Particle accelerator

Particle accelerator, research tools used to accelerate electrically charged subatomic particles to high velocities. Physicists can focus the resulting particle beams to interact with othr particles or to break up atomic nuclei, in order to learn more about the fundamental nature of matter. Accelerators use electromagnetic fields to accelerate the particles in a straight line or in a circular or spiral path. The devices are rated according to the kinetic energy they impart, which is measured in electron volts (eV).

The first accelerator, designed by John Douglas Cockcroft and Ernest Thomas Walton in l932, accelerated protons to energies of 700 keV.

The first linear accelerator, or linac, was built in l928 by R. Wideröe. The largest linear accelerator still in operation, 2 mi long (3.2-km), at Stanford University, can accelerate electrons to energies of 20 GeV.

The first circular accelerator, the cyclotron, was built by E.O. Lawrence in l931. The particles were accelerated twice in each revolution, spiraling outward and eventually shot out toward a target; however the relativistic gain in mass tended to throw them out of phase with the acceleration pulses. The solution was the synchrocyclotron, or frequency modulated cyclotron, which varies the acceleration frequencies to keep them in phase with the particles. The largest synchrocyclotron today achieves energies of more than 700 MeV. As still higher energies continued to be sought, the synchrotron was developed, guiding the particles around a ring of magnets through a thin evacuated tube. The largest now operating, at the Fermi National Accelerator Laboratory in Batavia, III., achieves energies in excess of 500 GeV.

The latest accelerators are colliding-beam machines, in which positive and negative particles circle in opposite directions. The resulting head-on collisions yield much higher effective energies than collisions with stationary targets.