Nuclear fusion experiment overcomes two key operating hurdles

A nuclear fusion reaction has overcome two key barriers to operating in a “sweet spot” needed for optimal power production: boosting the plasma density and keeping that denser plasma contained. The milestone is yet another stepping stone towards fusion power, although a commercial reactor is still probably years away.

One of the main avenues being explored in efforts to achieve fusion power is using tokamak reactors. These have a doughnut-shaped chamber where plasma hotter than the surface of our sun is contained by vast magnets.

It had been thought that there was a point – known as the Greenwald limit – above which you couldn’t raise the density of the plasma without it escaping the clutches of the magnets, potentially damaging your reactor. But raising density is crucial to increasing output, as experiments have shown that the output of tokamak reactors rises proportionally with the square of the fuel density.

Now, Siye Ding at General Atomics in San Diego, California, and his colleagues have shown that there is a way to raise the plasma density, and proved that it can be stable, by running the DIII-D National Fusion Facility tokamak reactor for 2.2 seconds with an average density that is 20 per cent above the Greenwald limit. While this barrier has been passed before, with less stability and for shorter durations, this experiment crucially also ran with a metric known as H98(y,2) of above 1.

H98(y,2) is a complex blend of measurements and values that shows how well the plasma is contained by the magnets, says Gianluca Sarri at Queen’s University Belfast, with a value of 1.0 or above signifying that plasma is being successfully held in place.

“You’re now starting to show some sort of stable operation where you can consistently be in the sweet spot,” says Sarri. “This was done in a small machine. If you take these results and extrapolate it to a larger machine… that is expected to put you in a situation where gain and significant power production can be achieved over a significant amount of time.”

The DIII-D experiment relied on a mix of approaches that aren’t themselves new, says Sarri, but together seem to have created a promising approach. The team used higher density in the core of the doughnut shaped plasma, to increase output, while allowing it to dip at the edges nearest the containment vessel to drop below the Greenwald limit, therefore avoiding any plasma escape. They also puffed deuterium gas into the plasma to calm reactions in specific spots.

DIII-D’s plasma chamber has an outside radius of just 1.6 metres, and isn’t yet know whether the same method would work for ITER, the next-generation tokamak under construction in France, which will have a radius of 6.2 metres and is expected to create plasma as soon as 2025.

“These plasmas are very complicated,” says Sarri. “A small change in conditions leads to a big change in behaviour. And experimentally it has been more like a trial-and-error sort of approach, where you try many different configurations and basically see which one is best. It’s all about forcing the plasma to do something that is completely against its nature, that it really doesn’t want to do.”

Ding says the experiment bodes well for the future of fusion power. “Many reactor designs require simultaneous high confinement and high density. Experimentally, this is the first time it is realised,” he says. “The next step is expensive, and currently research is going in many different directions. My hope is that this paper will help focus the efforts worldwide.”

The work is another step towards a practical fusion power plant, says Sarri, but nobody should expect to see a commercial reactor in the next five, or even 10, years.

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