A sustained, stable nuclear fusion reaction in South Korea proves once again that nuclear fusion is no longer a physical problem, but a technical one.
Researchers have allowed a nuclear fusion reaction to last for 30 seconds at temperatures over 100 million degrees Celsius. Although duration and temperature are not records in themselves, achieving high temperatures and stability at the same time one step closer to a viable fusion reactor. However, the technology used still needs to be scaled up.
Most scientists agree that useful fusion energy is still decades away, but there has been steady progress in understanding and results. An experiment in 2021 produced a reaction that produced enough energy to sustain itself. Work is also underway on conceptual designs for a commercial reactor, and work on the large experimental fusion reactor ITER in France continues unabated.
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Stable for 30 seconds
plasma physicist Yong-Su Na of Seoul National University in South Korea and his colleagues have now succeeded in eliciting a response at the extremely high temperatures required for a viable reactor. They also managed to keep the hot, ionized plasma stable for 30 seconds.
Controlling this plasma is vital. When it hits the walls of the reactor, it cools quickly, quenching the reaction. This also causes damage to the reactor wall.
Researchers use different types of magnetic fields to control the plasma. An example of this is a edge transport barrier (ETB), which shapes the plasma with a sharp transition in pressure at the reactor wall. This prevents heat and plasma from escaping. Another approach uses an internal transport barrier (ITB), which creates a higher pressure in the center of the plasma. Both can cause instability.
Plasma density
Na’s team used a modified ITB technique in the Korea Superconducting Tokamak Advanced Research (KSTAR) facility. In doing so, they achieved a much lower plasma density. This approach appears to increase the temperatures in the core of the plasma, while lowering those at the periphery, which will extend the life of the reactor components.
Theoretical physicist Dominic Power of Imperial College London says you can increase the energy a reactor produces in three ways: you can make the plasma really hot, increase its density, or increase the time you keep it locked up.
“This team concludes that density is somewhat lower than with the traditional approach, which is not necessarily a bad thing, as it is compensated by higher temperatures in the core of the plasma,” he says. “It’s an exciting result, but there’s great uncertainty about how well our understanding of physics scales to larger devices. A device like ITER is much bigger than KSTAR’.
FIRE
Researcher Na says low plasma density was the breakthrough, and that fast, more energetic ions in the core of the plasma — so-called fast ion-regulated gain (FIRE) — are integral to stability. But the team doesn’t fully understand the mechanisms involved yet.
The reaction was stopped after 30 seconds due to equipment limitations. Longer periods should be possible. KSTAR has now been shut down for upgrades, replacing carbon parts on the reactor wall with tungsten, which should improve the reproducibility of the experiments.
technical physicist Lee Margetts of the University of Manchester (UK) says we understand the physics of fusion reactors better, but many technical hurdles still need to be overcome before we can build a working plant.
Generate electricity
Part of this is developing methods for extracting heat from the reactor and using it to generate electrical power. ‘That’s not physics, that’s technology,’ he says. ‘Think of it as a gas or coal-fired power station: if you don’t have anything to dissipate the heat, the staff will quickly say: ‘We have to switch it off because it gets too hot, the reactor will soon melt.’ That’s exactly the situation here.’
fusion physicist Brian Appelbe from Imperial College London agrees that the scientific challenges must be achievable, and that FIRE is a step forward, but also that commercialization will be difficult.
“The magnetic confinement fusion approach has a long history of evolving to solve the next problem,” he says. ‘What makes me a little nervous or insecure are the technical challenges of building an economically viable power plant.’