Heating the plasma
Whatever the way in which the plasma was created inside the confinement structure, it never initially has the temperature required for fusion. Three methods are possible to heat the plasma up:
• the current flowing in the plasma is also used to heat the plasma by Joule effect (ohmic heating). The latter is effective up to a temperature around 10 million degrees. Beyond that, plasma resistivity becomes too weak and effectiveness of this method decreases. In a Stellarator, there is no central current and therefore no ohmic heating.
• heating by injection of neutrals consists in creating and accelerating a beam of ions, outside the confinement machine. This beam is then neutralised before entering the plasma where the particles are ionised and confined by the magnetic field. The collisions redistribute energy and the temperature of the plasma rises.
• the plasma may absorb energy from electromagnetic waves at frequencies characteristic of the environment. This heating by electromagnetic waves is transmitted to the plasma by antennas covering part of the confinement area. The choice of frequency defines the type of particles (ions or electrons) that will be heated up and the area through which the wave and thus the heating will be absorbed.
In a thermonuclear fusion reactor by magnetic confinement, the temperature of the plasma may be raised to a suitable level by a combination of the methods presented above. When there are a great number of fusion reactions, the energy carried by the helium nuclei remains confined in the plasma and contributes to heating it. If this contribution becomes equal to the energy lost by the plasma, then the heating methods above are no longer necessary. The thermonuclear plasma is thus self-maintained, and we say that it is in ignition. If we define the amplification factor as being the ratio between the total power generated by the plasma and the heating power injected into the plasma, then this amplification factor is infinite if the plasma is self-maintained. When this factor is equal to one, the plasma supplies as much energy as is injected into it. This last condition is called "break even". The European tokamak JET has achieved plasmas close to "break even".
The above animation shows plasma start-up sequences