A sustained, stable experiment is the latest demonstration that nuclear fusion
is moving from being a physics problem to an engineering one.
A nuclear fusion reaction has lasted for 30 seconds at temperatures in
excess of 100 million°C. While the duration and temperature alone aren’t
records, the simultaneous achievement of heat and stability brings us a step
closer to a viable fusion reactor – as long as the technique used can be
scaled up.
Most scientists agree that viable fusion power is still decades away, but
the incremental advances in understanding and results keep coming. An
experiment conducted in 2021 created a reaction energetic enough to be
self-sustaining, conceptual designs for a commercial reactor are being drawn
up, while work continues on the large ITER experimental fusion reactor in
France.
Now Yong-Su Na at Seoul National University in South Korea and his
colleagues have succeeded in running a reaction at the extremely high
temperatures that will be required for a viable reactor, and keeping the
hot, ionised state of matter that is created within the device stable for 30
seconds.
Controlling this so-called plasma is vital. If it touches the walls of the
reactor, it rapidly cools, stifling the reaction and causing significant
damage to the chamber that holds it. Researchers normally use various shapes
of magnetic fields to contain the plasma – some use an edge transport
barrier (ETB), which sculpts plasma with a sharp cut-off in pressure near to
the reactor wall, a state that stops heat and plasma escaping. Others use an
internal transport barrier (ITB) that creates higher pressure nearer the
centre of the plasma. But both can create instability.
Na’s team used a modified ITB technique at the Korea Superconducting Tokamak
Advanced Research (KSTAR) device, achieving a much lower plasma density.
Their approach seems to boost temperatures at the core of the plasma and
lower them at the edge, which will probably extend the lifespan of reactor
components.
Dominic Power at Imperial College London says that to increase the energy
produced by a reactor, you can make plasma really hot, make it really dense
or increase confinement time.
“This team is finding that the density confinement is actually a bit lower
than traditional operating modes, which is not necessarily a bad thing,
because it’s compensated for by higher temperatures in the core,” he says.
“It’s definitely exciting, but there’s a big uncertainty about how well our
understanding of the physics scales to larger devices. So something like
ITER is going to be much bigger than KSTAR”.
Na says that low density was key, and that “fast” or more energetic ions at
the core of the plasma – so-called fast-ion-regulated enhancement (FIRE) –
are integral to stability. But the team doesn’t yet fully understand the
mechanisms involved.
The reaction was stopped after 30 seconds only because of limitations with
hardware, and longer periods should be possible in future. KSTAR has now
shut down for upgrades, with carbon components on the wall of the reactor
being replaced with tungsten, which Na says will improve the reproducibility
of experiments.
Lee Margetts at the University of Manchester, UK, says that the physics of
fusion reactors is becoming well understood, but that there are technical
hurdles to overcome before a working power plant can be built. Part of that
will be developing methods to withdraw heat from the reactor and use it to
generate electrical current.
“It’s not physics, it’s engineering,” he says. “If you just think about this
from the point of view of a gas-fired or a coal-fired power station, if you
didn’t have anything to take the heat away, then the people operating it
would say ‘we have to switch it off because it gets too hot and it will melt
the power station’, and that’s exactly the situation here.”
Brian Appelbe at Imperial College London agrees that the scientific
challenges left in fusion research should be achievable, and that FIRE is a
step forwards, but that commercialisation will be difficult.
“The magnetic confinement fusion approach has got a pretty long history of
evolving to solve the next problem that it comes up against,” he says. “But
the thing that makes me kind of nervous, or uncertain, is the engineering
challenges of actually building an economical power plant based on this.”
Reference:
Han, H., Park, S.J., Sung, C. et al. A sustained high-temperature fusion
plasma regime facilitated by fast ions. Nature 609, 269–275 (2022).
DOI: 10.1038/s41586-022-05008-1