Sustained nuclear fusion has yet to be achieved and remains a hope for abundant future energy yet the energy that has sustained us has been the energy of nuclear fusion. How do you explain this?

Answer 1

In the sun, and other stars, it is nuclear fusion which provides the power which enables life on earth to flourish. The nuclear reaction in the sun is called the proton-proton reaction, which produces helium and releases energy. This is promoted by the huge pressure at the sun's centre, and of course it started naturally, presumably as the sun attracted matter from space and gradually grew in size. The sun's reactions are at the core, and the energy spreads out so that the surface which we see is sending out light and infra-red radiation because it is incandescent.

On earth, in man made equipment, it is not possible to produce the conditions in the sun's core, and it is necessary to heat a gas plasma to much higher temperatures before fusion will occur. The power density in the sun's core is surprisingly low in fact, and to make a reaction chamber with those conditions it would have to be too large to be possible or economic. So for man made fusion the gas plasma has to be heated to hundreds of millions of degrees C, and the most promising way to do this is using magnetic confinement in a tokamak or torus shaped chamber. Plasma just means an ionised hot gas. Theoretically the reaction requiring the least energy to start it is between nuclei of deuterium and tritium (both hydrogen isotopes) and that is what is being experimented with. So far fusion has been produced but only for a fraction of a second (see JET in Wikipedia).