Fusion reactors could provide much more power than previously thought, study suggests

Scientists have revised a fundamental law “foundational” to fusion energy research that could allow for more hydrogen fuel in reactors, and potentially help obtain more energy from fusion than previously thought.

The study, published in the journal Physical Review Letters, showed that an upcoming international megaproject can operate with twice the amount of hydrogen fuel, and thereby generate considerably more energy.

Nuclear fusion involves two atomic nuclei combining into one, thereby releasing enormous amounts of energy – a process that occurs every day naturally in the sun, whose warmth comes from hydrogen nuclei fusing into heavier helium atoms. Conventional nuclear power plants rely instead on fission, whereby a uranium atom is split with a neutron, releasing a large amount of energy in the form of heat and radiation.

The International Thermonuclear Experimental Reactor (ITER) is an upcoming fusion megaproject that aims to replicate the fusion processes of the sun to create energy on earth.

Researchers in the project, including those from the Swiss Plasma Center (SPC), say they aim to create high-temperature plasma – the fourth state of matter – that provides the right environment for fusion to occur.

Plasma is an ionised state of matter, similar to a gas, that is made up of positively charged nuclei and negatively charged electrons, and is almost a million times less dense than the air we breathe, scientists explain.

It is created by subjecting “the fusion fuel” – hydrogen atoms – to extremely high temperatures, about 10 times that of the core of the sun, and forcing electrons to separate from their atomic nuclei.

In fusion reactors, the process takes place inside a donut-shaped structure called a tokamak.

In the new study, scientists showed that the upcoming ITER tokamak can theoretically operate with twice the amount of hydrogen and therefore generate more fusion energy than previously thought.

“In order to create plasma for fusion, you have to consider three things: high temperature, high density of hydrogen fuel, and good confinement,” Paolo Ricci, a co-author of the study from the Swiss Plasma Center said in a statement.

“One of the limitations in making plasma inside a tokamak is the amount of hydrogen fuel you can inject into it,” Dr Ricci added.

Since the early days of fusion, scientists theorised that as the fuel density is increased at some point there would be “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,” Dr Ricci explained.

In 1988, fusion scientist Martin Greenwald came up with a law correlating fuel density to the tokamak’s minor radius, and the current that flows in the plasma inside the tokamak.

The “Greenwald limit”, scientists say, has been a foundational principle of fusion research with ITER’s tokamak-building strategy based on it.

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

“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,” he added.

To test the theory, scientists ran simulations using highly sophisticated technology to precisely control the amount of fuel injected into a tokamak.

Researchers found that as they added more fuel into the plasma, parts of it moved from the outer cold layer of the tokamak, the boundary, back into its core, since 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 it is for current to flow in the plasma, possibly leading to a disruption,” Dr Ricci explained.

Researchers could then derive a new equation for fuel limit in a tokamak, which they say aligns well with experiments.

Based on the new research, scientists say the Greenwald limit can be raised almost two-fold in terms of fuel in ITER.

They say tokamaks like ITER can use almost twice the amount of fuel to produce plasmas without worries of 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,” Dr Ricci added.

“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,” he added.