Electrostatic Accelerators
 
Van de Graaff Accelerator
Van de Graaff Accelerator

How do they work?

The first particle accelerator was built by two physicists, Ernest Walton (1903-1995) and John Cockcroft (1897-1967) and used the principle of electrostatic acceleration.

Electrostatic accelerators work by accelerating a charged particle through a constant potential difference. If a particle has a charge q and mass m and move through a potential difference of V then it will gain a kinetic energy of:

mv2/2 = qV

The Cockcroft Walton accelerator performed the first nuclear disintegration by artificial means. Cockcroft and Walton accelerated protons up to voltage of 700kV and bombarded them onto a target of lithium producing the reaction:

1 1P+37Li -> 2 4He + 2 4He

This was the first experiment to show that one element (lithium) could be artificially transformed or transmuted into another element (helium).

 
Robert J Van de Graff
Robert J Van de Graff

The Van de Graaff Accelerator

The accelerating voltage achievable by a Cockcroft-Walton accelerator is limited by the physical properties of the capacitors and diodes. To obtain higher accelerating voltages a Van de Graaff accelerator is used. The Van de Graaff machine was designed by an American physicist Robert Jemison Van de Graaff (1901-1967)

Here a continually moving belt of insulating material runs between two pulleys which are separated by an insulated column. The lower pulley is earthed and attached to the pulley is a sharp metallic comb which is maintained at a potential difference of a few kV between itself and the pulley. A high electric field is produced at the time of the comb and an electrical discharge occurs through the belt from the comb to the pulley, removing electrons from the belt, making it positively charged. The belt carries the charge up to the top pulley which is inside a large ball shaped metal electrode. The electrode then acquires an increasing positive charge with correspondingly high electric potential. The voltage rises rapidly until an equilibrium is established where the rate of loss of negative charge balances the positive charge current carried by the moving belt. In this way, the Van de Graaf machine can reach very high electric potentials of a few million volts.

 
The inner workings of a Tandem accelerator
A Tandem accelerator

In a Van de Graaff accelerator, the high electric potential is coupled to an accelerating tube at the top of which is an ion source that produces the particles to be accelerated. The maximum accelerating potential is limited by breakdown across the accelerating tube and charge leakage from the high voltage electrode. However, with careful design, Van de Graaff accelerators operating at accelerating potentials in excess of 25 MV have been built.


The Tandem Accelerator

A variation of the Van de Graaff accelerator is the tandem accelerator, which can produce accelerating potentials twice as high as that from a single Van de Graaff accelerator. Negative ions are produced which are then accelerated to a certain electrode which froms part of a Van de Graaff machine. The ions are then stripped of two of their electrons and emerge as positive ions, accelerating away from the central electrode towards the target. The ions thus gain kinetic energy twice, once as negative ions and then as positive ions in a tandem fashion. Accelerators of this kind are able to accelerate protons to energies in excess of 40 MeV.


Links

A biography of Robert J Van de Graaff
About the Van de Graaff accelerator
Very useful introduction to the Van de Graaff Accelerator