Introduction

8 - 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.

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

9 - The main results

Since the arrival of the tokamaks around 1970, plasma fusion power generated by various installations throughout the world has increased by 10,000 million. Many significant results have been obtained in all fields, whether in physics or in the technologies used.  

 
The progress in fusion power through the years

If we only look at the main results they are:

To obtain high performance plasma, it must meet criteria of density (there must be enough nuclei) and of temperature (these nuclei must be at temperatures of several million degrees). The energy carried by the helium nuclei must also remain confined in the plasma for a sufficient time. The period during which the energy stays confined inside the plasma is called the " energy confinement time " and this varies according to the square of the major radius of the plasma. This size effect is one of the (intrinsic) features of fusion installations. High performance plasmas are obtained in large-scale installations. 

The criteria above have been obtained independently for density, temperature and confinement time in the various current experimental installations. The community of researchers and engineers involved in studies on controlled magnetic fusion is now ready to take another step: demonstrate control of sustained combustion of deuterium-tritium plasma over long durations. This will be the next step and the main goal of the next international experimental machine (ITER).