In a tokamak, plasma is like a crowd on a narrow street: once someone starts running, everyone takes off at once. In a stellarator with a non-circular cross-section, the street is winding, with dead ends and pockets. The crowd can't gather into a single wave — people get stuck in different corners. So do plasma waves: in three-dimensional geometry, they localize like sound in a maze.
Scientists have shown that instability in a stellarator turns into a network of individual wave packets. Each packet sits in its own pocket and can't merge with its neighbors. A global disruption is when waves break through all the pockets, like water through cracks in a dam. If there are many pockets and they are isolated, no disruption occurs. This is like water seeping through a porous material: as long as the pores don't connect into a through-channel, the water stays put.
This approach explains why stellarators don't suffer from the ballooning disruptions that plague tokamaks. But there's a nuance: in quasi-symmetric stellarators, which are deliberately made to resemble tokamaks, pockets can connect. Then the protection weakens. This makes the percolation threshold an important parameter in reactor design. Engineers will be able to assess in advance how stable the plasma will be in a given shape.
Plasma in a stellarator is like water in a sponge: as long as the pores aren't connected, there's nowhere to flow. In a tokamak, it's like water in a pipe: one crack, and the flow gushes out. Three-dimensional geometry prevents cracks from merging into one big breach.
The word 'stellarator' comes from the Latin 'stella'—star: the shape of the magnetic field resembles a star, and plasma is confined as it is in a star.
In Larry Niven's novel 'Ringworld', a fusion reactor based on magnetic confinement is a dream that stellarators are bringing closer to reality.