Energy-Saving Tokamak Heat Barrier Solves 'Achilles Heel' in Nuclear Fusion

Scientists have overcome one of the "greatest uncertainties" in the operation of future fusion reactors.

In order to shield the fusion plasma from impurities in the reactor walls, which greatly reduces the efficiency of energy generation, researchers were able to create an environment that optimizes the thin heat barrier surrounding it.

"It cannot be stressed how important it is that the plasma is able, under the right conditions, to spontaneously exhibit this phenomena of impurity 'screening,'" Anthony Field, lead scientist in the study published in the journal Nuclear Fusion in January, told Newsweek.

This discovery demonstrates how fusion reactions can be made more efficient under the right conditions, which will be important in the development of future large-scale nuclear fusion reactors, including the ITER reactor—what aims to be the largest fusion reactor of its kind—which is currently under construction in France.

What Is Nuclear Fusion?

Nuclear fusion is a technology that creates energy in the same way as the sun: it occurs when two atoms are thrust together with such force that they combine into a single, larger atom and release huge amounts of energy in the process.

Unlike nuclear fission—the nuclear reaction that is currently used in the energy sector—fusion does not create radioactive waste. It produces three to four times more energy than fission, as estimated by the U.S. Department of Energy, and does not release carbon dioxide into the atmosphere, like burning fossil fuels. What is more, fusion is a very fragile process that will shut down in a fraction of a second if the correct conditions are not maintained. Therefore, there is no risk of nuclear meltdown from this reaction.

There is, however, one problem: fusion requires vast amounts of energy to achieve the required conditions, and so far we have not managed to get significantly more energy out of a fusion reaction than what we have put in. Across the sector, the focus is therefore on making the reaction as streamlined and efficient as possible to minimize waste and maximize energy gains.

The breakthrough was made by members of the United Kingdom Atomic Energy Authority (UKAEA) and the EUROfusion consortium working at the Joint European Torus (JET) plant in Oxfordshire at the UKAEA's Culham Campus.

The plant uses a machine called a toroidal tokamak, a donut-shaped contraption that uses powerful magnets to contain a ring-like flow of super-hot plasma.

​​Plasma is the next state of matter after solids, liquids and gasses. It is like a flame but much hotter. Plasma is formed at super-high temperatures and is basically a soup of negatively charged electrons and positively charged ions of elements that have been pulled apart by the extremely hot temperature.

The flavor of fusion used at the JET plant involves whacking hydrogen atoms together until they fuse (although different methods of fusion can use different elements in this reaction).

Your standard hydrogen atom contains one positively charged particle, called a proton, and one negatively charged particle, an electron. When hydrogen atoms are superheated into a plasma, they are pulled apart from their electrons and become positively charged particles, called ions, which repel each other.

Withstanding Intense Temperatures

In the sun, intense gravitational forces create extremely high pressures that overcome this repulsion. But such high pressures are nearly impossible to replicate on Earth. Therefore, we must heat the plasma to an even higher temperature—in the case of JET, to 10 times hotter than the center of the sun—to get these particles to actually fuse.

To withstand these intense temperatures, the metals used to line the machine's inner walls need to have an incredibly high melting point. The part of the reactor that comes into direct contact with the plasma is called the divertor, which is sort of like an exhaust system for the reaction chamber. It is this component of the machine that must be most resistant to the high temperatures of the fusion plasma.

"Tungsten is used as the material for the divertor targets [...] because it has the highest melting point of any metal, at 3,400 C [6,152 F]," said Field, senior physicist at the UKAEA.

However, tungsten comes with its own problems, described as its "Achilles heel" by the UKAEA: when the hot plasma is allowed to interact with the divertor's walls, the tungsten can lose some of its electrons and be swept up in the plasma.

Because tungsten atoms are so heavy—each atom contains 74 protons and 74 electrons—it is very difficult to tear away all of their electrons. This is a problem because the electrons that stay bound to the tungsten can take energy away from the electrons in the plasma, which makes the overall process much harder to sustain. And, if it is harder to sustain, it compromises the ability to get a sufficient amount of energy out of the reaction to outweigh the amount that has been put in.

"If there is more than a tiny amount of tungsten in the confined plasma it becomes impossible to sustain [the reaction]," said Field.

This can be avoided if a barrier is produced around the outside of the plasma, which can prevent the tungsten impurities from getting inside in the first place.

Decades ago, it was hypothesized that an extreme drop in temperature between the plasma core and the diverter walls would be able to act as a sort of "heat barrier" to shield the plasma from this kind of contamination. Now, Field and his team have shown that the theory actually works in practice, at the heat barrier that forms around the plasma edge.

"As this impurity screening phenomena had been predicted but never previously observed in an actual tokamak at the plasma edge, this discovery came as a very exciting surprise," Field said.

"This observation eases our concerns about one of the greatest uncertainties surrounding operation of a future tokamak fusion reactor," he added.

"The heat barrier is a thin, insulating layer of strongly sheared flow about 2 to 3 centimeters [roughly an inch] across, that forms just inside the plasma edge. The mechanism is analogous to the way the 'Jet Stream' in the upper atmosphere prevents eddies of colder air from the arctic regions moving down into the temperate zones and vice versa."

For this heat barrier to effectively screen out impurities, there must be a large enough temperature difference between the confined plasma and the plasma edge, which Field said was "equivalent to 22 million degrees Celsius across the thickness of a triple-glazed window!"

Field said that the confirmation of this hypothesis was an important step towards the "Holy Grail" of fusion power generation.

This method was trialed as part of a series of experiments that allowed JET to break the world record for sustained fusion energy in February 2022, with 59 megajoules of sustained fusion produced over a period of 5 seconds.

While tungsten is not used in all nuclear fusion reactors, this demonstration is a major boost for the nuclear fusion industry as a whole. "Every challenge overcome is another step towards our collective goal of clean, limitless fusion energy for the world," Steven McNamara, Tokamak science director at Tokamak Energy, another fusion company that uses magnets to support the fusion process, told Newsweek. "We congratulate the UKAEA and EUROfusion teams on this result, and will apply any relevant learnings to Tokamak Energy's future devices."

These small-scale tests at JET have been conducted to optimize the efficiency of the world's largest tokamak machine, ITER, in France which is currently under construction. ITER is expected to produce its first plasma at the end of 2025, with full-scaled operations beginning in 2035, according to ITER's website.

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