Nuclear fusion energy

Fusion is the reaction which occurs in stars, including our Sun, and which results in the enormous amount of energy we experience as heat and light. Most fusion in our Sun is the formation of helium (atomic weight 4) from hydrogen (atomic weight 1). However, there are two isotopes of hydrogen: deuterium (atomic weight 2) and tritium (atomic weight 3).

These heavier isotopes are used in the fusion experiments. Physicists have invested a great deal of effort and money in the attempt to make fusion a viable source of energy, for the production of electricity. The main problem is not how to cause the fusion to occur, but how to contain the enormous heat it generates. This is the containment problem.

Many designs of containment chambers have been proposed. The first successful controlled release of fusion energy was achieved in 1991, at the Joint European Torus facility (JET), England: the Preliminary Tritium Experiment. Current experimental designs include the tokamak and the inertial confinement fusion laser. These experiments are still not producing enough energy to be commercially viable.

A helium nucleus has a lower mass than the composite lighter nuclei. During the fusion of deuterium and tritium to a helium atom, the missing mass is converted to energy, according to Einstein’s famous equation: E = mc2

However, to get the hydrogen protons to join together to form a helium nucleus, requires the electrostatic forces (like charges repel) of the two positive protons to be overcome, so that the strong nuclear force can hold the protons together. The strong nuclear force is much more powerful than the electrostatic forces, but only operates over very short distances. Beyond its effective range, the electrostatic force is dominant, and forms a Coulomb barrier.

Therefore, a large amount of energy is needed to cause fusion. This energy can be kinetic, such as can be obtained by a particle accelerator. Or it can be provided by heat. The heat strips off the single electron from the hydrogen atom, ionising it. The cloud of ions, with separated electrons, is the plasma. The ions are controllable, because they are positive. By placing them in a magnetic field, they can be positioned and held in place as they are heated under pressure.

Fusion of deuterium and tritium to helium releases 17.6 MeV of energy.

21H + 31H → 42He + 10ν