Plasma in a fusion reactor is like a swarm of fireflies that you need to keep inside an invisible jar. It used to be thought that the jar's walls had to be perfectly smooth, or the fireflies would find a crack and escape. New work shows that the jar can be glued together from pieces with noticeable seams, and the swarm will still stay inside.
Conventional magnetic traps, tokamaks and most stellarators, are built on the principle of "omnigenity": the field is designed so that charged particles, wherever they drift, return to their orbits. It's like a maze where each corridor smoothly transitions into another. But perfectly smooth mazes are hard to build. The authors proposed a "piecewise omnigenous" field: the maze is assembled from sections, and at the joints, corridors don't align perfectly. A particle has to hop over a small threshold.
Computer tests showed that such a trap not only holds hot gas but is also robust across different operating regimes. The plasma mixes less, loses fewer fast particles, and barely generates its own currents that spoil confinement. Moreover, the design is compatible with an island divertor — a device that removes impurities and fusion ash.
The list of candidates for future reactors has grown. Now engineers can choose not only among perfectly smooth mazes but also among those assembled from pieces. This gives fusion energy more chances to find a workable and not-too-expensive form.
The word "stellarator" comes from the Latin stella, meaning star. These traps are named so because, like in stars, plasma is confined by a magnetic field.