Accelerator complex at Fermilab. Photo credit: Fermi National Accelerator Laboratory

An accelerator is a ring-shaped or linear (straight-line) device that accelerates charged particles to high velocities and energies and through the collision of these particles allows the study of matter at very small scales (less than 10-12 cm). Modern accelerators can produce collisions that mimic the conditions of the early universe. Creating tiny fireballs of high density and high temperature, physicists are able to produce the particles that were abundant in the early universe, a trillionth of a second after the Big Bang. The largest accelerator in the world is the Large Hadron Collider at CERN.


How accelerators work

An accelerator increases the velocity and hence the energy of charged particles through the use of alternating electric fields in an evacuated chamber. The particles must enter the high-frequency field as it begins increasing (negative particles) or decreasing (positive particles) in order to achieve the maximum energy increase. Magnetic fields are used to focus the particles into a narrow stable beam and to maintain the required curvature of the beam. As the particle velocity rises a relativistic increase in mass occurs.


In a linear accelerator, the particles travel in a straight line, usually accelerated by an electric field. In a cyclotron, particles are accelerated in a spiral path between pairs of D-shaped magnets with an alternating voltage between them. In a synchrotron, the accelerating voltage is synchronized with the time it takes the particle to make one revolution. A synchrotron consists of an (often very large) circular tube with magnets to deflect the particles in a curve and radio-frequency fields to accelerate them. The most advanced modern accelerators are colliders, in which beams of particles moving in opposite directions are allowed to collide with each other, thus achieving higher energies of interaction.


Development of the accelerator

The earliest accelerators, called electrostatic accelerators, consisted simply of vacuum tubes in which electrons were accelerated by the voltage difference between two oppositely charged electrodes. The Van de Graaff generator and Cockcroft-Walton machine (see Cockcroft, John) work on the same principle but are larger and more elaborate. A further extension of the same idea is the linear accelerator, or linac, which is a sophisticated machine with many scientific and medical uses. The world's most power linear accelerator is the Stanford Linear Collider in Palo Alto. All straight-line accelerators are handicapped by the finite length of the flight path, which limits the particle energies that can be achieved.


A major breakthrough in accelerator technology came in 1920 when Ernest O. Lawrence invented the cyclotron. In this device, magnets guide the particles along a spiral path, allowing a single electric field to apply many cycles of acceleration. Soon unprecedented energies were achieved, and the steady improvement of Lawrence's simple machine has led to today's proton synchrotrons, whose endless circular flight paths allow protons to gain huge energies by passing millions of times through the electric fields that accelerate them.


Until the 1960s, all accelerators were fixed-target machines, in which the speeding particle beam was made to hit a stationary target of some chosen substance. But then physicists began to build colliders, in which two carefully controlled beams are made to smash into each other at a chosen point. Colliders are more demanding to build, but the effort is well worthwhile. In a fixed-target machine, most of the projectile particles continue the forward motion with the debris after impact on the target. In a collider, on the other hand, two particles of equal energy coming together have no net motion, and collision makes all their energy available for new reactions and the creation of new particles.