The Van de Graff Generator
The Van de Graff particle accelerator uses the large voltages generated by a Van de Graff generator to accelerate charged particles to high speeds and energies for use in particle experiments. A high voltage DC supply supplies charge to a spraycomb consisting of sharp conducting points adjacent to a moving insulating paper belt.
Charge is sprayed from the spraycomb onto the belt and carried up to a large hollow conducting terminal at the top of an insulating column. At the top the charge is removed from the belt by another set of spraycombs and moves to the terminal. The charge collects on the outside surface of the terminal. The voltage can be bui;t to to a level – 10 million Volts or more - limited only by the breakdown field of the air, but this can be increased by enclosing the whole apparatus in a nitrogen or freon pressure vessel.
Positive ions can be accelerated down a tube (the accelerator tube above right). It the botton of the tube a magnetic field is applied causing the protons to curve in a circle. The magnetic field bends the paths of the ions to an extent which depends on the ir speeds. Only particles with a narrow range of speeds with emerge from the end of the magnet.