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Why does a toroidal, doughnut-shaped plasma reduce end losses in fusion devices?

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How scientists use lasers to learn about the universe — plasma Learn about the PHELIX (Petawatt High-Energy Laser for Heavy Ion Experiments) laser at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany. PHELIX is used for plasma and atomic physics research.

In a linear fusion device, plasma is confined within a straight magnetic field, which allows charged particles to stream freely out of the open ends, resulting in significant energy loss.12

By forming the plasma and the magnetic field into a closed, doughnut-shaped configuration known as a torus, these end losses are eliminated entirely. In this toroidal geometry, the magnetic field lines wrap around the device, ensuring that particles remain confined within the magnetic structure rather than escaping through an open boundary.12

To maintain a stable equilibrium within this shape, the magnetic field must be helical, consisting of both a toroidal component (pointing the long way around the torus) and a poloidal component (pointing the short way around). This configuration prevents particles from drifting to the walls and keeps the plasma contained, which is essential for achieving the conditions necessary for nuclear fusion.31