Nuclear fusion researchers have surpassed the theoretical “Greenwald limit” for plasma density while making strides in ensuring reactors don’t tear themselves apart, bringing the goal of limitless clean power another step closer.
Generating denser and better-confined plasma is crucial to realising fusion energy on a commercial scale.
But scientists using tokamak reactors, one of the two main technologies being used to develop fusion energy, have to date been thwarted in generating super dense plasma while keeping it in what is known as high confinement mode.
They had been running up against the so-called Greenwald limit. This is a point it was theorised you could not raise the density of the plasma past without it escaping the magnets – potentially doing major damage to the tokamak itself.
Researchers at US energy and defence firm General Atomics have claimed in a paper published in Nature to have broken through that limit on a small tokamak reactor.
Tokamak reactors are shaped like doughnuts and use huge magnets that are used to contain plasma hotter than the surface of the sun.
They seek to harness fusion, the same process that generates light and heat from stars, to unlock unlimited zero-carbon electricity. Fusion is sometimes referred to as the ‘Holy Grail’ of the energy transition.
To be economically viable, the researchers said that most tokamak designs require reaching a plasma density above the Greenwald limit while achieving better confinement than in the typical high confinement mode.
Previous attempts to break the Greenwald limit have they said caused a “strong decrease of the confinement quality or even a sudden, complete loss of plasma energy."
Another challenge when trying to surpass this limit is that “instabilities” in the plasma can “severely damage” components in the reactor itself.
Breaking the limit while maintaining confined plasma was therefore a “great challenge for the magnetic confinement fusion community.”
Now, the researchers, working at a San Diego-based facility General Atomics runs for the US Department of Energy, say they have managed to generate stable plasma roughly 20% denser than the Greenwald limit with an energy confinement quality around 50% better than the standard high-confinement mode.
“The achievement of entering this previously uncharted regime” can the researchers said help support “critical requirements in many fusion reactor designs all over the world and opens a potential avenue to an operating point for producing economically attractive fusion energy.”
One of the General Atomics researchers, Siye Ding, told Recharge that his team has helped show the “unique synergy between high density and high confinement, and the conditions that are required to achieve this synergy.”
The experiments also provided “potential solutions” to the challenge tokamak reactors face in keeping the “fusion plasma edge cool enough that it doesn't damage the wall of the tokamak, while keeping the plasma core hot enough in order to produce sufficient fusion power in a reactor,” he said.
The results he said therefore provide experimental evidence and an understanding of the physics involved that are “essential” to realising the most compact fusion power plant designs.
