Fusion reactor designs with 'long legs' show promise

Magnetic fusion is all about managing the interface between hot plasma and ordinary materials. The strong magnetic field in a tokamak—the vessel used in this fusion approach—is a very effective insulator; it is able to reduce the plasma temperature by a factor of 100, from over 100 million degrees Celsius at the center to "only" 1 million degrees at the edge. However, this is not low enough. Therefore, it's the job of the boundary plasma to reduce the temperature by another factor of