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Why do magnetic confinement reactors use a doughnut-shaped, or toroidal, design?

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3D illustration of the interior reaction chamber of a tokamak fusion reactor, showing a large, cylindrical, metallic structure with curved walls and bright light panels. — fusion reactor A tokamak is a device used in nuclear-fusion research for magnetic confinement of plasma.

Magnetic confinement reactors utilize a doughnut-shaped, or toroidal, design primarily to eliminate the problem of end loss found in linear systems. In a straight magnetic field configuration, plasma is free to stream out of the ends, making it impossible to maintain the necessary conditions for sustained fusion.12

By forming the plasma and magnetic field into a closed torus, researchers can effectively contain the plasma away from material boundaries. Within this toroidal geometry, magnetic field lines follow a helical path, which is essential for stability. This helical field consists of two components:123
* Toroidal component: Points the long way around the torus.3
* Poloidal component: Points the short way around the machine.3

Both components are required for the plasma to remain in a stable equilibrium; without the poloidal field, particles would drift to the walls, and without the toroidal field, the plasma would develop distortions that carry it into the wall. Thus, the toroidal design provides the necessary topology to balance pressure and maintain stable confinement.32