A new theoretical upgrade could unlock vastly greater amounts of fusion power

Raising the tokamak fuel limit to new heights.
Chris Young
The tokamak reactor at Swiss Plasma Center.Alain Herzog/EPFL

The world's largest fusion experiment, ITER, may be able to unleash more power than previously thought.

That's because a team of scientists from the Swiss Plasma Center, one of the world's leading nuclear fusion research institutes, released a study updating a foundational principle of plasma generation, a press statement reveals.

Their research shows that the upcoming ITER tokamak can operate using twice the amount of hydrogen that was believed to be its full capacity, meaning it could generate vast amounts more nuclear fusion energy than previously thought.

Raising the bar for nuclear fusion

"One of the limitations in making plasma inside a tokamak is the amount of hydrogen fuel you can inject into it," explained Paolo Ricci, from the Swiss Plasma Center at the Swiss Federal Institute of Technology Lausanne (EPFL).

"Since the early days of fusion, we've known that if you try to increase the fuel density, at some point there would be what we call a 'disruption' — basically you totally lose the confinement, and plasma goes wherever," Ricci continued. "So in the eighties, people were trying to come up with some kind of law that could predict the maximum density of hydrogen that you can put inside a tokamak."

In 1988, fusion scientist Martin Greenwald published a famous law correlating fuel density with a tokamak's minor radius (the radius of the spherical reactor's inner circle) as well as the current that flows in the plasma maintained in the tokamak. The law, named the "Greenwald limit", became a foundational principle of research into nuclear fusion, and it has guided the strategy behind the world's largest fusion experiment, Europe's ITER.

Now, the EPFL team's new study, published in Physical Review Letters, highlights the fact that Greenwald's limit was derived from experimental data. 

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"Greenwald derived the law empirically, that is completely from experimental data — not a tested theory, or what we'd call 'first principles,'" Ricci explained. "Still, the limit worked pretty well for research. And, in some cases, like DEMO (ITER's successor), this equation constitutes a big limit to their operation because it says that you cannot increase fuel density above a certain level."

Working with other international tokamak teams, the EPFL team designed a state-of-the-art experiment that allowed them to precisely measure the amount of fuel injected into a tokamak. The investigation was carried out at the world's largest tokamaks: the Joint European Torus (JET) in the UK, the ASDEX Upgrade in Germany (Max Plank Institute), and EPFL's own TCV tokamak. The joint experiments were coordinated by the EUROfusion Consortium.

While those experiments were taking place, Maurizio Giacomin, a Ph.D. student in Ricci's group, analyzed the physics processes limiting the density in tokamaks to derive a first-principles law that correlates fuel density with tokamak size. To do so, they had to run simulations through some of the largest computers in the world, including some from the CSCS, the Swiss National Supercomputing Center.

"What we found, through our simulations," Ricci explained, "was that as you add more fuel into the plasma, parts of it move from the outer cold layer of the tokamak, the boundary, back into its core, because the plasma becomes more turbulent."

In the opposite fashion to a copper wire, which becomes more resistant as it heats up, the researchers say plasma becomes more resistant as it cools. This means that the more fuel you add in at the same temperature, the more of it cools down — making the flow of current in the plasma more difficult.

A new equation for the fuel limit in a tokamak

Though simulating turbulence in plasma was a great challenge, Ricci and his team were able to do so, and they wrote a new equation for the fuel limit in a tokamak based on their investigation. According to the researchers, the new equation does justice to Greenwald's limit, at the same time as updating it substantially.

Crucially, the new equation posits that the Greenwald limit can be raised to almost double its current figure when it comes to the fuel used in ITER, meaning it can use almost double the fuel without disruption.

ITER and other global tokamak projects aim to unleash the power of nuclear fusion, which has the potential to produce almost limitless energy using the same method as the Sun and the stars. ITER is expected to start operating with low-power hydrogen reactions in 2025. 

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