What Is a Tokamak? Inside the Fusion Reactor
A tokamak is a doughnut-shaped (toroidal) machine designed to do something the Sun does naturally: fuse hydrogen nuclei together and release energy. It is currently the world’s most advanced approach to building a fusion power plant.
The problem it solves
To fuse, hydrogen isotopes must be heated to over 100 million °C — far hotter than the Sun’s core. At that temperature matter becomes a plasma: a soup of charged particles. No solid container can touch it. So how do you hold something ten times hotter than the Sun? With magnetic fields.
How a tokamak works
The word tokamak comes from a Russian acronym for “toroidal chamber with magnetic coils.” Its key features:
- A toroidal chamber — the doughnut where the plasma lives.
- Powerful magnetic coils — they create a helical (twisting) magnetic field that traps the charged plasma particles, keeping the plasma away from the walls.
- Heating systems — electric current, microwaves and particle beams push the plasma to fusion temperatures.
Because the plasma is made of charged particles, magnetic fields can steer and confine it, holding it suspended in the vacuum chamber like an invisible bottle.
The goal: net energy gain
The prize is Q > 1 — getting more energy out than you put in. In 2022, the National Ignition Facility achieved net gain using a different (laser) method, proving fusion ignition is possible. Tokamaks aim to do it continuously.
- ITER, in France, is the largest tokamak ever built — a global collaboration designed to produce 500 MW from 50 MW of heating input.
- Private companies like Commonwealth Fusion Systems (SPARC) use new high-temperature superconducting magnets to build smaller, stronger tokamaks faster.
Why it matters
Fusion promises abundant energy with no carbon emissions, no long-lived radioactive waste, and fuel (hydrogen isotopes) drawn from seawater. The engineering is brutally hard — but the tokamak is how much of the world is trying to get there.
Related: how does nuclear fusion work? and what is a superconductor?
FAQ
What is a tokamak in simple terms?
A doughnut-shaped machine that uses magnetic fields to hold superheated plasma so hydrogen nuclei can fuse and release energy.
Why is it doughnut-shaped?
A torus lets the magnetic field loop back on itself with no ends, so the plasma can circulate continuously without leaking out.
Has a tokamak produced net energy yet?
Not yet in continuous operation. ITER aims to demonstrate large net gain; fusion ignition was first achieved in 2022 using lasers, not a tokamak.