The challenge
Scientists have long seen a puzzling pattern in tokamaks, the doughnut-shaped machines that could one day reliably generate electricity from fusing atoms. When plasma particles escape the core of the magnetic fields that hold the plasma in its doughnut shape, they stream down toward the exhaust system, known as the divertor. There, plasma particles strike metal plates, cool down and bounce back. (The returning atoms help fuel the fusion reaction.) But experiments consistently show that far more particles hit the inner divertor target than the outer one.
Understanding what drives this lopsided distribution matters for designing future fusion systems: Engineers need to know where exhaust particles will land to build divertors that can handle the heat. The leading explanation centered on what's known as cross-field drifts within the divertor itself, the sideways movement of particles across magnetic field lines. However, computer simulations that included only this kind of drift couldn't reproduce the uneven striking pattern in experiments, making it difficult to trust that simulations could reliably guide divertor design for future machines.
The science
New simulations show that the toroidal rotation - the motion of the particles as they move around the tokamak - plays a key role in determining exactly where the plasma fuel lands in the machine's exhaust system. A team of researchers used the modeling code SOLPS-ITER to simulate the path of the particles under different conditions. Their findings, which were published in Physical Review Letters, show that when plasma core rotation is combined with cross-field drifts, simulations finally match experimental measurements. Aligning simulations with experimental results is critical to designing fusion power plants that can withstand the demands of real-world operations.
"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."
The team modeled plasma in the DIII-D tokamak in California, testing four scenarios: with and without cross-field drifts and with and without plasma rotation. The simulations couldn't come close to matching experiments until the team added one ingredient: the measured core rotation of 88.4 kilometers per second.
The combined effect proved far greater than either component alone. The finding suggests that accurately predicting exhaust behavior in future fusion systems will require accounting for how the rotating plasma core influences edge flows, a connection that could help engineers design divertors that can better handle reality.
The team
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.
The funding
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.
