Thermonuclear fusion is the basic energy process of the stars and the sun. There the fusion reaction occurs spontaneously. Now the problem is to make this reaction controllable. Such a reaction can only take place at very high temperatures (about 140 million degrees Celsius). So the problem is how to keep plasma (hot ionized gas) at temperatures which are about ten times the temperature of the sun.
Obviously no solid substances can be used to withstand such heat. If plasma would touch the walls of the reactor it would immediately lose all of its energy and fusion would stop. Therefore the only way to make thermonuclear reaction possible on the earth is to control plasma by a magnetic field. Scientists carry out a great deal of research to solve this problem. They look into the future and hope to demonstrate fusion as a viable energy source for producing electricity.
Fusion has a number of advantages over fission. It is able to produce much more energy than any other energy source and it may use materials that are not harmful to the environment.