A tokamak is a doughnut-shaped magnetic-confinement device designed to hold extremely hot plasma using strong magnetic fields so controlled nuclear fusion reactions can be studied and sustained.
Why a tokamak is shaped like a torus
Plasma particles are electrically charged, so magnetic fields can guide their motion. Bending the plasma into a closed ring avoids the particle losses that would occur at the ends of a straight magnetic bottle. A torus—the familiar doughnut shape—provides that closed path.
A purely toroidal magnetic field is not enough for stable confinement. Tokamaks combine fields in different directions so magnetic field lines twist around the torus.
Where the magnetic fields come from
Large external coils produce the main toroidal field. A central solenoid acts somewhat like the primary winding of a transformer, inducing a large electric current in the plasma. That plasma current creates another magnetic-field component and also contributes to heating.
Additional coils control plasma shape and position, while auxiliary heating systems raise the temperature further.
The blanket, divertor and vacuum vessel
The vacuum vessel encloses the plasma. A divertor at the bottom of many tokamaks helps exhaust helium “ash,” impurities and heat. Future power reactors would surround the vessel with blankets that absorb neutron energy and may breed tritium from lithium.
These components face intense heat and neutron loads, making materials and remote maintenance major engineering challenges.
ITER and what it is meant to demonstrate
ITER is a large international tokamak under construction in France. Its purpose is to study burning plasma and technologies needed for future fusion power, not to generate electricity for the grid.
A successful tokamak power plant would still need reliable long-duration operation, a tritium fuel cycle, durable materials and an efficient system for turning fusion heat into electricity.
