Particle accelerator - Vande Graff Generator

Van de Graaff Generator

In 1929, Robert J. Van de Graaff designed an electrostatic
machine which produces large electrostatic potential difference of the
order of 10^7 V.
File:Az első magyar gyorsító Van de Graaff-generátora(2).jpg

Van de Graaff generator of the first Hungarian linear particle accelerator. It achieved 700 kV in 1951 and 1000 kV in 1952. (Constructor: Simonyi Károly; Sopron, 1951.)

The working of Van de Graaff generator is based on the principle
of electrostatic induction and action of points.

Normal van de graff generator

A hollow metallic sphere A is mounted on insulating pillars as shown in the Fig.1.32. A
pulley B is mounted at the centre of the sphere and another pulley C is mounted near the
bottom. A belt made of silk moves over the pulleys. The pulley C is driven continuously by
an electric motor. Two comb−shaped conductors D and E having number of needles, are mounted
near the pulleys. The comb D is maintained at a positive potential of the order of 10^4 volt by a
power supply. The upper comb E is connected to the inner side of the hollow metal sphere.

Effects of van de graff generator on human

Because of the high electric field near the comb D, the air gets ionised due to action of points, the negative charges in air move towards the needles and positive charges are repelled on towards the
belt. These positive charges stick to the belt, moves up and reaches near the comb E.
As a result of electrostatic induction, the comb E acquires negative charge and the sphere acquires positive charge. The acquired positive charge is distributed on the outer surface of the sphere. The high electric field at the comb E ionises the air. Hence, negative charges are repelled to the belt, neutralises the positive charge on the belt before the belt passes over the pulley. Hence the descending belt will be left uncharged. Thus the machine, continuously transfers the positive charge to the sphere. As a result, the potential of the sphere keeps increasing till it attains a limiting value (maximum). After this stage no more charge
can be placed on the sphere, it starts leaking to the surrounding due
to ionisation of the air.

The leakage of charge from the sphere can be reduced by
enclosing it in a gas filled steel chamber at a very high pressure.

The high voltage produced in this generator can be used to
accelerate positive ions (protons, deuterons) for the purpose of nuclear