggest challenges for thermonuclear
magnetic fusion is understanding how plasma in the fusion reactor modifies the
inner wall,” says 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. The machines are
ideal for materials testing.
The materials analysis particle probe, or MAPP, will be
connected to the underside of the tokamak. The students custom designed the
probe 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 says. “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 says. “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.”
The probe will provide information about 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 says. “Some
completely break down and melt. We need to understand how to operate and
control the wall itself and the plasma together as they interacting.”
The effects of plasma on surface materials is now analyzed
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
to analyze tiles used in the Princeton
tokamak. This approach shows only the cumulative results of hundreds of
experiments, whereas scientists would prefer seeing the fine details associated
with individual experiments.
“That’s what this new probe can do,” he says.
“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 would be used to validate
computational models and guide design of new materials.
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 Allain since he was a junior in undergraduate research. Additional
students involved in the work are nuclear engineering students: doctoral
students Zhangcan Yang and Chase Taylor, senior Sean Gonderman, junior Miguel
Gonzalez, and seniors Sami Ortoleva and Eric Collins.
Heim and Gonderman are spending six weeks at Princeton this summer to set up the instrument.
“The device is completely remote controlled, in
principle from anywhere in the world,” Allain says.
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 says. “Therefore, someone
from overseas will have the opportunity to use MAPP without leaving their home