# Particle accelerators

A particle accelerator is a piece of apparatus used by physicists to accelerate sub atomic particles to very high speeds and then use these rapidly moving particles to investigate the structure of matter by letting these beams of high energy particles collide with a target.

## Electron gun

The simplest form of particle accelerator is the electron gun. Here a high voltage is used to attract the electrons so that they pass through the cylindrical anode at high speed. To get higher speeds you simply need a higher anode voltage. If you put a target in the beam the electrons collide with it and their kinetic energy is used to break up other particles – the so-called 'atom smasher'.

## Linear accelerator

The problem is that if we want the electrons to move faster we have to increase the anode voltage. As they move faster and faster they are more and more difficult to accelerate (see the file called Relativity). You can compare a simple electron gun to a person with a bat hitting a ball - the harder they hit it the faster the ball moves. There is clearly a limit to how hard they can do this. Imagine now the ball on a track with a line of people standing beside it. The first person hits the ball and as it passes each of the others they also hit it. Its speed increases in steps – each person only increasing its energy a little but the ball gains energy from each hit. This is the principle of the linear accelerator. It is a series of electron guns in a line – each one gives the electron a 'kick' as it passes and so the electrons coming out of the other end have a very high speed. Of course it is not quite that simple and if you want to finds out more read the file called Linear accelerator in the 16-19/Nuclear physics/Text section of the site. At Stanford University in the USA electrons are accelerated down an evacuated tube 3 km long and reach speeds close to that of the speed of light.

## Cyclotron

To reach higher and higher energies linear accelerators had to be built longer and longer. Then in 1900 Lawrence had the idea of bending the stream of electrons in a magnetic field – sort of wrapping up the accelerating electron beam.

### schoolphysics: Cyclotron animation

To see an animation of the operation of a cyclotron click on the animation link.

## Synchrotron

The enormous machine (LHC) at CERN near Geneva, due to start operation in November 2007 will accelerate protons to close to the speed of light and the allow two beams travelling in opposite directions round a 28 km ring to collide with each other! These experiments can take us back to the start of the universe – high energy means a long time ago.

### schoolphysics: Synchrotron animation

To see an animation of the operation of a cyclotron click on the animation link.