Boron "detergent" for nuclear fusion

Scientists at the U.S. Department of Energy's Princeton Plasma Physics Laboratory have found a way to add a dash of boron to a tungsten tokamak fusion reactor to keep its plasma clean during operations.

Because of its high melting point, tungsten is a metal of choice for helping tokamak components withstand the intense heat of the fusion process – often upwards of 150 million degrees Kelvin, or ten times the temperature at the core of the Sun.

While it is not new to use boron to shield tungsten from plasma, there lies a simple issue of having to shut down the reactor to clean and recoat the parts every time.

Not only does the plasma reaction degrade the tungsten, but once contaminants enter the plasma, it can reduce the output of the reactor. Boron is used because it not only can partly shield tungsten from plasma, but it can also prevent tungsten from leaking into the plasma.

It can also absorb any stray elements like oxygen that may be in the plasma from other sources, undesirable impurities that can essentially cool down the plasma and ultimately quench fusion reactions.

"We need a way to deposit boron coatings without turning off the tokamak's magnetic field, and that's what the powder dropper allows us to do," said Grant Bodner, postdoctoral researcher at Princeton Plasma Physics Laboratory and lead author of the study.

The research was performed using the W Environment in Steady-State Tokamak, or simply WEST, operated by France's Atomic Energy Commission.

"WEST is one of the few full-tungsten environments that can help us test this technology as long pulses," Bodner added.

Another significant reason why the physicists chose WEST is that its magnets are made of superconducting material that is the closest representation to future fusion devices.

Upon picking the ideal tokamak, PPPL needed a way to replenish the boron coatings while the machines are operating because future fusion facilities will not be able to shut down often for re-coating.

"Dropping boron into a tokamak while it is operating is like cleaning your apartment while doing all the other things that you usually do in it," said CEA scientist Alberto Gallo, who contributed to the research. "It's very helpful – it means you don't have to take extra time out of your usual activities to do the cleaning."

The powder dropper device is mounted to the top of the tokamak and uses precise actuators to move powdered material from their reservoirs to the tokamak's vacuum chamber.

This mechanism allows researchers to precisely set the rate and duration of the powder drops, which in other fusion facilities could also include performance-boosting materials like lithium.

"Because of that flexibility, the dropper has the potential to be really useful in the future," Bodner said.

Additionally, the researchers were surprised to find that the boron spread by the dropper did more than condition the inner tungsten surfaces.

"We saw that when we dropped in the powder, the plasma confinement increased, meaning that it retains more of its heat, which aids the fusion process," added Bodner.

The increased confinement was especially helpful because it occurred without the plasma entering a state known as H-mode (high-confinement mode), in which the confinement improves but the plasma is more likely to erupt with what are known as edge-localized modes, or ELMs.

These ELMs move heat out of the plasma, reducing the efficiency of the fusion reactions and sometimes damaging internal components.

"If we can use the dropper to get the good confinement of H-mode without actually entering H-mode and risking ELMs, that would be great for fusion reactors," continued Bodner.

In the future, the researchers want to test using the dropper only when necessary to maintain good plasma performance.

"Adding any extra impurities, even boron, can reduce how much fusion power you get because the plasma becomes less pure," added the research lead. "Therefore, we have to try to use the smallest amount of boron that can still produce the effects we want."

Upcoming experiments will focus on how much boron is actually coating the tungsten surfaces.

"We want to measure these amounts so we can really quantify what we're doing and extend these results in the future," Bodner finished.