Putting an end to extravagant behavior in fusion reactors

Fusion researchers are increasingly turning to the element tungsten when searching for an ideal material for components that will directly face the plasma inside fusion reactors called tokamaks and stellarators. But under the intense heat of the fusion plasma, the tungsten atoms in the wall can spit out and enter the plasma. Too much tungsten in the plasma would cool it considerably, making it very difficult to sustain fusion reactions.

Now, researchers at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have experimental results suggesting that sprinkling boron powder into the tokamak could solve the problem. Boron partly protects the reactor wall from the plasma and prevents atoms from the wall from penetrating the plasma.

A new computer modeling framework, also led by PPPL researchers, shows that the powder may only need to be sprayed from one location. The experimental results and computational modeling framework will be presented this week at the 66th Annual Meeting of the Plasma Physics Division of the American Physical Society in Atlanta.

Joseph Snipes, deputy director of Tokamak Experimental Science, is optimistic about the solid boron injection system, based on experiments demonstrating reduced tungsten sputtering after solid boron injection. The experiments were carried out in three tungsten-walled tokamaks around the world: one in Germany, one in China and one in the United States.

“Boron is sprinkled into the tokamak plasma in the form of a powder, like that of a salt shaker, which is ionized at the edge of the plasma and then deposited on the internal walls of the tokamak and in the exhaust region,” he said. he explained. “Once coated with a thin layer of boron, it will prevent tungsten from penetrating the plasma and radiating plasma energy.”

Snipes and his colleagues are working on the boron injection system with the ultimate goal of potentially using it in the ITER organization’s reactor-scale tokamak. The injection system is well suited to the task because it can add boron while the machine is running. It also makes it possible to precisely control and limit the quantity of boron injected. The deposited boron layers retain the radioactive element tritium, which must be minimized in the ITER tokamak to respect nuclear safety. Scientists and engineers from ITER and Oak Ridge National Laboratory also collaborated on this project.

Florian Effenberg, a research physicist at PPPL, led a separate project to create a computational modeling framework for the boron injection system in the DIII-D tokamak. The framework suggests that sprinkling the boron powder from a single location can provide a sufficiently uniform distribution of boron across the reactor components considered in the simulation domain.

“We have developed a new way to understand how injected boron behaves in a fusion plasma and how it interacts with the walls of fusion reactors to keep them in good condition during operation,” Effenberg said.

The researchers’ approach combines three different computational models to create a new framework and workflow. “One model simulates plasma behavior, another shows how boron powder particles move and evaporate in plasma, and the third examines how boron particles interact with the walls of the tokamak, including how they stick and wear and mix with other materials,” Effenberg said.

“This information is crucial for optimizing boron injection strategies to achieve efficient and uniform wall conditioning of ITER and other fusion reactors,” Effenberg said.

While the modeling framework focused on DIII-D, a tokamak operated by General Atomics in San Diego, the next phase of this research is to extend the modeling framework to ITER. Although the walls of DIII-D are made of carbon, ITER plans to have walls made of tungsten. It will therefore be important to study the differences in the way boron protects the walls.