Physicists just solved a strange fusion mystery that stumped experts

When the particles reach the divertor, they hit metal plates, cool, and rebound. (The returning atoms help fuel the fusion reaction.) However, experiments have consistently revealed an unexpected imbalance. Far more particles strike the inner divertor target than the outer one.

This uneven distribution is more than just a curiosity. It has major implications for future fusion reactors. Engineers must know exactly where particles will land in order to design divertors that can withstand extreme heat and stress. Until now, the leading explanation focused on cross-field drifts, which describe how particles move sideways across magnetic field lines within the divertor. But simulations that included only this effect failed to reproduce what experiments were showing, raising doubts about whether models could reliably guide reactor design.

Plasma Rotation Emerges as the Missing Factor

New research has uncovered a key piece of the puzzle. Scientists found that toroidal rotation, the motion of plasma as it circles around the tokamak, strongly influences where particles ultimately end up in the exhaust system.

Using the SOLPS-ITER modeling code, researchers simulated particle behavior under a range of conditions. Their results, published in Physical Review Letters, showed that simulations only matched real-world measurements when plasma rotation was included alongside cross-field drifts. This alignment between models and experiments is essential for designing fusion systems that can operate reliably outside the lab.

"There are two components to flow in a plasma," said Eric Emdee, an associate research physicist at the U.S. Department of Energy's (DOE) Princeton Plasma Physics Laboratory (PPPL) and lead author of the study. "There's cross-field flow, where particles drift sideways across the magnetic field lines, and parallel flow, where they travel along those lines. A lot of people said cross-field flow was what created the asymmetry. What this paper shows is that parallel flow, driven by the rotating core, matters just as much."

Simulations Match Reality at Last

To test their idea, the team modeled plasma behavior in the DIII-D tokamak in California. They ran four different scenarios, toggling cross-field drifts and plasma rotation on and off. The results were clear. None of the simulations matched experimental data until one critical ingredient was added: the measured core rotation speed of 88.4 kilometers per second.

Once both effects were included, the models closely reproduced the uneven particle distribution seen in real experiments. The combined influence of sideways drift and rotation proved much stronger than either factor on its own.

Designing Fusion Systems for Real Conditions

The findings highlight an important connection between the rotating plasma core and the behavior of particles at the edge of the system. Accurately capturing this relationship will be essential for predicting how exhaust particles move in future reactors.

Better predictions mean better engineering. With a clearer understanding of where heat and particles will concentrate, designers can build divertors that are more resilient and better suited to 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's Office of Fusion Energy Sciences, using the DIII-D National Fusion Facility, a DOE Office of Science user facility, under awards DE-AC02-09CH11466, DE-FC02-04ER54698, DE-SC0024523, DE-SC0014264 and DE-SC0019130.