Researchers at the Princeton Plasma Physics Laboratory have identified plasma rotation as the key factor explaining why particles in fusion tokamaks strike one side of the exhaust system more than the other. Their simulations, which matched real experiments, combined rotation with sideways drifts. The discovery could improve designs for future fusion reactors.
Fusion experiments in tokamaks have long puzzled scientists with an imbalance in the divertor, the exhaust system where escaping plasma particles strike metal plates. Far more particles hit the inner divertor target than the outer one, complicating designs for heat-resistant components in reactors meant to generate electricity from atom fusion. Previous models relying only on cross-field drifts—sideways particle movement across magnetic lines—failed to replicate this pattern observed in experiments. The breakthrough came from including toroidal rotation, the plasma's circling motion around the tokamak. Eric Emdee, an associate research physicist at the U.S. Department of Energy's Princeton Plasma Physics Laboratory, led the study published in Physical Review Letters. Using the SOLPS-ITER code, the team simulated conditions in California's DIII-D tokamak. They tested scenarios toggling drifts and rotation, finding matches to data only when incorporating the core rotation speed of 88.4 kilometers per second alongside drifts. Emdee explained, 'There are two components to flow in a plasma... parallel flow, driven by the rotating core, matters just as much.' The team, including researchers from PPPL, MIT, and North Carolina State University, highlighted the link between core rotation and edge particle behavior. This understanding will aid in building resilient divertors for practical fusion systems, supported by the DOE's Office of Fusion Energy Sciences.
