An anonymous reader quotes a report from New Scientist: A nuclear fusion reaction has lasted for 30 seconds at temperatures in excess of 100 million degrees celsius. While the duration and temperature alone aren’t records, the simultaneous achievement of heat and stability brings us a step closer to a viable fusion reactor — as long as the technique used can be scaled up. […] Now Yong-Su Na at Seoul National University in South Korea and his colleagues have succeeded in running a reaction at the extremely high temperatures that will be required for a viable reactor, and keeping the hot, ionized state of matter that is created within the device stable for 30 seconds.
Controlling this so-called plasma is vital. If it touches the walls of the reactor, it rapidly cools, stifling the reaction and causing significant damage to the chamber that holds it. Researchers normally use various shapes of magnetic fields to contain the plasma — some use an edge transport barrier (ETB), which sculpts plasma with a sharp cut-off in pressure near to the reactor wall, a state that stops heat and plasma escaping. Others use an internal transport barrier (ITB) that creates higher pressure nearer the center of the plasma. But both can create instability. Na’s team used a modified ITB technique at the Korea Superconducting Tokamak Advanced Research (KSTAR) device, achieving a much lower plasma density. Their approach seems to boost temperatures at the core of the plasma and lower them at the edge, which will probably extend the lifespan of reactor components.
Dominic Power at Imperial College London says that to increase the energy produced by a reactor, you can make plasma really hot, make it really dense or increase confinement time. “This team is finding that the density confinement is actually a bit lower than traditional operating modes, which is not necessarily a bad thing, because it’s compensated for by higher temperatures in the core,” he says. “It’s definitely exciting, but there’s a big uncertainty about how well our understanding of the physics scales to larger devices. So something like ITER is going to be much bigger than KSTAR”. Na says that low density was key, and that “fast” or more energetic ions at the core of the plasma — so-called fast-ion-regulated enhancement (FIRE) — are integral to stability. But the team doesn’t yet fully understand the mechanisms involved. The reaction was stopped after 30 seconds only because of limitations with hardware, and longer periods should be possible in future. KSTAR has now shut down for upgrades, with carbon components on the wall of the reactor being replaced with tungsten, which Na says will improve the reproducibility of experiments. The research has been published in the journal Nature.