By Jonathan Spencer Jones

NOTE: This article was originally written for our sister publication, Power Engineering International.

A revision of a fundamental law for tokamak fusion energy has doubled the fuel potential, thereby increasing the output energy potential.

The revision to the so-called Greenwald’s law – after fusion scientist Martin Greenwald – which has been foundational to fusion research and tokamak development for over three decades, suggests that the amount of hydrogen fuel that can be injected into the tokamak can be increased almost two-fold, enabling operation at a higher power level.

Greenwald’s law correlates the fuel density to the radius of the toroidal shaped tokamak’s inner circle and the current that flows in the plasma inside the tokamak and sets a limit on its operation. With increasing fuel density, at some point a ‘disruption’ occurs, in which the confinement of the plasma is lost and the fusion process breaks down.

Greenwald’s law was used in for example the design of the forthcoming International Thermonuclear Experimental Reactor (ITER) tokamak, which now should be able to operate at a near doubled fuel density and generate more fusion energy than previously thought.

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“One of the limitations in making plasma inside a tokamak is the amount of hydrogen fuel you can inject into it,” says Paolo Ricci, a physicist at the EPFL Swiss Plasma Centre in Lausanne, who led the research.

“[The finding] is important because it shows that the density that you can achieve in a tokamak increases with the power you need to run it. 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.”

The finding was based on experiments using sophisticated technology to precisely control the amount of fuel injected into a tokamak, which were conducted at the Joint European Torus (JET) in the UK, at the ASDEX Upgrade in Germany and EPFL’s own TCV tokamak.

Alongside this, theoretical analysis was undertaken of the physical processes that limit the density in tokamaks.

Computer simulations showed that as more fuel is added 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. This in turn causes it to cool down, making it more difficult for the current to flow and possibly leading to a disruption.

The findings of the research are published in the journal Physical Review Letters.

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