Fusion is the source of energy in the sun and other stars. A star starts to shine when, under the force of gravity, the matter in its very heart attains sufficiently high densities and temperatures to set off thermonuclear reactions, which then release energy. Plasma’s tendency of dispersing, and therefore cooling down, is balanced out by gravitational force.
On Earth, gravitational confinement is impossible. Two paths have been studied to reproduce these reactions:

• raising a small volume of matter for a very short time to very high pressure and temperature, described as inertial confinement. We thus seek to obtain the greatest possible number of fusion reactions before the plasma disperses.
  

• trapping and maintaining a plasma at very high temperature. This plasma is confined in an intangible torus-shaped bottle created by magnetic fields, described as magnetic confinement. 

In order for the fuel in the form of plasma to produce enough thermonuclear reaction, it must be maintained in a limited volume and kept away from any structural material in order to maintain its high temperature. This is called confinement.

In the free state of plasma, the particles’ trajectory is random (picture1 opposite) and the particles escape.

As the plasma is made up of charged particles, the magnetic fields may act on them. If this same plasma is bathed in a rectilinear magnetic field (picture 2), the particles wind around the field lines and will no longer touch the side walls.

So as to avoid losses from the edges, we close off the magnetic bottle by creating a torus (picture 3). The magnetic field thus created by a series of magnets surrounding the plasma is called a toroidal magnetic field. The magnets generating this field are the toroidal magnets.

Here is shown that confinement is not quite enough and to minimise particle leakage even more, the field lines must be helicoidal (picture 4). This is achieved by adding another magnetic field to the toroidal field, which is perpendicular to it (the poloidal field). The method used to produce these helicoidal field lines has given birth to two types of machine:

• In a « tokamak », an assembly of coils produces a magnetic field in the direction of the torus, to which is added the magnetic field created by an intense axial current flowing in the plasma itself. The two fields generate the helicoidal structure of the field lines (picture 5). This configuration has made considerable progress since its invention in the 1960s by Russian researchers. This is currently the path of research that is being studied the most (an example of a tokamak : Tore Supra)

• In a « stellarator », magnetic configuration is entirely based on currents flowing in helicoidal coils (picture 6).

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