3. Magnetic plasma confinement - the Tokamak

Since a plasma comprises charged particles : ions (positive) and electrons (negative), powerful magnetic fields can be used to isolate the plasma from the walls of the containment vessel - thus enabling the plasma to be heated to temperatures in excess of 100 million Kelvin. This isolation of the plasma reduces the conductive heat loss through the vessel and also minimises the release of impurities from the vessel walls into the plasma that would contaminate and further cool the plasma by radiation.

In a magnetic field the charged plasma particles are forced to spiral along the magnetic field lines. The most promising magnetic confinement systems are toroidal (from torus : ring-shaped) and, of these, the most advanced is the Tokamak. Currently, JET is the largest Tokamak in the world although the future ITER machine will be even larger.

Other, non magnetic plasma confinement systems are being investigated - notably laser-induced inertial confinement fusion systems.

The Tokamak

In a Tokamak the plasma is heated in a ring-shaped vessel (or torus) and kept away from the vessel walls by applied magnetic fields. The basic components of the Tokamak's magnetic confinement system are :

• The toroidal field - which produces a field around the torus. This is maintained by magnetic field coils surrounding the vacuum vessel (see figure). The toroidal field provide the primary mechanism of confinement of the plasma particles.

• The poloidal field - which produces a field around the plasma cross section. It pinches the plasma away from the walls and maintains the plasma's shape and stability. The poloidal field is induced both internally, by the current driven in the plasma (one of the plasma heating mechanisms), and externally, by coils that are positioned around the perimeter of the vessel.

The main plasma current is induced in the plasma by the action of a large transformer. A changing current in the primary winding or solenoid (a multi turn coil wound onto a large iron core in JET) induces a powerful current (up to 5  Millon Amperes on JET) in the plasma - which acts as the transformer secondary circuit.