2. Conditions for a fusion reaction

Three parameters (plasma temperature, density and confinement time) need to be simultaneously achieved for sustained fusion to occur in a plasma. The product of these is called the fusion (or triple) product and, for D-T fusion to occur, this product has to exceed a certain quantity - derived from the so-called Lawson Criterion after British scientist John Lawson who formulated it in 1955. (See article of the month, December '05

Attaining conditions to satisfy the Lawson criterion ensures the plasma exceeds Breakeven - the point where the fusion power out exceeds the power required to heat and sustain the plasma.

Temperature

Fusion reactions occur at a sufficient rate only at very high temperatures - when the positively charged plasma ions can overcome their natural repulsive forces. Typically, in JET, over 100 million Kelvin is needed for the Deuterium-Tritium reaction to occur - other fusion reactions (e.g. D-D, D-He³) require even higher temperatures.

Density

The number of fusion reactions per unit volume is roughly proportional to the square of the density. Therefore the density of fuel ions must be sufficiently large for fusion reactions to take place at the required rate. The fusion power generated is reduced if the fuel is diluted by impurity atoms or by the accumulation of Helium ions from the fusion reaction itself. As fuel ions are burnt in the fusion process they must be replaced by new fuel and the Helium products (the "ash") must be removed.

Energy Confinement

The Energy Confinement Time is a measure of how long the energy in the plasma is retained before being lost. It is officially defined as the ratio of the thermal energy contained in the plasma and the power input required to maintain these conditions. At JET we use magnetic fields (see Section 3) to isolate the very hot plasmas from the relatively cold vessel walls in order to retain the energy for as long as possible. A significant fraction of losses in a magnetically-confined plasma is due to radiation. The confinement time increases dramatically with plasma size (large volumes retain heat much better than small volumes)- the ultimate example being the Sun whose energy confinement time is massive.

For sustained fusion to occur, the following plasma conditions need to be maintained (simultaneously).
* Plasma temperature: (T) 100-200 million Kelvin
* Energy Confinement Time: (t) 4-6 seconds
* Central Density in Plasma: (n) 1-2 x 10²⁰ particles m^-3 (approx. 1/1000 gram m^-3, i.e. one millionth of the density of air). Note that at higher plasma densities the required confinement time will be shorter but it is very challenging to achieve higher plasma densities in realistic magnetic fields.