A Fundamental New Law Unchains Fusion Energy

EPFL physicists have changed one of the fundamental equations that has been crucial to plasma and fusion research for over three decades, even governing the construction of megaprojects like ITER, as part of a major European effort. The new information shows that we may safely use more hydrogen fuel in fusion reactors, resulting in more energy than previously imagined.

One of the most promising future energy sources is fusion. It involves the merger of two atomic nuclei into one, releasing massive amounts of energy. The Sun's warmth originates from hydrogen nuclei fusing into heavier helium atoms, which we see every day.

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.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 order to create plasma for fusion, you have to consider three things: high temperature, high density of hydrogen fuel, and good confinement,” explains Paolo Ricci of the Swiss Plasma Center, one of the world's premier fusion research centers (EPFL).

Ricci's team, working as part of a large European collaboration, has now published a study that updates a fundamental principle of plasma generation, demonstrating that the upcoming ITER tokamak can operate with twice the amount of hydrogen and thus 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,” Ricci adds. “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.”

Martin Greenwald, a fusion scientist, proposed a renowned formula in 1988 that connects fuel density to the tokamak's minor radius (the radius of the donut's inner circle) and the current flowing in the plasma inside the tokamak. The "Greenwald limit" has been a core notion of fusion research ever since; in fact, it is the basis for ITER's tokamak-building technique.

“Greenwald derived the law empirically, that is completely from experimental data – not a tested theory, or what we’d call ‘first principles’,” Ricci adds. “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.”

The Swiss Plasma Center, in collaboration with other tokamak teams, devised an experiment in which very sophisticated equipment was used to precisely manage the amount of fuel fed into a tokamak. The world's largest tokamaks, 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, were used in the huge tests. The EUROfusion Consortium, a European organization that coordinates fusion research in Europe, made this big experimental effort possible, and EPFL currently participates in it through the Max Planck Institute for Plasma Physics in Germany.

At the same time, Maurizio Giacomin, a PhD student in Ricci's group, began to investigate the physics processes that restrict density in tokamaks in order to extract a first-principles law that can link fuel density to tokamak size. However, using advanced plasma simulations with a computer model was a part of that.

“The simulations exploit some of the largest computers in the world, such as those made available by CSCS, the Swiss National Supercomputing Center and by EUROfusion,” explains Ricci. “And what we found, through our simulations, 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. Then, unlike an electrical copper wire, which becomes more resistant when heated, plasmas become more resistant when they cool down. So, the more fuel you put into it at the same temperature, the more parts of it cool down – and the more difficult is for current to flow in the plasma, possibly leading to a disruption.”

This was challenging to simulate. “Turbulence in a fluid is actually the most important open issue in classical physics,” Ricci adds. “But turbulence in a plasma is even more complicated because you also have electromagnetic fields.”

Finally, Ricci and his colleagues cracked the code and put "pen to paper" to derive a new equation for the tokamak fuel limit that closely matches experiments. It was published in the journal Physical Review Letters on May 6, 2022, and while it is close to Greenwald's limit, it is significantly updated. 

According to the new equation, the Greenwald limit in terms of fuel in ITER can be nearly doubled, implying that tokamaks like ITER can build plasmas with nearly twice the quantity of fuel without causing disruptions. 

“This is important because it shows that the density that you can achieve in a tokamak increases with the power you need to run it,” explains Ricci. “Actually, DEMO will operate at a much higher power than present tokamaks and ITER, which means that you can add more fuel density without limiting the output, in contrast to the Greenwald law. And that is very good news.”

A Fundamental New Law Unchains Fusion Energy A Fundamental New Law Unchains Fusion Energy Reviewed by Lilit on May 23, 2022 Rating: 5
Powered by Blogger.