The triple product and ‘Q’ amplification factor
The triple product and amplification factor ‘Q ’The performance of a fusion plasma or its amplification factor ‘Q’ (ratio between the energy invested and the energy produced) is characterised by the product of three parameters: density, temperature and energy confinement time.
- The temperature must be high enough for the speed of the nuclei to overcome electrical repulsion, and not so high that the nuclei have time to fuse. The optimum value is around 150 to 200 million degrees.
- The density of matter within the plasma must be greater than 1020 particles per m3 to ensure that the nuclei have a sufficient probability of encountering each other.
- The confinement time, i.e. the time it takes for the plasma to lose its heat if its energy source is cut off, must be greater than 1 second. The higher the confinement time, the less energy the plasma loses. In a way, this data characterises the quality of the plasma’s thermal insulation.
When Q = 1, the power generated by the plasma is equal to the power supplied from outside to maintain it. This state is called the breakeven state.When Q = ∞ (infinity), this means that the power supplied from outside is zero. The plasma is self-heated: it is said to be igniting.
The ITER research facility has been designed to achieve an amplification factor 10 times greater than the injected power (Q = 10), over times of the order of 400 seconds: it should therefore supply 400 MW of energy from an injected power of 40 MW.
To be profitable, an industrial reactor would have to reach Q = 20-30.
Plasma heating
To reach a temperature in excess of 100 million °C in a plasma, several methods are used :
- The current flowing through the plasma helps to heat it up thanks to the electrical resistance effect (Joule effect). However, it is not sufficient to exceed 20 to 30 million °C.
- One additional heating method involves injecting highly accelerated neutral deuterium atoms into the plasma (at energies much higher than those of the plasma particles). Collisions between these particles and those in the plasma redistribute the energy and the plasma temperature rises.
- A complementary technique involves coupling electromagnetic waves to the plasma, via antennae installed at the periphery of the vacuum chamber. The absorption of the wave by certain particles in the plasma, depending on the frequency used, will also cause the plasma temperature to rise.
In practice, plasma confinement and heating are much more difficult to achieve than predicted by simple theoretical models based on particle collisions, partly because of the internal turbulence that develops within the plasma. In addition, the plasma is subject to various heat losses, by radiation and conduction, which make it difficult to obtain stable conditions with high amplification factors.