Future fusion reactions inside tokamaks could produce much more energy than
previously thought, thanks to groundbreaking new research that found a
foundational law for such reactors was wrong.
The research, led by physicists from the Swiss Plasma Center at École
Polytechnique Fédérale de Lausanne (EFPL), has determined that the maximum
hydrogen fuel density is about twice the "Greenwald Limit" – an estimate
derived from experiments more than 30 years ago.
The discovery that fusion reactors can actually work with hydrogen plasma
densities that are much higher than the Greenwald Limit they are built for
will influence the operation of the massive ITER tokamak being built in
southern France, and greatly affect the designs of ITER's successors, called
the Demonstration power plant (DEMO) fusion reactors, said physicist Paolo
Ricci at the Swiss Plasma Center.
"The exact value depends on the power," Ricci told Live Science. "But as a
rough estimate, the increase is on the order of a factor of two in ITER."
Ricci is one of the leaders on the research project, which combined
theoretical work with the results of about a year of experiments at three
different fusion reactors across Europe – EPFL's Tokamak à Configuration
Variable (TCV), the Joint European Torus (JET) at Culham in the United
Kingdom, and the Axially Symmetric Divertor Experiment (ASDEX) Upgrade
tokamak at the Max Planck Institute for Plasma Physics at Garching in
Germany.
He's also one of the lead authors of a study about the discovery published
May 6 in the journal
Physical Review Letters.
Future fusion
Donut-shaped tokamaks are the one of the most promising designs for nuclear
fusion reactors that could one day be used to generate electricity for power
grids.
Scientists have worked for more than 50 years to make controlled fusion a
reality; unlike nuclear fission, which makes energy from smashing apart very
large atomic nuclei, nuclear fusion could generate even more energy by
joining very small nuclei together.
The fusion process creates much less radioactive waste than fission, and the
neutron-rich hydrogen it uses for its fuel is comparatively easy to obtain.
The same process powers stars like the Sun, which is why controlled fusion
is likened to a "star in a jar"; but because the very high pressure at the
heart of a star isn't feasible on Earth, fusion reactions down here require
temperatures hotter than the Sun to operate.
The temperature inside the TCV tokamak, for example, can be more than 216
million degrees Fahrenheit (120 million degrees Celsius) – almost 10 times
the temperature of the fusion core of the Sun, which is about 27 million
Fahrenheit (15 million Celsius).
Several fusion power projects are now at an advanced stage, and some
researchers think the first tokamak to generate electricity for the grid
could be working by 2030, Live Science previously reported.
More than 30 governments around the world are also funding the ITER tokamak
("Iter" means "the way" in Latin) which is due to produce its first
experimental plasmas in 2025.
ITER, however, isn't designed to generate electricity; but tokamaks based on
ITER that will, called DEMO reactors, are now being designed and could be
working by 2051.
Plasma problems
At the heart of the new calculations is the Greenwald Limit, named after MIT
physicist Martin Greenwald who determined the limit in 1988.
Researchers were trying to find out why their fusion plasmas effectively
became uncontrollable (they expanded outside the magnetic fields they were
contained by within the tokamak chamber) when they increased the fuel
density past a certain point, and Greenwald derived an experimental limit
based on a tokamak's minor radius (the size of the donut's inner circle) and
the amount of electric current passing through the plasma.
Although scientists had long-suspected the Greenwald Limit could be improved
upon, it has been a foundational rule of fusion research for more than 30
years, Ricci said. For example, it's a guiding principle of the ITER design.
The latest study, however, expands on both the experiments and theory that
Greenwald used to derive his limit, resulting in a much higher fuel density
limit that will both increase the capacity of ITER and impact the designs of
the DEMO reactors that come after it, he said.
The key was the discovery that a plasma can sustain a greater fuel density
as the power output of a fusion reaction increases, he said.
It's not yet possible to know how such a large increase in fuel density will
affect the power output of tokamaks, Ricci said, but it's likely to be
significant; and research shows greater fuel density will make fusion
reactors easier to operate.
"It makes safe, sustainable fusion conditions easier to achieve," he said.
"It allows you to get to the regime that you want, so that the fusion
reactor can work properly."
This article was originally published by
Live Science.
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original article here.
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