Nuclear Fusion Can Unleash Even More Power Than We Realized, Scientists Say

Future fusion events within tokamaks might generate far more energy than previously imagined, due to ground-breaking new study that discovered a fundamental rule for such reactors was incorrect.

The study, led by physicists from the Swiss Plasma Center at École Polytechnique Fédérale de Lausanne (EFPL), discovered that the maximum hydrogen fuel density is roughly double the "Greenwald Limit" — a figure based on studies conducted more than 30 years ago.

The discovery that fusion reactors can work with hydrogen plasma densities much higher than the Greenwald Limit will have an impact on the operation of the massive ITER tokamak being built in southern France, as well as the designs of ITER's successors, known as Demonstration power plant (DEMO) fusion reactors, according to Swiss Plasma Center physicist Paolo Ricci.

"The exact value depends on the power," Ricci said to Live Science. "But as a rough estimate, the increase is on the order of a factor of two in ITER." 

Ricci was one of the project's leaders, combining theoretical work with the results of about a year of experiments at three different fusion reactors across Europe – EPFL's Tokamak à Configuration Variable (TCV), the Joint European Torus (JET) at Culham in the United Kingdom, and the Axially Symmetric Divertor Experiment (ASDEX) Upgrade tokamak at the Max Planck Institute for Plasma Physics in Garching, Germany.

Future fusion 

One of the most promising designs for nuclear fusion reactors that might one day be utilized to generate electricity for power networks is donut-shaped tokamaks.

Unlike nuclear fission, which generates energy by smashing together very large atomic nuclei, scientists have been working for more than 50 years to make controlled fusion a reality; unlike nuclear fission, which generates energy by joining very small nuclei together, nuclear fusion could generate even more energy by joining very small nuclei together.

Fusion generates far less radioactive waste than fission, and the neutron-rich hydrogen it utilizes as fuel is very inexpensive to get.
The same mechanism fuels stars like the Sun, which is why controlled fusion is referred to as a "star in a jar"; however, because the extremely high pressure at the center of a star isn't possible on Earth, fusion processes must function at temperatures higher than the Sun.

The temperature inside the TCV tokamak, for example, may reach over 216 million degrees Fahrenheit (120 million degrees Celsius) — about ten times that of the Sun's fusion core, which is at 27 million degrees Fahrenheit (15 million Celsius).

According to Live Science, several fusion power projects are currently at an advanced level, and some experts believe the first tokamak to generate electricity for the grid might be operational by 2030.

The ITER tokamak ("Iter" means "the way" in Latin) is likewise being funded by more than 30 nations throughout the world, and it is expected to create its first experimental plasmas in 2025.

ITER, on the other hand, isn't intended to generate power; nevertheless, tokamaks based on ITER, known as DEMO reactors, are now being constructed and might be operational by 2051.

Plasma problems

The Greenwald Limit, named after MIT physicist Martin Greenwald, who discovered it in 1988, lies at the core of the new computations.

Greenwald derived an experimental limit based on a tokamak's minor radius (the size of the donut's inner circle) and the amount of electric current passing through the plasma when researchers were trying to figure out why their fusion plasmas effectively became uncontrollable (they expanded outside the magnetic fields that contained them within the tokamak chamber) when they increased the fuel density past a certain point.

The Greenwald Limit has been a core guideline of fusion research for more than 30 years, Ricci said, despite scientists' long suspicions that it might be improved. It's a guiding idea in the ITER design, for example.

The new study, on the other hand, builds on both the tests and theory that Greenwald used to come up with his limit, resulting in a significantly larger fuel density limit that will boost ITER's capacity while also influencing the designs of future DEMO reactors, he added.

The key was the revelation that when the power output of a fusion reaction grows, a plasma can sustain a higher fuel density, he added.

It's impossible to say how such a substantial increase in fuel density would effect tokamak power production, but it's likely to be considerable, according to Ricci, and research suggests that higher fuel density makes fusion reactors easier to operate.

"It makes safe, sustainable fusion conditions easier to achieve," he added. "It allows you to get to the regime that you want, so that the fusion reactor can work properly."
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