Scientists have produced a tokamak plasma that is stable at 10 times the Greenwald limit.

Physicists at 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. The researchers caution, however, that their plasma is not directly comparable to that in a fusion reactor.

It is believed that to obtain positive energy for nuclear fusion, density is crucial: the more atomic nuclei collide with 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. Over the decades, the so-called Greenwald limit has been exceeded at most twice.

Tokamak devices are a prime candidate for a nuclear fusion reactor.

“Tokamak devices are considered a major contender in the race to build a fusion reactor that will generate energy in the same way as the sun,” said Noah Hurst, a scientist at Wisconsin Plasma Physics Laboratory (WiPPL) and lead author of the study.

“Our discovery of this extraordinary ability to operate well above the Greenwald limit is important for increasing fusion energy production and preventing damage to the machines.”

The new study, published in Physical Review Letters, is part of the tokamak experiments in the Madison Symmetric Torus, which have increased the density to an unprecedented level about ten times that limit. This is believed to be possible in part because of a thick, stabilizing, conductive wall and a high-voltage, feedback-controlled power supply that drives the plasma current.

The radial profile of the toroidal current flattens to about twice this boundary, without any edge collapse as is routinely observed in other experiments.

For many years, MST has been one of the leading programs investigating the reverse field pinch phenomenon, a toroidal configuration closely related to the tokamak.

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

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

Scientists looked at the plasma density

Scientists at the University of Wisconsin studied the density of the plasma, trying to destabilize it by blowing more and more gas into it.

They set the power supply to provide the voltage needed to maintain a constant current of 50,000 amps in each plasma (as the plasma density increases, it becomes more resistive, and more voltage is needed to keep the current constant). They measured the resulting plasma density using interferometers, observing the plasma along 11 different lines of sight.

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

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

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

“Future reactor-sized tokamaks will likely need to operate near or above the Greenwald limit, so if we better understand what causes the density limit and understand the physics of how we got to ten times that limit, then we might have a chance of doing something about it.”

The researcher also stressed that it’s unlikely that these results can be directly applied to fusion reactors like ITER and others being built in the hopes of becoming the first tokamaks to produce net positive energy. But he and his team are cautiously optimistic.

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

“We will continue to study these plasmas and believe our knowledge can help higher-performance fusion devices operate at the higher densities needed to operate effectively.”