A fusion reactor aims to create a hot plasma in which light nuclei fuse, confine or compress that plasma long enough to release useful energy, then capture the energy in a form that can ultimately produce electricity.
Create a plasma hot enough for fusion
Fusion fuel must be heated until atoms are ionized into plasma and nuclei collide at energies where fusion becomes likely. For deuterium-tritium fuel, temperatures of many millions of degrees are required. The challenge is not simply reaching high temperature but maintaining the right combination of temperature, density and confinement.
Heating methods can include electric current in the plasma, neutral-particle beams and radio-frequency waves, depending on the reactor concept.
Confine the fuel
Magnetic-confinement reactors use magnetic fields because charged particles spiral around field lines. A tokamak combines toroidal and poloidal magnetic fields to form nested magnetic surfaces. Stellarators produce the twisting field mainly with external magnets.
Inertial-confinement systems instead compress very small fuel capsules rapidly so inertia holds the fuel together for the brief interval in which reactions occur.
Capture fusion energy
In deuterium-tritium magnetic-fusion concepts, most fusion energy leaves the plasma in fast neutrons. A surrounding blanket absorbs the neutron energy as heat. A future power plant would transfer that heat to a working fluid and drive a turbine or another conversion system to produce electricity.
The blanket may also contain lithium so neutron reactions can produce tritium fuel, creating a closed or partially closed fuel cycle.
A reactor needs far more than a hot plasma
Engineering systems must handle neutron damage, remove heat, maintain a high vacuum, control impurities, protect magnets, replace components and operate with high availability. A power plant also needs enough net electrical output to cover its own pumps, magnets, heating and support equipment.
That is why a physics experiment demonstrating fusion gain is different from a commercially useful fusion reactor.
