I.

Introduction

Particle Accelerators, devices used to accelerate charged elementary particles or ions to high energies. Particle accelerators are the largest and most expensive instruments used by physicists. They all have the same three basic parts: a source of elementary particles or ions, a tube pumped to a high vacuum in which the particles can travel freely, and some means of speeding up the particles.

Charged particles can be accelerated by an electrostatic field. For example, by placing electrodes with a large potential difference at each end of an evacuated tube, the British scientists John D. Cockcroft and Ernest Walton were able to accelerate protons to 250,000 electronvolts. (1 electronvolt, or 1 eV, is the energy acquired by a particle with 1 unit charge—for example, an electron—when accelerated by a potential difference of 1 volt.) Another electrostatic accelerator is the Van de Graaff accelerator, which was developed in the early 1930s by the American physicist Robert Jemison Van de Graaff. This accelerator uses the same principles as the Van de Graaff generator, building up a potential between two electrodes by transporting charges on a moving belt. Modern Van de Graaff accelerators can accelerate particles to energies as high as 15 MeV (15 million electronvolts).