Why are toroidal devices used?
The effect of the magnetic field is to confine the charged plasma particles by applying a force that opposes the motion across the field. There is no such force in the direction parallel to the magnetic field so, if the magnetic field lines were to connect the ends of a linear device, for instance, particles would be able to escape rapidly to the ends. In a toroidal device, the particles primarily spiral along the field lines, travelling around the machine typically a million times before escaping.
