Tokamak

A tokamak confines hot plasma by combining toroidal and poloidal magnetic fields. The toroidal field is produced by large coils surrounding the chamber, while a powerful current driven through the plasma generates much of the poloidal field. Together, these fields create helical magnetic surfaces that keep charged particles away from the vessel wall.¹

The plasma is formed in a doughnut-shaped vacuum vessel evacuated of ordinary gas. Auxiliary heating systems, including neutral-beam injection and radio-frequency waves, raise the plasma to temperatures at which deuterium and tritium nuclei can overcome their electrical repulsion. A central solenoid commonly induces the plasma current, although future reactors are expected to rely more heavily on non-inductive current drive.

A power-producing tokamak would use deuterium–tritium fusion because this reaction reaches useful rates at lower temperatures than most other fusion fuels. Its main products are a helium nucleus, which helps heat the plasma, and a high-energy neutron, which carries most of the released energy into surrounding structures.²

The neutron energy would be absorbed in a blanket containing lithium, where it could generate heat and help breed tritium fuel. Heat would then drive turbines or another power-conversion system. Reactor designs must also handle intense neutron damage, exhaust helium ash and impurities, disruption events, superconducting magnets, and remote maintenance inside an activated machine.³

Tokamak research began in the Soviet Union during the 1950s, and the design gained international prominence after experiments demonstrated comparatively strong plasma confinement. The approach became the basis of major devices such as the Joint European Torus, Japan's JT-60, the United States' DIII-D, and the Chinese Experimental Advanced Superconducting Tokamak.⁴

ITER, being constructed in France by an international partnership, is intended to demonstrate sustained burning-plasma conditions rather than produce commercial electricity. Its design uses superconducting magnets and a large vacuum vessel to investigate whether self-heating by fusion-produced alpha particles can dominate external heating.⁵ Commercial deployment would still require separate power-plant designs and solutions to materials, fuel-cycle, reliability, and economic challenges.

Tokamak performance depends not only on temperature but also on density, confinement time, and plasma purity. These quantities are often summarized by the fusion triple product, a measure of how closely a plasma approaches the conditions needed for net fusion gain. Turbulence and small-scale instabilities can transport heat across magnetic surfaces and substantially reduce confinement.

The tokamak's plasma current creates both useful shaping and serious operational risks. A sudden disruption can generate large electromagnetic forces and deposit intense heat on plasma-facing components, so modern experiments use massive shaping coils, feedback control, shattered-pellet mitigation, and sacrificial divertor materials.⁶ Alternative magnetic-confinement concepts, including stellarators and spherical tokamaks, address some of these problems differently, but the tokamak remains the most extensively developed configuration.

Plasma

An electrically conducting state of matter composed of charged particles.

Toroidal field

The magnetic field directed around the long circumference of a tokamak.

Divertor

A component that exhausts plasma impurities, helium ash, and heat from the confined plasma.

Fusion triple product

A combined measure of plasma density, temperature, and energy-confinement time.

Tritium breeding

The production of tritium from lithium using fusion-generated neutrons.