EPFL researchers update fusion’s “Greenwald limit”

June 7, 2022, 3:00PMANS Nuclear Cafe

A newly released study led by physicist Paolo Ricci has revised a fundamental, foundational law of plasma generation and nuclear fusion by showing that more hydrogen fuel can safely be used in fusion reactors, thereby generating more energy than previously thought possible. Ricci, of the Swiss Plasma Center at the Swiss Federal Institute of Technology in Lausanne (EPFL), explains that his team’s results indicate that tokamaks, such as the international collaborative project ITER, could use almost twice the amount of hydrogen fuel in their plasmas without the danger of disruption, or loss of confinement of the plasma.

The research team’s findings amend one of the long-time limitations (the so-called Greenwald limit) in generating and sustaining the high-temperature plasma needed to produce fusion energy.

Experiment methods: Researchers with the study, which was published in Physical Review Letters, employed sophisticated technology to precisely control the amount of hydrogen fuel that was injected into tokamaks. The tokamaks used in the experiment were the Joint European Torus in the United Kingdom, the ASDEX Upgrade in Germany, and the TCV tokamak at the EPFL. The international collaboration was carried out under the umbrella of the EUROfusion Consortium.

The researchers also analyzed the physics-related constraints to fuel density in tokamaks in order to correlate the density with tokamak radius. That analysis involved the use of advanced computer simulations at the Swiss National Supercomputing Center.

Findings: The results of the experiments allowed the researchers to develop a new equation that updates the well-known law, developed by Martin Greenwald in 1988, that set a limit on the fuel density inside a tokamak. With this new equation, the Greenwald limit is increased “almost two-fold in terms of fuel in ITER,” according to EPFL.

Ricci added that DEMO, the planned successor to ITER, “will operate at a much higher power than present tokamaks and ITER, which means that you can add [even] more fuel density without limiting the output, in contrast to the Greenwald law. And that is very good news.”


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