Van De Graaff Generator

Van De Graaff Generator 

In this electrostatic accelerator, electric charge is carried continuously to an insulated hollow spherical conductor (S) and delivered to it.

The potential of the hollow sphere can thus be gradually raised.

When the potential has attained a sufficiently high value, it can be used to accelerate positive ions in the accelerator tube.

A well-insulated endless belt B is rapidly rotated between two pulleys P₁  and P₂ by means of a motor [Fig 1].

Fig 1

The upper pulley P₂ is placed inside a hollow spherical conductor S.

Near the lower pulley P₂, there is a metallic comb C connected to a source of steady voltage of 50 kV.

Positive charge is sprayed over the moving belt by the metallic comb C.

The belt moving upward carries this charge into the hollow sphere.

The positive charge, on entering the sphere S. induces an equivalent negative charge on its inside and a corresponding positive charge on its outside.

The negative charge is sprayed by the comb C on the the belt, thus neutralizing the positive charge on it.

The net result is that the positive charges are in effect transferred from the moving belt to the outside of the sphere. The belt, returning to the bottom, is charged again and delivers up the charge, when inside the sphere.

Charges are thus accumulated and distributed on the surface of the sphere. As the charges on the sphere accumulates, its potential rises to a very high value.

The charged particles which are to be accelerated are produced by a gas discharge ion source I placed inside the hollow sphere S over the acceleration tube G.

The accelerating tube is evacuated and contains a number of insulated metal cylinders (C) arranged with short gaps between them [Fig 2].

 

Fig 2

 

The H.T. is applied to the topmost cylinder near the ion source and the bottom-most cylinder is earthed.

 A potential gradient exists from the top to the bottom. The ions are accelerated at the gaps between the cylinders, where intense electric field exists.

The highly accelerated ions are deviated by a suitable and adjustable magnetic field M and made to strike the target (T) at the bottom of the tube.

By reversing the potential of the spray voltage, the Van De Graaff generator can also be used to accelerate electrons.