Scientists produce tokamak plasma that’s stable at 10x Greenwald limit

Physicists from the University of Wisconsin–Madison have developed a tokamak plasma that is stable at ten times the Greenwald limit. Their achievement is expected to have implications for tokamak fusion reactors. However scientists have cautioned that their plasma is not directly comparable to that in a fusion reactor.

It’s believed that if net-positive fusion energy is ever to be achieved, density is key: the more atomic nuclei crashing into each other, the more efficient the reaction will be.

Almost 40 years ago, Martin Greenwald identified a density limit above which tokamak plasmas become unstable, and the so-called Greenwald limit has, at best, been exceeded by a factor of two in the ensuing decades.

Tokamak devices are leading contender for nuclear fusion reactor

“ Tokamak devices are considered a leading contender in the race to build a nuclear fusion reactor that generates power in the same way as the Sun,” says Noah Hurst, a scientist with the Wisconsin Plasma Physics Laboratory (WiPPL) and lead author on the study.

“Our discovery of this unusual ability to operate far above the Greenwald limit is important for boosting fusion power production and preventing machine damage.”

The new study published in Physical Review Letters is part of tokamak experiments in the Madison Symmetric Torus with a density up to an unprecedented level of about ten times this limit. This is thought to be made possible in part by a thick, stabilizing , conductive wall, and a high-voltage, feedback-controlled power supply driving the plasma current.

The radial profile of the toroidal current flattens around twice the limit without the edge collapse routinely observed in other experiments.

MST has operated for many years as one of the leading programs studying the reversed field pinch, a toroidal configuration closely related to the tokamak.

MST was designed to anticipate operation as a tokamak, allowing direct comparison of the two toroidal configurations in the same device. Unlike other tokamaks, the metal donut that houses the MST plasmas is thick and highly conducting, allowing for more stable operation, according to University of Wisconsin.

“My job was to try to find ways to make the plasma go unstable,” said Hurst. “I tried, and I found that, well, in many cases, it doesn’t. It was surprising.”

Researchers looked into plasma density

Researchers at the University of Wisconsin looked into plasma density, trying to destabilize the plasma by puffing in more and more gas.

They set the power supply to provide whatever voltage was needed to maintain a steady 50000 amps of current in each plasma (as plasma density increases, it becomes more resistive, and more voltage is needed to keep the current steady). They measured the achieved plasma density with interferometers viewing the plasma along 11 different lines of sight.

According to the University of Wisconsin , the Greenwald limit is just the ratio of the plasma density to the product of the plasma current and plasma size, a simple metric that allows comparison of different devices and operating conditions. Since the limit was defined, only a handful of devices have operated above it, and by at most a factor of two.

Future tokamaks will likely need to operate near or above the Greenwald limit

“Here, we were at a factor of ten,” Hurst said.

“Future reactor-scale tokamaks will likely need to operate near or above the Greenwald limit, so if we can better understand what’s causing the density limit and understand the physics of how we got to ten times the limit, then maybe we have a shot at doing something about it.”

The researcher also underlined that these results are unlikely to be directly applicable to fusion reactors, such as ITER and others that are being built in the hopes of being the first net-positive energy production tokamaks. But he and the team are cautiously optimistic.

“Our results were obtained in a low magnetic field, low temperature plasma, which is not capable of fusion power production. Still, we were the first ones to be able to do this, and you have to start somewhere,” added Hurst.

“We’re going to keep studying these plasmas, and we think that what we learn might help higher-performance fusion devices to operate at the higher densities they need to be successful.”