Good News About Energy From Physicists

Good News About Energy From Physicists
Tokamak thermonuclear fusion reactor at the Swiss Plasma Center. Credit: Alain Herzog (EPFL)

Fusion is one of the most promising sources of future energy. It is formed by the fusion of two atomic nuclei, thus releasing a large amount of energy. In fact, we experience fusion every day: The sun's heat comes from hydrogen nuclei breaking down into heavier helium atoms. Currently, an international fusion research project called ITER is underway, which aims to replicate the fusion processes of the Sun to create energy on Earth.

Its purpose is to create high-temperature plasma, which provides the right environment and generates energy for fusion to occur. Plasmas are an ionized state of matter similar to a gas.

It consists of positively charged nuclei and negatively charged electrons. It is almost a million times less dense than the air we breathe.

Plasmas are created by exposing "fusion fuel" - hydrogen atoms - to extremely high temperatures (10 times that of the Sun's core), forcing electrons to leave their atomic nuclei. The process takes place inside a (“toroidal”) structure called a “tokamak”.

“To create plasma for fusion, you have to consider three things: high temperature, high hydrogen fuel density and good entrapment,” says Paolo Ricci of the Swiss Plasma Center, one of the world's leading research institutes in fusion, located at Epfl.

Ricci's team has now published a study that updates the basic principle of plasma production. The study shows that it can produce more fusion energy than previously thought.

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

“We knew from the early days of fusion that if you tried to increase fuel density, at some point what we call 'degradation' would happen. The resulting plasma is dispersed. Back in the eighties, people were trying to come up with some sort of law that could predict the maximum hydrogen density you could put in a tokamak."

In 1988, fusion scientist Martin Greenwald formulated a theory about fuel density. It was a theory that relates the small radius of the tokamak and the current flowing in the plasma inside the tokamak. Since that time, "Greenwald border” has been a fundamental principle of fusion research.

In fact, ITER's tokamak creation strategy is based on this. “Greenwald derived the law empirically, that is, purely from empirical data,” Ricci said. It's not a tested theory, or what we call 'first principles,'” explains Ricci.

“Still, it was pretty successful for frontier research. And in some cases, this equation creates a big limit for operations because it says you can't increase fuel density above a certain level.”

Working with other tokamak teams, the Swiss Plasma Center designed an experiment in which it was possible to use highly advanced technology to precisely control the amount of fuel injected into a tokamak.

Major experiments were carried out on the world's largest tokamaks, the Joint European Torus (JET) in the UK and the ASDEX Upgrade (Max Plank Institute) in Germany and EPFL's own TCV tokamak.

This major experimental effort was made possible by the EUROfusion Consortium, the European organization that coordinates fusion research in Europe and is joined by EPFL now through the Max Planck Institute for Plasma Physics in Germany.

At the same time, Maurizio Giacomin, a PhD student in Ricci's group, began analyzing physics processes that limit density in tokamaks to come up with a first-principles law that could relate fuel density and tokamak size.

However, part of this involved using advanced simulation of the plasma performed with a computer model.

“The simulations use some of the largest computers in the world, such as those provided by CSCS, the Swiss National Supercomputing Center and EUROfusion,” says Ricci.

“And what we found with our simulations is that as we add more fuel to the plasma, its fragments come back from the outer cold layer of the tokamak, from the boundary to its core, because the plasma becomes more turbulent.

Then, plasmas become more resistant when cooled, unlike an electrical copper wire that becomes more resistive when heated. That is, the more fuel you put in at the same temperature, the more of it cools down – and the harder it is for the current to flow in the plasma, which is likely to cause a breakdown.”

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

Finally, Ricci and his colleagues were able to crack the code. They embarked on an effort to derive a new equation for the fuel limit in a tokamak that fit very well with the experiments.

He published the work in Physical Review Letters. The values ​​they put forward in their work do justice to being close to Greenwald's limit, but update it significantly.

The new equation suggests that the Greenwald limit could be increased almost twice in fuel terms in ITER.

This means tokamaks like ITER can use nearly twice as much fuel to produce plasmas without worrying about downtime.

"This is important because it shows that the intensity you can achieve in a tokamak increases with the power you need to operate it," says Ricci.

“Actually the DEMO (successor to ITER) will run at much higher power than existing Tokamaks and ITER, meaning you can add more fuel density without limiting output, unlike Greenwald's law. And that is very good news.”

Source: EPFL

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