The U.S. scientific community is currently conceptualizing the first nuclear
fusion power plants, which will revolutionize energy production. Like the
sun and stars, a fusion power plant will produce energy by fusing light
elements, like hydrogen, into heavier ones, like helium, at temperatures
higher than 25 million F. Fusing hydrogen to produce helium releases about 4
million times more energy than a chemical reaction, such as the burning of
coal, oil, or gas. Yet, the fusion reaction neither releases carbon dioxide
in the atmosphere, nor results in radioactive biproducts, which makes it one
of the most promising ways to produce clean energy on Earth.
At such high temperatures, atoms decompose into positively charged nuclei
(or ions) and negatively charged electrons, creating a state of matter
called plasma. In the donut-shaped fusion device, called a tokamak, the
plasma is held inside the vessel with the help of strong magnetic fields
that can control the paths of the charged particles. Efficiently generating
fusion energy requires containing the plasma long enough for the ions to
collide and fuse.
These plasmas, however, can harbor electromagnetic oscillations, known as
Alfvén waves, which cause the magnetic fields to vibrate much like plucking
a guitar string. The vibrations can carry energetic ions out of the hot core
region of the plasma before fusion occurs, much like a surfer riding a wave.
Studying and controlling this fast ion flow in a fusion reactor require an
understanding of the paths these ions travel and where they end up. The
individual migration trajectories of the ions have been a long-standing,
unresolved mystery—until now.
Recently, scientists at the DIII-D National Fusion Facility in San Diego
developed a new instrument, called the imaging neutral particle analyzer,
that works as follows. First, a beam of neutral (uncharged) particles is
injected in the core of the plasma, where the fast ion flow occurs. When an
injected neutral particle collides with a fast ion from the core, the two
particles exchange electrons. In this charge-exchange process, the fast ions
accept electrons and turn into fast neutrals. These fast neutrals can escape
the core plasma because their lack of charge means they are not trapped by
the magnetic fields. As they escape, they are captured by the imaging
neutral particle analyzer, which can reconstruct information about the
energy, trajectory and orientation of the original fast ions.
Combining this information, the instrument has for the first time revealed
details about the fast ion flow driven by the Alfvén waves. Researchers
observed that some ions moved toward the colder external regions of the
plasma, while others surprisingly gained energy from the Alfvén waves and
traveled towards the hotter plasma core regions (Figure 1).
Like birds, this particle migration occurs along specific routes, and as
with birds, the routes are determined by the environment the particles move
through, specifically the wave frequency, wave number, and other factors
associated with particle motion. These routes, which are induced by multiple
waves, also intersect with each other. That suggests the possibility that
fast ions experience a large-scale migration by transitioning among
different routes, and some ions follow routes that allow them to escape the
core, while others return to it. The observed migration paths were also
confirmed by state-of-the-art plasma simulations.
It is important to note that a strong fast ion flow is destructive to the
operation of fusion plasmas, because if the particles exit the plasma too
quickly to transfer their energy, this will limit the achievable
temperatures needed to keep the fusion process going. To reduce this
transport, scientists are examining possible ways to reduce migration routes
and their intersections, and to remove the "last mile" route for fast ions
to escape the plasma.
The relevant paper is currently accepted and to be published in Phys.
Rev. Lett. (2021).
https://journals.aps.org/prl/accepted/85073Y61O561a17bc2a97d34e912a47ec6ae386da
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