Controlled fusion

Controlled fusion is one of physicists' dreams. The problem is not simply to reproduce the thermonuclear fusion reactions that make the stars shine on Earth - which has already been done in the H bomb or in accelerators - but to control these reactions to produce energy. If, for example, fusion reactions between deuterium and tritium, two hydrogen isotopes, in stable conditions were achieved, there would then be an almost clean and almost inexhaustible source of energy.

There are essentially two approaches to achieve this:

• The first is the basis of the ITER project. The idea is to maintain a very high temperature in a low density plasma for a relatively long time of the order of a second. To do this intense magnetic fields inside a toroidal cavity must be used: this is fusion by confinement or magnetic fusion. It is known how to trigger a fusion reaction, but the plasma trapped in the magnetic fields is highly unstable and the conditions required for fusion cannot be maintained for long enough for the reaction to become self-sustaining and produce more energy than it consumes.
• The second approach is the basis of the HiPER project, for High Power laser Energy Research, which resumes research carried out in the 1970s when the first really important work on laser fusion was being carried out. Today there are still military installations in the United States, the NIF (National Ignition Facility) at the Lawrence Livermore National Laboratory, and in France the LMJ (Laser Mégajoule) in Bordeaux, used to simulate nuclear weapons and to explore the physics of lasers capable of achieving what is known as inertial fusion. For this technique a very high density plasma must be produced (greater by a factor of 10⁹ than that of fusion by confinement) and the reaction time is extremely short (of the order of a billionth of a second). To achieve these conditions Europe wants to build the most powerful laser in the world in the hope of making nuclear fusion a reality within the next two decades. The location of the research centre which will house this laser has not yet been decided but the United Kingdom is currently the favourite candidate.

Briefly, the mechanism involved in inertial fusion is as follows.

(Credit: www.ilp.u-bordeaux1).

A mixture of tritium (T) and deuterium (D) is enclosed in a tiny sphere a few millimetres in diameter and is exposed to the cross-fire of several long and very high energy laser pulses to exert a uniform pressure over the entire surface of the sphere. The high temperature that is reached vaporises the material which, escaping from the sphere in all directions, exerts an addition isotropic pressure. The result is that a density of 300 g/cm³ and a temperature of 100 million degrees Celsius is reached. The fusion reaction, or its ignition, can then begin and the only problem is to obtain more energy than is consumed by the ignition of the fusion reaction.

The inertial fusion approach is also being explored using the famous Z-machine.