A top scientist at the U.S. Department of Energy’s Princeton Plasma Physics
Laboratory (PPPL) has developed a model for predicting the outflow of heat
during fusion experiments, which may help overcome a key barrier to the fusion
process.
The model, designed by Robert Goldston, a Princeton professor of astrophysical
sciences and former PPPL director, predicts the width of what physicists call
the “scrape-off layer” in tokamaks, the most widely used fusion
facilities. Such devices confine hot, electrically charged gas, or plasma, in
powerful magnetic fields. But heat inevitably flows through the system and
becomes separated, or scraped off, from the edge of the plasma and flows into
an area called the divertor chamber.
The challenge is to prevent the thin and highly concentrated layer of heat
from reaching and damaging the plate that sits at the bottom of the divertor
chamber and absorbs the scrape-off flow in fusion facilities. Such damage would
halt fusion reactions, which take place when the atomic nuclei, or ions, inside
the plasma merge and release energy. “If nothing was done and you took
this right on the chin, it could be a knockout blow,” says Goldston, who
published his model in Nuclear Fusion.
“This [model] allows you to depict the size of the challenge so you can
think through what needs to be done to overcome it,” Goldston says.
Goldston was among physicists who recently presented aspects of the model at
the 20th Annual International Conference on Plasma Surface Interactions. Some
400 researchers from around the world attended the conference. Results of the
model have been “eerily close” to the data from actual fusion
experiments, says Thomas Eich, a senior scientist at the Max Planck Institute
for Plasma Physics in Garching, Germany, who gave an invited talk on his
measurements of scrape-off layers. The agreement appears too close to have
happened by chance, Eich adds.
Solving the heat layer problem will be vital for future machines like ITER,
the world’s most powerful tokamak, which the European Union, the United States,
and five other countries are building in France to demonstrate fusion as a
source of clean and abundant energy. The project is designed to produce 500 MW
of fusion power in 400-sec-long pulses, which will require researchers to
spread the scrape-off heat as much as possible to protect the divertor
plate.
Goldston’s model could help guide such efforts. He began pondering the width
of the heat flux during a physics conference in South Korea in 2010. Looking at
the latest scrape-off layer data based on improved measurements, he
estimated—literally on an envelope—that the new widths could be produced
without plasma turbulence, a factor that is typically considered but is
notoriously difficult to calculate. This led him to search for a way to
estimate the width of the surprisingly thin layer, and to gauge how the width
would vary as conditions such as the amount of electrical current in the plasma
varied.
The way plasma flows inside tokamaks provided the major clue. The ions
within the charged gas gyrate swiftly along the magnetic field lines while
drifting slowly across the lines. At the same time, the electrons also in the
plasma travel very rapidly along the lines and carry away most of the heat.
Goldston arrived at his prediction by determining how fast these subatomic
particles flow into the divertor region, and how long it therefore takes them
to reach it. The result “is what we call a ‘heuristic’ estimate, based on
the key aspects of the physics, but not a detailed calculation,” Goldston
says.
His estimate confirmed what Goldston had suspected: The width of the
scrape-off layer nearly matched the results of a calculation, made without
considering turbulence, for determining how far the ions drift away from their
field lines. “What’s stunning is how closely the values correspond to the
data, both in absolute value and in variation with the plasma current, magnetic
field, machine size and input power,” Goldston says. “This does not
mean that turbulence plays no role, but it suggests that for the highest
performance conditions, where turbulence is weakest, the motion of the ions is
dominated by non-turbulent drift effects.” This will be true in the case
of ITER, he added, since it is designed to operate in high-performance
conditions.
Researchers are developing techniques for widening the scrape-off layer.
Such methods include pumping gas into the divertor region to keep some heat
from reaching the plate. Physicists use deuterium, a form of hydrogen, to block
the heat, and are injecting nitrogen to turn other parts of the heat into
ultraviolet light. (While charged deuterium ions are already in the plasma, the
deuterium gas that is injected into the divertor region to block the heat is
not electrically charged.)
These strategies look promising. “We know that they will work,”
Goldston says. “The outstanding question is whether they will work
completely enough” to mitigate the heat flux at ITER’s highest power
levels, without introducing so much gas that it cools the fuel. Physicists
around the world are conducting experiments to understand the process better.
For Goldston, calculating the width of the scrape-off layer marks the latest
research effort in a 40-year career at PPPL, which began when he was a graduate
student. Along the way he helped pioneer techniques for heating the plasma, and
developed a widely used method called “Goldston scaling” for
predicting how long heat is retained in a tokamak plasma.
“First, heat is injected into the plasma,” Goldston says of how
tokamaks operate. “Second, that heat is retained while much more heat is
generated by fusion reactions. Finally, the resulting heat has to come out of
the plasma. Without thinking about it, I have been following heat along this
trajectory throughout my whole research career,” he adds. “We have
made great progress on the first two steps, and now the most exciting
challenge, to me, is the one that comes because of our success so far. Now we
need to learn to handle the the outflow of heat from a high-power fusion energy
source.”
