Accelerators

The term accelerators most commonly refers to particle accelerators, devices for increasing the velocity of subatomic particles such as protons, electrons, and positrons. Particle accelerators were originally invented for the purpose of studying the basic structure of matter, although they later found a number of practical applications. Particle accelerators can be subdivided into two large sub-groups: linear and circular accelerators. Machines of the first type accelerate particles as they travel in a straight line, sometimes over very great distances. Circular accelerators move particles along a circular or spiral path in machines that vary in size from less than a few feet to many miles in diameter.

The simplest particle accelerator was invented by Alabama-born physicist Robert Jemison Van de Graaff (1901-1967) in about 1929. The machine that now bears his name illustrates the fundamental principles on which all particle accelerators are based.

In the Van de Graaff accelerator, a silk conveyor belt collects positive charges from a high-voltage source at one end of the belt and transfers those charges to the outside of a hollow dome at the other end of the belt located at the top of the machine. The original Van de Graaff accelerator operated at a potential difference of 80,000 volts, although later improvements raised that value to 5,000,000 volts.

The Van de Graaff accelerator can be converted to a particle accelerator by attaching a source of positively charged ions, such as protons or He⁺ ions, to the hollow dome. These ions feel an increasingly strong force of repulsion as positive charges accumulate on the dome. At some point, the ions are released from their source, and they travel away from the dome with high energy and at high velocities. If this beam of rapidly-moving particles is directed at a target, the ions of which it consists may collide with atoms in the target and break them apart. An analysis of ion-atom collisions such as these can provide a great deal of information about the structure of the target atoms, about the ion "bullets" and about the nature of matter in general.