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.
