Scientists have puzzled over a pattern in tokamaks, which are doughnut-shaped machines designed to create electricity by fusing atoms. In these devices, plasma - a hot, charged gas - escapes the magnetic fields holding it in place and heads to the divertor, an exhaust system. There, particles hit metal plates, cool off, and return to fuel the fusion process.
Experiments show more particles strike the inner plate than the outer one. Knowing why this uneven distribution happens is key for building divertors that can withstand the heat in future fusion systems. Past ideas focused on cross-field drifts, where particles move sideways across magnetic lines in the divertor. But simulations using only this effect did not match real results, raising doubts about using them for designs.
Solving the puzzle
New computer models reveal that toroidal rotation - the spinning motion of plasma around the tokamak - works with cross-field drifts to cause the imbalance. Researchers used a code called SOLPS-ITER to test particle paths under various conditions. The results, published in Physical Review Letters, show that adding measured core rotation of 88.4 kilometers per second makes simulations align with experiments. By "combining drifts, rotation, and viscous coupling, existing boundary plasma models can achieve a satisfactory agreement with experimentally measured neutral asymmetries." This rotation drives parallel flow, where particles move along magnetic lines, matching the importance of cross-field flow.
Physicists at the Princeton Plasma Physics Laboratory (PPPL) noted that plasma flow has two parts: sideways drifts and along-field movement from the spinning core. Models of the DIII-D tokamak in California tested scenarios with and without drifts and rotation. Only combining both matched data. The effect together is stronger than either alone. This insight means future predictions must include how core spin affects edge flows, helping engineers create tougher divertors.
These findings could make fusion reactors more reliable for clean energy. The work involved researchers from Princeton Plasma Physics Laboratory, MIT, and North Carolina State University.