What is the connection between boron, an element in a common household cleaner, and tokamaks, ring-shaped fusion facilities that heat fuel to millions of degrees? Scientists at the US Department of Energy’s (DOE) Plasma Physics Laboratory (PPPL) have conducted research showing that a powder dropper developed by PPPL can successfully drop boron powder into the high-temperature plasma inside 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 apply boron to tungsten parts, because bare tungsten walls can impair plasma performance 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 the tungsten from flowing into the plasma; it also absorbs any stray elements such as oxygen that may be in the plasma from other sources. These unwanted impurities can cool the plasma and quench fusion reactions.
“We need a way to deposit boron coatings without turning off the tokamak’s magnetic field, and this is what allows us to make dust droplets,” said Grant Bodner, a postdoctoral researcher at PPPL, who was lead author of the paper. researcher who reported the results to us Nuclear fusion. The research was carried out using the W Environment at the Steady-State Tokamak (WEST), operated by the French Atomic Energy Commission (CEA). “WEST is one of the few full-tungsten environments that can help us test this long-pulse technology,” Bodner said.
Another reason physicists conduct their experiments using WEST is that its magnets are made of superconducting material that will appear in magnets inside future fusion devices. This material conducts electricity with little or no resistance and produces little excess heat, so 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 restrain the plasma so that it can undergo fusion.
Fusion, the power 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 — that generates massive amounts of energy. Scientists are looking to replicate fusion on Earth for a virtually inexhaustible supply of energy to generate electricity.
Scientists need a way to refill the boron coatings while the machines are operating, because future fusion devices won’t be able to shut down frequently for recoating. “Putting boron 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 research. “It’s very useful — it means you don’t have to take extra time out of your regular activities to do the cleaning,” he said.
The dust dropper is mounted on top of the tokamak and uses precision actuators to move the dust material from their reservoirs into the tokamak’s vacuum chamber. This mechanism allows researchers to precisely determine the rate and duration of dust fall, which in other fusion facilities may 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 surfaces of the tungsten. “We saw that when we got into the dust, the insulation of the plasma increased, which means it retains more of its heat, which helps the fusion process,” Bodner said.
The increased confinement was particularly useful because it occurred without entering a plasma state known as H mode (high confinement mode), in which confinement is improved but the plasma is more likely to explode in what are known as localized modes on the edge. or ELM. 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 jeopardizing the ELMs, that would be great for fusion reactors,” Bodner said.
In the future, the researchers want to test using droppers 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 amounts so we can really quantify what we’re doing and extend these results in the future,” Bodner said.
Collaborators included scientists from CEA, Oak Ridge National Laboratory, and France’s École Polytechnique. Funding was provided by the DOE Office of Science (Fusion Energy Sciences).
Materials provided by DOE/Princeton Plasma Physics Laboratory. Original written by Raphael Rosen. Note: Content may be edited for style and length.