Magnetic confinement

Fusion plasmas are composed
of charged particles,
that if immersed in a magnetic field
will move spirally along the field lines

In most fusion devices based on magnetic confinement, large magnets are placed around a toroidal vacuum chamber. These magnets create a magnetic field. The particles that make up the plasma, contained in the doughnut (torus), then move by orbiting along these field lines.

The magnetic field prevents a charged particle from passing through it, but cannot block its movement along the field itself, so the particle is forced to follow the field line, like a train follows the tracks. By creating a doughnut-shaped, toroidal magnetic field, the plasma remains confined to a finite space.