While no fusion reactors are net producers of power yet, a finding about instability in materials surfaces may lead to the design of improved structural materials for plants. (Image courtesy of ITER.) |
A new discovery about the dynamic impact of individual energetic
particles into a solid surface improves our ability to predict surface
stability or instability of materials under irradiation over time.
The
finding may lead to the design of improved structural materials for
nuclear fission and fusion power plants, which must withstand constant
irradiation over decades. It may also accelerate the advent of fusion
power, which does not produce radioactivity.
Sun in a bottle
Fusion,
also known as “sun in a bottle,” re-creates on earth the sun’s nuclear
process for converting hydrogen into helium in a plasma.
Unlike
all nuclear power plants operating today, which are based on
fission—the breaking up of heavy nuclei into lighter ones—fusion
consumes only water and creates no radioactive waste.
While no fusion reactors are net producers of power yet, both the U.S. and Europe have built demonstration reactors.
Publishing
in Nature Communications, Michael Aziz, Gene and Tracy Sykes Professor
of Materials and Energy Technologies, and Michael Brenner, Glover
Professor of Applied Mathematics and Applied Physics, both at the
Harvard School of Engineering and Applied Sciences (SEAS), and
colleagues developed a new rigorous mathematical theory that is “fed”
the measured shape of the average crater resulting from the impact of an
energetic particle.
The
impacts, lasting a few trillionths of a second, are simulated using
intensive computer calculations. The theory then “up-scales” the
cumulative effect of individual energetic particle impacts to predict
surface topography evolution over thousands of seconds or longer.
“Our
results illustrate how large-scale computer simulations can be combined
with rigorous mathematical analysis to yield precise predictions of new
phenomena on length and timescales that would otherwise be
computationally impossible,” says Brenner.
The
researchers were surprised to discover that stability/instability is
not determined by the atoms that are blasted away, but instead by the
atoms that are knocked around and re-settle nearby.
“Our
discovery overturns a long-held paradigm about what causes surfaces to
erupt into patterns under energetic particle bombardment. The blasting
away of individual atoms from energetic particle impacts has long been
thought to determine whether a surface is stable or unstable,” says
Aziz.
“The
effect of atoms blasted away turns out to be so small that it is
essentially irrelevant. The lion’s share of the responsibility of what
makes a surface stable or unstable under irradiation comes from the
cumulative effect of the much more numerous atoms that are just knocked
to a different place but not blasted away.”
The
team found that the cumulative effect of these displacements can be
either ultra-smoothening, which may be useful for the surface treatment
of surgical tools, or topographic pattern-forming instabilities, which
can degrade materials. The outcome depends on the type of material,
energetic particle, and irradiation conditions.
The
discovery, while interesting in its own right, may also help to solve a
mysterious degradation problem in tungsten plasma-facing reactor walls
in prototype fusion reactors.
Tungsten
is used for two reasons. First, it is inert to nuclear reactions that
could arise from the bombardment by hydrogen and helium from the plasma.
Additionally, Aziz says, “The material is so strongly bound that this
bombardment does not lead to the blasting away of any tungsten atoms.”
Yet,
under some conditions the tungsten inexplicably becomes “foamy” or
begins to degrade. Armed with insight from the new theory, the
researchers considered the implications for foamy tungsten.
“Even
under plasma conditions where no tungsten atoms are blasted away, they
are indeed displaced to new positions,” says Aziz. “We conjectured that
the way the helium moves the tungsten atoms around is causing the
instability. While more research will be needed to know for sure, we
think we’re finally in the position of being able to develop a
predictive theory for tungsten by extending the one we have presented
here for simpler materials.”
Extending
the study to more complex materials such as tungsten is an important
future challenge, as it could lead to better design criteria for
materials that must remain stable during exposure to irradiation, both
for cleaner, safer fusion and for conventional fission reactors.
Aziz
and his colleagues developed an experimental data set using ion beam
irradiation of silicon because it is the simplest prototype material for
such studies. Their theory was validated by detailed comparison of its
predictions to their experimental results for surface ultra-smoothening,
instabilities, and pattern formation in silicon.
“This
could help solve a technical problem with nuclear fusion power, which
holds promise for nuclear power without radioactivity,” says Aziz.
Co-authors
included Scott A. Norris, a former postdoctoral research fellow in SEAS
and now on the Faculty at Southern Methodist University; Juha Samela,
Larua Bukonte, Marie Backman, Djurabekova Flyura, Kai Nordlund, all at
the Department of Physics and Helsinki Institute of Physics; and Charbel
S. Madi, a Ph.D. student in SEAS.
The
research was supported by the Department of Energy; the National
Science Foundation (NSF); the NSF-funded Materials Research Science and
Engineering Center at Harvard; the Kavli Institute for Bionano Science
and Technology at Harvard; and the Academy of Finland and the University
of Helsinki. The authors also acknowledge grants of computer time from
the Center for Scientific Computing in Espoo, Finland.
