What is the connection between boron, an element of a common household cleaner, and tokamaks, ring-shaped smelters that heat fuel to temperatures of a million degrees? Scientists from the Princeton Plasma Physics Laboratory (PPPL) of the United States Department of Energy (DOE) have conducted research showing that a powder dropper developed by PPPL can successfully deposit boron powder in plasma at high temperature in tokamaks whose parts are made of heat-resistant material. known as tungsten. 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.
Due to its high melting point, tungsten is increasingly used in tokamaks to help components withstand the intense heat of the melting process. The boron partly protects the tungsten from the plasma and prevents the tungsten from leaking into the plasma; it also absorbs any parasitic 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 need 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, postdoctoral researcher at PPPL who was the lead author of the research paper reporting the leads to nuclear fusion. The research was carried out using the W Environment Steady State (WEST) tokamak, operated by France’s 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 physicists performed their experiments with WEST is that its magnets are made of a superconducting material that will feature in the magnets inside future fusion devices. This material conducts electricity with little or no resistance and produces little excess heat, so the magnets can run non-stop for long periods of time, as future fusion reactors will have to do. The magnets create the forces that hold the plasma together 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 composed of free electrons and atomic nuclei – which generates enormous amounts of energy . Scientists seek to replicate fusion on Earth for a virtually inexhaustible supply of energy to generate electricity.
Scientists need a way to replenish boron coatings while machines are running, because future smelting facilities won’t be able to stop often to coat. “Dropping boron into a tokamak while it’s running is like cleaning your apartment while doing all the other things you usually do there,” said CEA scientist Alberto Gallo, who contributed to the research. “It’s very helpful – it means you don’t have to take extra time from your usual activities to do housework,” he said.
The powder dropper device is mounted on the top of the tokamak and uses precise actuators to move the powder material from their reservoirs to the tokamak’s vacuum chamber. This mechanism allows researchers to precisely set the rate and duration of powder drops, which in other fusion setups may include other performance-enhancing materials like 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 internal surfaces of the tungsten. “We saw that when we dropped the powder, the confinement of the plasma increased, which means it holds its heat more, which aids the fusion process,” Bodner said.
The increased confinement was particularly useful because it occurred without the plasma entering a state known as H mode (high confinement mode), in which confinement improves but the plasma is more susceptible to burst with what are called edge-localized modes, or ELMs. These ELMs conduct heat away from the plasma, reducing the efficiency of fusion reactions and sometimes damaging internal components. “If we can use the dropper to get the right H-mode confinement without actually going into H-mode and risking ELMs, that would be great for fusion reactors,” Bodner said.
In the future, the researchers want to test the use of the dropper only when necessary to maintain good plasma performance. “Adding additional impurities, even boron, can reduce the fusion power you get because the plasma becomes less pure,” Bodner said. “Therefore, we have to 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 coats tungsten surfaces. “We want to measure these quantities so that 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 École polytechnique de France. Funding was provided by the Office of Science of DOE (Fusion Energy Sciences).
