Inside the DIII-D tokamak fusion reactor
Ruth Wilcox (CC BY-SA 4.0)
Fusion reactions have overcome two important barriers to operating in the “sweet spot” needed for optimal power production: increasing plasma density and keeping a denser plasma confined. This milestone marks another stepping stone toward fusion power, although commercial reactors are likely still years away.
One of the main avenues being considered in efforts to achieve fusion power generation is the use of tokamak reactors. These have donut-shaped chambers in which plasma, hotter than the surface of the sun, is trapped by giant magnets.
It was thought that there was a point known as the Greenwald limit, beyond which the plasma could not become denser without escaping the influence of the magnets, potentially damaging the reactor. However, experiments have shown that the power of a tokamak reactor increases proportionally to the square of the fuel density, so increasing the density is important to increase the power.
now, Siye Din General Atomics in San Diego, California, and colleagues have shown that there is a way to increase plasma density and have shown that plasma density can be stabilized. DIII-D National Fusion Facility The tokamak reactor was operated for 2.2 seconds at an average density 20 percent above the Greenwald limit. This barrier has been crossed before with low stability and short duration, but this experiment was also, importantly, performed with a metric greater than 1, known as H98(y,2) .
H98(y,2) states that it is a complex combination of measurements and values that indicate how well the plasma is contained by the magnet. gianluca sarri At Queen’s University Belfast, a value of 1.0 or higher indicates normal plasma retention.
“He’s starting to show some sort of stable behavior where he can consistently be in the sweet spot,” Sarri says. “This was done on a small machine. If we apply these results to a larger machine…we would expect to find a situation where we can achieve gains and significant power production over a significant period of time.”
Sarri said the DIII-D experiment relied on a combination of approaches that are not new in themselves, but that together they appear to have produced a promising approach. The researchers increased the power output by increasing the density in the center of the donut-shaped plasma, while avoiding the escape of the plasma by sinking it below the Greenwald limit at the edge closest to the containment vessel. They also injected deuterium gas into the plasma to calm the reaction in certain areas.
DIII-D’s plasma chamber has an outer radius of just 1.6 meters, but it remains to be seen whether the same method will work for ITER, the next generation tokamak being built in France. ITER has a radius of 6.2 meters and is expected to be completed. It is planned to generate plasma by 2025 at the earliest.
“These plasmas are very complex,” Sarri says. “Small changes in circumstances lead to large changes in behavior. Experimentally, it was a trial-and-error kind of approach, trying out many different configurations and basically seeing which one worked best. It was all about It’s about forcing the plasma to do something that is completely against its nature and that it doesn’t really want to do.”
Ding says the experiment bodes well for the future of fusion power generation. “Many reactor designs require high confinement and high density at the same time. Experimentally, this is the first time this has been achieved,” he says. “Next steps will be costly and research is currently moving in many directions. We hope this document will help focus efforts around the world.”
Sarri says this work is another step toward a practical fusion power plant, but no one should expect a commercial reactor to be completed within the next five or even 10 years.
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