New law breaks restrictions on integration

Physicists at EPFL, in a major European collaboration, have revised one of the fundamental laws that has been central to 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 fuel in fusion reactors, thus getting more energy than previously thought.

Fusion is one of the most promising sources of future energy. It includes two atomic nuclei that combine into one nucleus, thus releasing huge amounts of energy. In fact, we are testing fusion Every day: comes from the warmth of the sun hydrogen nuclei Fusion into heavier helium atoms.

There is currently a massive international fusion research project called ITER, which aims to replicate the fusion processes of the Sun to generate energy on Earth. Its goal is to produce a high-temperature plasma that provides the right environment for fusion to occur, and for energy production.

Plasma — a gas-like state of ionized matter — consists of positively charged nuclei and negatively charged electrons, and is a million times less dense than the air we breathe. Plasma is formed by subduing ‘fusion fuel’ –hydrogen atoms—to extremely high temperatures (10 times the temperature of the Sun’s core), forcing electrons to separate from it atomic nuclei. The process takes place within a doughnut-shaped structure (“annular”) calledtokamak. “

Says Paolo Ricci at the Swiss Plasma Center, one of the world’s leading research institutes in the field of fusion located in EPFL.

With a major European collaboration, Ricci’s team has now released a study to update the foundational principle of plasma generation – and to show that the upcoming ITER tokamak can actually operate with twice the amount of hydrogen and thus generate more fusion power than previously thought.

“One of the limitations in making the plasma inside the tokamak is the amount of hydrogen fuel you can inject into it,” Ritchie says. “From the early days of fusion, we’ve known that if you try to increase the density of the fuel, at some point there will be what we call a ‘disruption’ – you basically just lose the limit completely, and the plasma goes wherever it is. In the ’80s, people were trying to come up with some kind of law that could predict At the maximum density of hydrogen you can fit inside a tokamak.”

The answer came in 1988, when fusion scientist Martin Greenwald published a famous law relating fuel density to the small tokamak radius (the radius of the inner circle of a donut) and the current flowing in the plasma inside the tokamak. Since then, the “Greenwald limit” has become a central tenet of fusion research; In fact, ITER’s tokamak-building strategy is based on this.

“Greenwald derived the law empirically, i.e. from experimental data“It’s not a tested theory, or what we call ‘first principles,'” explains Ritchie. However, the limit worked well in the research. And in some cases, like DEMO (the successor to ITER), this equation is pretty much a limit to its running because it states that you can’t increase the intensity of fuel above a certain level.”

Working with the tokamak teams, the Swiss Plasma Center designed an experiment where highly advanced technology could be used to precisely control the amount of fuel injected into the tokamak. The massive trials were conducted at the world’s largest tokamak, the Joint European Tokamak (JET) in the UK, as well as the ASDEX upgrade in Germany (Max Planck Institute) and EPFL’s TCV tokamak. This major experimental effort was made possible by the EUROfusion Consortium, the European organization coordinating fusion research in Europe and in which EPFL is now involved through the Max Planck Institute for Plasma Physics in Germany.

At the same time, Maurizio Giacomene, Ph.D. A student in Ricci’s group, he began to analyze the density-limiting physical processes in tokamaks, in order to derive a first-principles law that could relate the density of fuels to the volume of tokamaks. Part of that involves using an advanced simulation of plasma using a computer model.

“The simulations take advantage of some of the largest computers in the world, such as those made possible by CSCS, the Swiss National Center for Supercomputing, and EUROfusion,” says Ritchie. “And what we found, through our simulations, is that as you add more fuel to the plasma, parts of it travel from the tokamak’s outer cold layer, the boundary, to its core, because the plasma becomes more turbulent. Then, unlike the electrical copper wires, which It gets more resistant when heated, plasma becomes more resistant when it cools. So, the more fuel you put in it at the same temperature, parts of it cool down—and it gets harder for current to flow in the plasma, which can lead to turbulence.”

This was a challenge to simulate. “Turbulence in a fluid is actually the most important open issue in classical physics,” Ritchie says. “But the turmoil in plasma More complicated because you also have electromagnetic fields.”

In the end, Ritchie and his colleagues were able to crack the code and put “pen on paper” to derive a new equation for the maximum fuel limit at the tokamak, which aligns well with the experiments. Posted in Physical Review Lettersit does justice to Greenwald’s limits, by getting close to it, but updates them in important ways.

The new equation assumes that the Greenwald limit can be raised approximately twice in terms of fuel at ITER; This means that tokamaks like ITER can actually use twice as much fuel to produce plasma without worrying about turbulence. “This is important because it shows that the intensity you can achieve in a tokamak increases with the power you need to run it,” Ritchie says. In fact, DEMO will run at a much higher power than existing tokamaks and ITER, which means you can add more fuel Density without limiting production, contrary to Greenwald’s law. This is very good news.”