Tokamaks confine plasma in a doughnut-shaped magnetic field where light atomic nuclei can fuse and release energy. When some plasma particles escape the core, they travel along magnetic field lines into the divertor region, hit specially designed metal plates, cool, and then bounce back as neutral atoms that can help fuel the fusion reaction. For years, measurements have shown a strong asymmetry in how these particles deposit on the inner versus outer divertor targets, raising concerns about how to design exhaust systems that can survive real-world conditions.
Earlier studies largely focused on cross-field drifts inside the divertor, where particles move sideways across magnetic field lines, as the main driver of the asymmetric exhaust pattern. However, simulations that included only these drifts consistently failed to match the observed distribution of particles on the divertor plates. That mismatch made it difficult to trust numerical models as reliable tools for predicting where heat and particles will land in future power-producing reactors.
New simulations now show that rotation of the plasma core around the tokamak, known as toroidal rotation, plays a crucial role in shaping where exhaust particles eventually land. Using the SOLPS-ITER modeling code, a research team tracked plasma behavior under different conditions and tested how various combinations of rotation and cross-field drifts affected the exhaust pattern. Their study demonstrates that only when both effects are included together do the simulations reproduce the strong imbalance between the inner and outer divertor targets seen in experiments.
"There are two components to flow in a plasma," said Eric Emdee, an associate research physicist at the U.S. Department of Energy's 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 focused on the DIII-D tokamak in California, a major U.S. fusion research facility, and ran simulations for four distinct scenarios: with and without cross-field drifts, and with and without plasma rotation. In each case, they compared the calculated divertor particle loads with experimental measurements.
The models only came into close agreement with experiments when they incorporated the measured core rotation speed of about 88.4 kilometers per second, revealing that rotation can strongly influence the direction and strength of flows along magnetic field lines near the plasma edge.
By combining toroidal rotation with cross-field drifts, the simulations produced a much larger asymmetry in particle deposition than either effect generated on its own. This combined influence proved essential for reproducing the experimentally observed pattern in which the inner divertor target receives far more particles than the outer target.
The result implies that any realistic attempt to predict exhaust behavior in future fusion power plants must include accurate information about how the core plasma rotates and how that rotation couples to edge and divertor flows.
Understanding and predicting this behavior is critical for designing divertors that can safely manage the extreme conditions expected in commercial fusion systems. If engineers underestimate where heat and particles will concentrate, components could wear out or fail much sooner than planned. With the improved modeling approach, designers can better estimate the distribution of particle and heat loads and develop exhaust systems that stand up to demanding reactor environments over many years of continuous operation.
In addition to Emdee, the research team included Laszlo Horvath, Alessandro Bortolon, George Wilkie and Shaun Haskey of PPPL; Raul Gerru Miguelanez of the Massachusetts Institute of Technology; and Florian Laggner of North Carolina State University. Their combined expertise in plasma physics, numerical modeling and tokamak experiments helped link detailed simulation results with measurements from the DIII-D facility.
The study, published in Physical Review Letters, highlights the growing importance of integrated modeling that treats core and edge plasma behavior as a single coupled system rather than separate domains. By showing that core rotation can strongly influence conditions in the scrape-off layer and divertor, the work points to new strategies for tailoring rotation profiles to optimize exhaust performance. It also underscores the value of high-quality measurements of plasma flows throughout the machine to validate and refine predictive models.
The research used data from the DIII-D National Fusion Facility, a U.S. Department of Energy Office of Science user facility dedicated to advancing magnetic fusion energy. Funding for the work came from the DOE's Office of Fusion Energy Sciences under several awards supporting both PPPL and collaborating institutions. Together, these efforts aim to develop the scientific understanding and engineering tools needed to make practical fusion energy a reality.
Research Report:Combined Influence of Rotation and Scrape-Off Layer Drifts on Recycling Asymmetries in Tokamak Plasmas
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Princeton Plasma Physics Laboratory
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