Science

A new fundamental law releases fusion energy

ITER Fusion Reactor
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Illustration of cloud-like ionized plasma in the tokamak of the ITER fusion reactor. Credit: ITER

EPFL physicists, within a broad European collaboration, have revised one of the fundamental laws that was the basis of[{” attribute=””>plasma and fusion research for over three decades, even governing the design of megaprojects like ITER. The update demonstrates that we can actually safely utilize more hydrogen fuel in fusion reactors, and therefore obtain more energy than previously thought.

Fusion is one of the most promising future energy sources . It involves two atomic nuclei merging into one, thereby releasing enormous amounts of energy. In fact, we experience fusion every day: the Sun’s warmth comes from hydrogen nuclei fusing into heavier helium atoms.

There is currently an international fusion research megaproject called ITER that seeks to replicate the fusion processes of the Sun to create energy on the Earth. Its goal is to generate high-temperature plasma that provides the right environment for fusion to occur, producing energy.

Plasmas — an ionized state of matter similar to a gas – are made up of positively charged nuclei and negatively charged electrons, and are almost a million times less dense than the air we breathe. Plasmas are created by subjecting “the fusion fuel” – hydrogen atoms – to extremely high temperatures (10 times that of the core of the Sun), forcing electrons to separate from their atomic nuclei. In a fusion reactor, the process takes place inside a donut-shaped (“toroidal”) structure called a “tokamak.”

Swiss Plasma Center Tokamak Thermonuclear Fusion Reactor

The tokamak thermonuclear fusion reactor at Swiss Plasma Center. Credit: Alain Herzog (EPFL)

“In order to create plasma for fusion, you have to consider three things: high temperature, high density of hydrogen fuel, and good confinement,” says Paolo Ricci at the Swiss Plasma Center, one of the world’s leading research institutes in fusion located at École polytechnique fédérale de Lausanne (EPFL).

Working within a large European collaboration, Ricci’s team has now released a study updating a foundational principle of plasma generation – and showing that the upcoming ITER tokamak can actually operate with twice the amount of hydrogen and therefore generate more fusion energy than previously thought.

“One of the limitations in making plasma inside a tokamak is the amount of hydrogen fuel you can inject into it,” says Ricci. “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. 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.”

An answer came in 1988, when fusion scientist Martin Greenwald published a famous law that correlates fuel density with the minor radius of the tokamak (the radius of the donut’s inner circle) and the current flowing through the plasma inside the tokamak. Since then, the “Greenwald limit” has been a fundamental tenet of fusion research; in fact, the ITER tokamak construction strategy is based on this.

“Greenwald derived the law empirically, meaning entirely from experimental data — not from a tested theory, or what we would call ‘first principles,’” says Ricci. “Still, the limit has worked pretty well for search. And, in some cases, like DEMO (ITER’s successor), this equation is a big limit to how they work because it says you can’t increase the fuel density beyond a certain level.

In collaboration with other tokamak teams, the Swiss Plasma Center designed an experiment to use highly sophisticated technology to precisely control the amount of fuel injected into a tokamak. The massive experiments were carried out in the largest tokamaks in the world, the Joint European Torus (JET) in the UK, as well as the ASDEX Upgrade in Germany (Max Plank Institute) and EPFL’s own TCV tokamak. This vast experimental effort was made possible by the EUROfusion Consortium, the European organization that coordinates fusion research in Europe and in which EPFL now participates via the Max Planck Institute for Plasma Physics in Germany.

At the same time, Maurizio Giacomin, a doctoral student in Ricci’s group, began to analyze the physical processes that limit density in tokamaks, in order to derive a first-principles law that could correlate fuel density and fuel size. tokamak. However, part of this involved the use of advanced plasma simulation performed with a computer model.

“The simulations exploit some of the largest computers in the world, such as those made available by the CSCS, the Swiss National Center for Supercomputing and by EUROfusion,” explains Ricci. “And what we found, through our simulations, is that when you add more fuel to the plasma, parts of it move from the outer cold layer of the tokamak, the boundary, to its core, because the plasma becomes more turbulent.Then, unlike an electrical copper wire which becomes more resistant when heated, plasmas become more resistant when cooled.So the more fuel you put into it at the same temperature, the more the parts get colder – and the harder it is for current to flow through the plasma, which can lead to disruption.

It was hard to fake. “Turbulence in a fluid is actually the most important open problem in classical physics,” says Ricci. “But the turbulence in a plasma is even more complicated because you also have electromagnetic fields.”

In the end, Ricci and his colleagues were able to crack the code and put “pen to paper” to derive a new equation for the fuel limit in a tokamak, which lines up very well with the experiments. Published in the journal Physical examination letters on May 6, 2022, it does justice to the Greenwald limit, coming closer to it, but updating it significantly.

The new equation postulates that the Greenwald limit can be almost doubled in terms of fuel in ITER; this means that tokamaks like ITER can actually use almost twice as much fuel to produce plasmas without worrying about disturbances. “It’s important because it shows that the density you can achieve in a tokamak increases with the power you need to run it,” Ricci says. “In fact, DEMO will operate at much higher power than current tokamaks and ITER, which means you can add more fuel density without limiting production, contrary to Greenwald’s law. And that’s a very good news. “

Reference: “First-Principles Density Limit Scaling in Tokamaks Based on Edge Turbulent Transport and Implications for ITER” by M. Giacomin, A. Pau, P. Ricci, O. Sauter, T. Eich, the ASDEX Upgrade team, JET Contributors , and the TCV team, on May 6, 2022, Physical examination letters.
DOI: 10.1103/PhysRevLett.128.185003

List of contributors

  • EPFL Swiss Plasma Center
  • Max Planck Institute for Plasma Physics
  • EPFL TCV team
  • ASDEX Upgrade Team
  • JET Contributors

Funding: EUROfusion Consortium (Euratom research and training programme), Swiss National Science Foundation (SNSF)


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