THE LINEAR ACCELERATOR

This type of accelerator was invented by Wideroe, and is the first of the important modern accelerators. It accelerates particles down a straight tube without the use of very high voltages. Some of these machines are very large, the one at Stanford, USA, being over 2 miles long.



The particles move down a vacuum tube under the influence of a travelling wave, which appears regularly with correct phase at the electrode gaps. As the particles reach each gap they are given a "kick" by the accelerating field.

Alternate pairs of electrodes are connected together and an a.c. signal applied to them.

Notice that the electrodes have to get longer as the particle gets further down the tube, since the particles will travel further during each cycle of the field. At very high velocities relativistic effects have to be allowed for.

With electron linear accelerators the particles are injected at about 4 MeV from a small electrostatic accelerator, and in the Stanford machine they are accelerated to energies in excess of 10 GeV (1000 MeV). The average beam current is about 15 mA.

(See also Cyclotron, Synchrotron)