Physicists have recently solved a mysterious fusion mystery that baffled scientists

When the particles reach the divertor, they hit the metal plates, cool, and bounce back. (The returning atoms help fuel the combined reaction.) However, experiments have repeatedly revealed unexpected imbalances. More particles hit the inner divertor target than the outer one.

This uneven distribution is not just a matter of curiosity. It has major implications for future fusion reactors. Engineers must know exactly where the particles will fall in order to design materials that can withstand extreme heat and stress. So far, the main explanation has focused on the grid, which describes how the particles move laterally across the lines of gravity in the divertor. But simulations that included only this effect failed to reproduce what the experiments showed, raising doubts about whether models can reliably guide reactor design.

The Plasma Cycle Appears to Be Absent

New research has uncovered an important piece of the puzzle. The scientists found that the toroidal rotation, the motion of the plasma as it rotates in the tokamak, strongly influences where the particles end up in the discharge system.

Using the SOLPS-ITER model code, the researchers simulated the particle’s behavior under a number of conditions. Their results, published in Physical Examination Lettersshowed that the simulations were only consistent with real-world measurements when plasma rotation was included along with cross-field drifts. This connection between models and experiments is essential for designing fusion systems that can reliably operate outside the lab.

“There are two parts to the plasma flow,” said Eric Emdee, a research physicist at the US Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL) and lead author of the study. “There is cross-field flow, where particles are swept sideways across the lines of gravity, and parallel flow, where they travel along those lines. Many people say that cross-field flow is what caused the asymmetry. What this paper shows is that parallel flow, driven by a rotating core, is very important.”

Simulations Match Reality Finally

To test their hypothesis, the team modeled the behavior of plasma in a DIII-D tokamak in California. They went through four different levels, turning on and off drifts and rotating plasma. The results were clear. None of the simulations matched the experimental data until one key ingredient was included: the core’s rotational speed of 88.4 kilometers per second.

Once these two effects were included, the models produced the uneven distribution of particles seen in real experiments. The combined effect of lateral and rotational drift has proven to be more powerful than either element alone.

Creating Fusion Systems for Real-Time Situations

The findings highlight the important relationship between circulating plasma composition and the behavior of particles at the boundary of the system. Accurately capturing this relationship will be important for predicting how steam particles behave in future reactors.

Better estimates mean better engineering. With a clear understanding of where heat and particles will concentrate, designers can build divertors that are more stable and suitable for real operating conditions.

In addition to Emdee, the research team included Laszlo Horvath, Alessandro Bortolon, George Wilkie and Shaun Haskey of PPPL; Raúl Gerrú Migueláñez of the Massachusetts Institute of Technology; and Florian Laggner of North Carolina State University.

This work was supported by the DOE Office of Fusion Energy Sciences, through the DIII-D National Fusion Facility, a DOE Office of the User Facility, under awards DE-AC02-09CH11466, DE-FC02-04ER54698, DE-SC0024523, DE-SC00142069 and DE-SC0013SC069 and DE-SC00142069