Nuclear Fusion: Nuclear fusion occurs when two lightweight nuclei fuse (combine) and form a nucleus of a heavier element. The products of the fusion weigh less than the combined weights of the original nuclei. The lost matter has therefore been changed into energy. Fusion reactions that produce large amounts of energy can be created only by means of extremely intense heat. Such reactions are called thermo-nuclear reactions. Thermonuclear reactions produce the energy of both the sun and the hydrogen bomb. A thermo-nuclear reaction can occur only in plasma, a special form of matter which has free electrons and free nuclei. Normally, nuclei repel one another. But if a plasma containing lightweight atomic nuclei is heated many millions of degrees, the nuclei begin moving so fast that they break through one another’s electrical barriers and fuse. Problems of Controlling Fusion: Scientists have not yet succeeded in harnessing the energy of fusion to produce power. In their fusion experiments, scientists generally work with plasmas that are made from one or two isotopes of hydrogen. Deuterium is considered an ideal thermo-nuclear fuel because it can be obtained from ordinary water. A given weight of deuterium can supply about four times as much energy as the same weight of uranium. To produce a controlled thermo-nuclear reaction, a plasma of deuterium or tritium or of both isotopes must be heated many millions of degrees. Bui scientists have yet to develop a container than can hold superhot plasma. Most experimental fusion reactors are designed to contain superhot plasma in “magnetic bottles” twisted into various coil-like shapes. The walls of the bottles are made of copper or some other metal. The walls are surrounded by a magnet. An electric current is passed through the magnet and creates a magnetic field on the inside of the walls. The magnetism pushes the plasma away from the walls and toward the centre of each coil. This technique is called magnetic confinement All the fusion devices thus far developed; however, use much more energy than they create. The most successful fusion reactor, called a tokamak, was originally designed by Russian scientists. Tokamak means strong current in Russian. Like other experimental fusion reactors, a tokamak uses a magnetic field to push plasma away from its containing walls. It also passes a strong current through the plasma. The current acts with the magnetic field to help confine the plasma. India has developed a tokamak Aditya, for research purposes at the Institute of Plasma Research, Ahmedabad. Another experimental method to achieve fusion uses beams of laser to compress and heat tiny pellets of frozen deuterium and tritium. This process creates miniature thermo-nuclear explosions that release energy before the pellets reach the containing walls. But all experiments with this method have not yet produced usable amounts of energy.