To attain the perfect fusion conditions, we need to combine three parameters; those of, Temperature, Density, and Time.
By multiplying these together we form what is known as the Fusion or Triple Product. At a certain value of the Triple Product (which is expressed as Q), the reaction becomes self-sustaining, this is referred to as ignition. At this point, the internal reactor heating systems can be turned off, as the heat generated by the reaction is enough to keep the plasma hot and self-sustaining.
The fuel used in the reaction is Deuterium – Tritium (DT), however, these DT fusion reactions require temperatures in excess of 100 million degrees.
To achieve these temperatures in tokamak fusion reactors, three separate heating systems (Ohmic, Neutral Beam and Radio-Frequency Heating) are currently used, each system is capable of delivering well over a million watts of power, and when combined, they are enough to generate and sustain the plasma, required for fusion to occur.
To contain the plasma density at high enough values and ensure enough collisions occur, the plasma vessel is surrounded by powerful electromagnets.
These create magnetic fields 10,000 times stronger than the Earth’s magnetic field and confine the plasma to perpetually circulating within the ring-shaped vessel. However if the plasma gets too dense then collisions of a different kind – between nuclei and electrons – begin to create large amounts of radiation. This radiation, called Bremsstrahlung (deceleration or braking radiation) which reduces energy from the plasma and prevents fusion occurring – the optimum density value is contained in a vacuum at around one millionth of our atmosphere at ground level.
In the reaction itself, approximately 80% eighty percent of the fusion energy is carried away by neutrons, and the remaining 20% twenty percent by the helium nuclei which remains in the plasma.
Helium formed in the reaction, is deflected around the vessel colliding with Neutrons in the D-T fuel, heating them up, to a point where the heat generated becomes self-sustaining.
Depending on the density and temperature of the plasma the confinement time (denoted by τ, the Greek letter tau), or the length of time for which particles are confined within the plasma, yield enough energy to achieve Q. For the smaller reactors such as JET, these are in the order of a second, although the larger reactors such as the ITER should be approaching four seconds.