Fusion Q&A
How does a tokamak work?
Short answer
A tokamak uses powerful magnetic fields shaped like a torus (donut) to confine a plasma of hydrogen isotopes at over 100 million Kelvin. The combined magnetic field — toroidal from external coils plus poloidal from current driven in the plasma itself — creates twisted field lines that trap the plasma long enough for deuterium and tritium nuclei to fuse.
The three magnetic fields
External D-shaped coils generate the toroidal field — the dominant field that wraps the long way around the donut.
A central solenoid drives a current through the plasma itself, which generates the poloidal field — the short-way component.
The combination produces twisted, helical field lines. Particles spiral along these lines, kept off the walls long enough to fuse.
Why this architecture wins on performance
Tokamaks have achieved the highest fusion performance ever — JET's 69 MJ pulse in 2023, EAST's long-duration plasma records — because the symmetric geometry maximises confinement per unit magnetic energy.
The tradeoff: the plasma current is a source of instability. Disruptions are a real risk and a major R&D focus.
Frequently asked
- How hot is a tokamak plasma?
- Above 100 million Kelvin — roughly 6× hotter than the centre of the Sun.
- What fuel does a tokamak use?
- Deuterium and tritium — both isotopes of hydrogen. Deuterium comes from seawater; tritium is bred from lithium.
- What stops the plasma from melting the walls?
- The magnetic field — the plasma never physically touches the wall. The wall material near the plasma exhaust (the divertor) does need active cooling.
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