fusion Fusion is the process of formation of heavy nucleus by combination of lighter nuclei (A < 20), resulting in the release of energy.  The  release  of energy in fusion can be understood from the  fact  that binding energy per nucleon in lighter nuclei is less  compared  to  that  of the heavier ones, upto mass number ~ 50 (see  fig. b4).  Since  binding energy is negative, the product of fusion  has  lower  rest  mass than the combined rest mass of the original nuclei. The lost rest mass appears as released energy.

Consider for example the formation of helium nuclei from two deuterium nuclei,

1H₂ + 1H₂ ® 2He₄

The energy released,

Q = (2 x 2.014102u - 4.002603u) x 931.5 MeV/u = 23.80 MeV.

This  energy  is  much  less compared to the energy released in a fission reaction of U₂₃₅. However the energy yield per unit mass is much greater in fusion.

Both  the  deuterium  nuclei are positively charged and the electrostatic repulsion  must be  surmounted in joining the two nuclei together. Taking distance between the nuclei ~ 10^-15 m, charge on  each  nuclei = 1.6x10^-19 C, the repulsion energy works out to be ~ 10⁶ eV. The only  way  to  give nuclei such high energies is to raise their temperature  to an extremely high value. From kinetic theory, the kinetic energy (KE) associated with thermal energy is

KE = (3/2) kT

where  k  is  the Boltzmann's constant (=8.6´ 10^-5 eV/K).This gives the temperature to be attained 10¹⁰ K. Even though such temperature is unattainable,  there  exists a finite probability of fusion at temperature  10⁶ K. The problem of building a fusion reactor is to attain  such  high  temperature and  to  confine  the completely ionized  atoms  (called  plasma) at  this  high temperature.  Recent attempt have been made to deliver very high energies exceeding 1000 joules in less  than  a  nanosecond  to pellets of deuterium and tritium by using  high  energy lasers. The high energy laser beam would cause the  pellet to  vaporize  and expand rapidly causing deuterium- tritium ions to fuse.

Another  attempt  is  being  made  by fusion  reactors known as tokamak, which uses bottle shaped magnetic fields to confine the plasma.

Fusion reaction in the sun: The enormous energy released by  the  sun  is believed to be due to continuous fusion reaction that  are  taking  place inside it. The high gravitational field confines  the  plasma  under  high pressure. Primarily it is the proton cycle which is converting the hydrogen into helium.

₁H¹ + ₁H¹ ® ₁H² + ₁e⁺ + n + g

₁H² + ₁H¹ ® ₂He³ + g

₂He³ + ₂He³ ® ₂He⁴ + ₁H¹ + ₁H¹

H. Bethe proposed another reaction called the Bethe cycle or CNO cycle.  In this  reaction carbon,  nitrogen  and oxygen act as catalysts. The net reaction is ,

4(₁H¹) ® ₂He⁴ + 2₁e+ + 24.7 MeV

This reaction is believed to be the primary process in hotter stars.