The Joint European Torus

Whilst the prospect of controllable fusion power remains out of reach, the technology employed to pursue this goal is quite fascinating.

Consider, for example, the Joint European Torus (JET), at UKAEA's Culham laboratory in Oxfordshire. This doughnut-shaped vessel, called a tokamak, is designed to confine a plasma of deuterium and tritium with the use of magnetic fields. The deuterium and tritium nuclei will fuse at the high temperatures created in a plasma, and release energy in two forms: neutrons and helium nuclei. The ultimate intention is that the energy carried by the neutrons can be used to heat the water in a jacket around the tokamak, and the steam energy from this can then be used to, say, power turbines that generate electricity. In principle, the energy carried by the helium nuclei is then sufficient to sustain the temperature of the plasma, and thereby make the fusion reaction self-sustaining once started.

What is particularly fascinating about JET is the way in which the magnetic fields are generated. To confine the plasma, a helical magnetic field is generated inside the tokamak. As a consequence, the charged deuterium and tritium nuclei in the plasma follow spiral trajectories inside the vessel. To generate this helical magnetic field, two distinct components are generated by distinct mechanisms:

Firstly, a 'toroidal' magnetic field is generated by superconducting magnets around the tokamak. This magnetic field is essentially generated in accordance with Ampere's law, in the same manner that a magnetic field is generated in the interior of a solenoid.

The toroidal field provides the longitudinal component to the resultant magnetic field. The total field is the sum of the toroidal field and a 'poloidal' field, which provides a latitudinal component to the resultant field. The poloidal field is generated by a changing current in a coil within the hole of the doughnut. To understand this requires some further background in electromagnetism:

An electric current creates a magnetic field, and in particular, creates something called magnetic flux, which is the integral of the magnetic field over a surface area. If a changing electric current is generated in a coil, then this generates a changing magnetic flux, and by magnetic induction, this will generate a current in another circuit. (This is the same principle by which a transformer operates).

In the case of a tokamak, the electrons in the plasma constitute the secondary circuit, and the changing current in the inner coil generates a toroidal electric field, which drives a longitudinal electron current in the plasma. Just like the current through a copper wire, this current generates a circular magnetic field in the perpendicular plane. This is the poloidal magnetic field, which generates the latitudinal component to the magnetic field controlling the trajectories of the deuterium and tritium nuclei.