VAN DE GRAAFF GENERATOR

VAN DE GRAAFF GENERATOR

In 1931, R.J. Van de Graaff designed an electrostatic generator capable of generation very high potential of the order of 5×10⁶ V, which was then made use in accelerating charged particles so as to carry out nuclear reactions.

Principle it is based on the following two electrostatic phenomena.

1.    The electric discharge takes place in air or gases readily at pointed conductors.

2.     If a hollow conductor is in contact with another conductor, then as charge is supplied to the conductor ,the hollow conductor continues accepting the charge irrecpective of the fact, howsoever large its potential may grow. The charge irrespective of the fact, howsoever large its potential may grow. The charge immediately shifts to outer surface the hollow conductor.

CONSTRUCTION AND WORKING

Van de Graff generator consists of a large hollow metallic sphere C mounted on two insulation columns C1 C2  as shown in figure. An endless belt of an insulating material is made to run on two pulleys P₁ and P₂ with the help of an electric motor. The metal comb B₁, called spray comb is held near the lower end of the belt.

When the spray comb is maintained at high positive potential with the help of E.H.T. source, it produces ions in its vicinity. The positive ions get sprayed on the belt due to the repulsive action of comb B₂, called collecting comb is positioned near the upper end of the belt, such that the pointed ends touch the belt and the other end is in contact with the inner surface of the metallic sphere S.

 The comb B₂ collects the positive ions and transfers them to the metallic sphere. The charge transferred by the comb B₂ immediately moves on to the outer surface of the hollow sphere, as the belt goes on moving, the accumulation of positive charge on the sphere also keeps on taking place continuously and its potential rises considerably with the increase of charge on sphere its leakages due to ionization of surrounding air also becomes faster, the maximum potential to which the sphere can be raised is reached, when the rate of loss of charge due to leakage becomes equal to the rate at which the charge is transferred to the sphere. 

To prevent the leakage of charge from the sphere the generator is completely enclosed inside an earth connected steel tank, which is filled with air under pressure. If the projectile such as protons, deutrons, is now generated in the discharge tube D with lower end earthed and upper end inside the hollow sphere, they get accelerated in downward direction along the length of the tube, at the other end they come to hit the target with large kinetic energy and bring about nuclear disintegration. 

Van de Graff generator of this type was installed at the Carnegie institute in Washington in 1937. One such generator was installed at Indian Institute of technology in kanpur in 1970 and it accelerates particles to 2 MeV energy.