Scientists Apply Boron to Tungsten Components in Fusion Facilities — ScienceDaily

What is the connection between boron, an element in a common household cleaner, and tokamaks, ring-shaped fusion devices that heat fuel to temperatures of a million degrees? Scientists at the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) have conducted research showing that a dust dropper developed by PPPL can successfully drop boron dust into high-temperature plasma within tokamaks that have parts made of a heat-resistant material known as tungsten. The scientists want to confirm that they can use this process to deposit boron on tungsten parts, since bare tungsten walls can harm plasma efficiency if the plasma damages the tungsten.

Because of its high melting point, tungsten is increasingly used in tokamaks to help components withstand the intense heat of the fusion process. Boron partially shields the tungsten from the plasma and prevents tungsten from leaking into the plasma; it also absorbs any stray elements such as oxygen that may be in the plasma from other sources. These unwanted impurities could cool the plasma and quench the fusion reactions.

“We needed a way to deposit boron coatings without turning off the magnetic field of the tokamaks, and that’s what the powder dropper allows us to do,” said Grant Bodner, a postdoctoral researcher at PPPL who was the lead author of the research paper reporting on leads to Nuclear fusion. The research was carried out using the W Environment in Steady-State Tokamak (WEST) operated by the French Atomic Energy Commission (CEA). “WEST is one of the few all-tungsten environments that can help us test this technology at long pulses,” Bodner said.

Another reason the physicists performed their experiments using WEST is that its magnets are made of a superconducting material that will be present in magnets in future fusion devices. This material conducts electricity with little or no resistance and produces little excess heat so that the magnets can operate without stopping for long periods of time, as future fusion reactors will need to do. The magnets create the forces that hold the plasma together so it can undergo fusion.

Nuclear fusion, the force that drives the sun and stars, combines light elements in the form of plasma – the hot, charged state of matter made up of free electrons and atomic nuclei – which generates enormous amounts of energy. Scientists are striving to replicate fusion on Earth for a virtually inexhaustible supply of energy to generate electricity.

Scientists need a way to restore the boron coatings while the machines are running, since future fusion facilities won’t be able to shut down frequently for recoating. “Putting pine into a tokamak while it’s running is like cleaning your apartment while doing all the other things you normally do in it,” said CEA scientist Alberto Gallo, who contributed to the study. “It’s very useful – it means you don’t have to take extra time out of your normal activities to clean,” he said.

The powder dropper device is mounted at the top of the tokamak and uses precision actuators to move powdered material from their reservoirs to the tokamak’s vacuum chamber. This mechanism allows researchers to precisely set the speed and duration of dust fall, which in other fusion facilities can include other performance-enhancing materials such as lithium. “Because of this flexibility, the dropper has the potential to be really useful in the future,” Bodner said.

The researchers were surprised to find that the boron deposited by the dropper did more than condition the inner tungsten surfaces. “We saw that when we released the dust, the retention of the plasma increased, meaning it retained more of its heat, which aided the fusion process,” Bodner said.

The increased confinement was particularly useful because it occurred without the plasma entering a state known as the H-mode (high confinement mode), where confinement is improved but the plasma is more likely to erupt with what is known as edge localized modes, or ELMs. These ELMs remove heat from the plasma, reducing the efficiency of fusion reactions and sometimes damaging internal components. “If we can use the dropper to get good H-mode confinement without actually going into H-mode and risking ELM, that would be great for fusion reactors,” Bodner said.

In the future, the researchers want to test using the dropper only when necessary to maintain good plasma performance. “Adding any additional impurities, even boron, can reduce the fusion power you get because the plasma becomes less clean,” Bodner said. “Therefore, we should try to use the smallest amount of boron that can still produce the effects we want.”

Future experiments will focus on how much boron actually covers the tungsten surfaces. “We want to measure these quantities so we can really quantify what we’re doing and expand on these results in the future,” Bodner said.

Collaborators include scientists from CEA, Oak Ridge National Laboratory, and France’s École Polytechnique. Funding was provided by the DOE Office of Science (Fusion Energy Sciences).

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Materials provided by DOE/Princeton Plasma Physics Laboratory. Original written by Raphael Rosen. Note: Content may be edited for style and length.

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