A New Law Unleashes Fusion Energy

A New Law Unleashes Fusion Energy


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EPFL physicists, in a broad European collaboration, have revised one of the fundamental laws that has underpinned plasma and fusion research for more than three decades, even governing the design of megaprojects like ITER. The update shows that we can actually safely use more hydrogen in fusion reactors, and therefore get more energy than previously thought.

Fusion is one of the most promising energy sources of the future. These are two atomic nuclei that combine into one, releasing huge amounts of energy. In fact, we experience fusion every day: the sun’s heat comes from hydrogen nuclei fusing into heavier helium atoms.

There is currently an international fusion research megaproject called ITER, which aims to replicate the fusion processes of the sun to create energy on Earth. Its goal is the creation of a high temperature plasma that provides the right environment for fusion to occur, producing energy.

Plasmas – a gas-like ionized state of matter – 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 Sun’s core), forcing electrons to separate from their atomic nuclei. The process takes place inside a doughnut-shaped (“toroidal”) structure called a “tokamak”.

“In order to create plasma for fusion, you need to consider three things: high temperature, high hydrogen density and good confinement,” says Paolo Ricci of the Swiss Plasma Center, one of the leading research institutes worlds in fusion located at EPFL.

Une nouvelle loi déchaîne l'énergie de fusion

Time plot of gas flux, electron density from Thomson scattering, radiation intensity and magnetic disturbances for JET discharge n° 80823. The MARFE event is identified by the sharp increase in radiation measured above of point X. The appearance of MARFE precedes the appearance of a locked mode, which finally leads to the disturbance of the plasma. The red dotted vertical line represents the time of appearance of MARFE, tM ≃ 20.9 sec. The start of the N=1 locked mode occurs at 21.95 s, while the disturbance time is at 21.1 s. Credit: Physical examination letters (2022). DOI: 10.1103/PhysRevLett.128.185003

Working within a large European collaboration, Ricci’s team has just published a study updating a fundamental principle of plasma generation and showing that the future ITER tokamak can in fact operate with twice as much hydrogen and thus generating more fusion energy than previously thought.

“One of the limitations of making plasma inside a tokamak is how much hydrogen you can inject into it,” Ricci says. “Since the early days of fusion, we’ve known that if you try to increase the density of the fuel, at some point there would be what we call a ‘disruption’ – basically you totally lose containment, and plasma goes everywhere. So in the 80s people were trying to find 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 research. 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 density of the fuel above 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 Ph.D. student in Ricci’s group, began to analyze the physical processes that limit density in tokamaks, in order to derive a first-principle law that can correlate fuel density and 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 CSCS, the Swiss National Center for Supercomputing and by EUROfusion,” says 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 they cool.So the more fuel you put into it at the same temperature, the more it cools the parts and the harder it is for current to flow through the plasma, which can lead to disturbance.”

It was hard to fake. “Turbulence in a fluid is in fact 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. Posted in Physical examination lettersit does justice to Greenwald’s limit, coming close to it, but updates 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,” says Ricci. “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.”

Toward fusion power, team models plasma turbulence on nation’s fastest supercomputer

More information:
M. Giacomin et al, First-Principles Density Limit Scaling in Tokamaks Based on Edge Turbulent Transport and Implications for ITER, Physical examination letters (2022). DOI: 10.1103/PhysRevLett.128.185003

Provided by the Federal Polytechnic School of Lausanne

Quote: A new law unchains fusion energy (May 17, 2022) retrieved May 17, 2022 from https://phys.org/news/2022-05-law-unchains-fusion-energy.html

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