Seen from the outside, the plasma machine is a confusing jumble of heating and measurement devices, and huge magnetic coils.
The heart of a fusion machine is a ring-shaped metal vessel. The inner wall of the vessel is lined with removable heat-resistant tiles, and has numerous openings for heating devices and measuring systems.
Equally spaced around the device are electromagnets, huge coils of wire wrapped around the vessel. These provide strong magnetic fields that keep the hot plasma away from the reactor walls. This is possible because charged particles can be controlled by magnetic forces.
To generate a plasma, the vessel is pumped down to high vacuum, a gram or so of gas fuel is injected and then heated. Electric currents are fed through the coils, creating the magnetic field to contain the plasma. The measuring systems begin to monitor the plasma’s heat, density and shape – if it begins to cool off or become unstable, the magnetic fields or the heating systems are tweaked to keep the plasma running.
Finally when the heating energy is used up, the plasma goes out. The leftover gas is pumped out and the system begins to charge up again, ready for another pulse.