Nuclear Fusion

In order to understand fusion we need to have a knowledge about fission.Fission is the process in which a heavy nucleus breaks up into two lighter nuclei.Fusion on the other hand is the reverse of fission.Thus,fusion is the process in which lighter nuclei merge one another to form a heavier nucleus.As in fission,fusion also is accompanied by a release of energy.
The binding energy per nucleus thus formed is greater than the binding energy per nucleon of the lighter elements which fuse to form the single nucleus.Taking an example,let us consider the fusion of the deuterium nuclei to form a single helium nucleus:
                                   Mass of  deuteron =2.01471 amu
                           so,mass of two deuterons=4.02942 amu
                                Mass of helium nucleus=4.00388 amu
                              so, mass defect=0.02554 amu
                  since 1 amu=931MeV
                     so,the energy liberated will be=0.02554*931 MeV=23.78 MeV
 Thus, a single helium nucleus formed out of fusion of two deuterons releases 24 MeV energy.In case a large number of helium nuclei are fused,we readily see that a tremendous amount of energy is released.
Since both the deuterons are similarly charged,therefore,we require a large amount of energy to bring the two together for fusion against coulomb repulsion.Though theoretically this energy may be given to them by accelerating them through strong electric field,practically it is not easy or convinient.The other alternative is to give them high thermal energies.In the sun and the stars such high temperatures are available which impart enough energy to the fusing particles which are protons or deuterons.Thus, such a fusion process is called a thermonucleur fusion.