History
Most of the development of particle accelerators has been motivated by research into the properties of atomic nuclei and subatomic particles. Starting with British physicist Ernest Rutherford’s discovery in 1919 of a reaction between a nitrogen nucleus and an alpha particle, all research in nuclear physics until 1932 was performed with alpha particles released by the decay of naturally radioactive elements. Natural alpha particles have kinetic energies as high as 8 MeV, but Rutherford believed that, in order to observe the disintegration of heavier nuclei by alpha particles, it would be necessary to accelerate alpha particle ions artificially to even higher energies. At that time there seemed little hope of generating laboratory voltages sufficient to accelerate ions to the desired energies. However, a calculation made in 1928 by George Gamow (then at the University of Göttingen, Ger.) indicated that considerably less-energetic ions could be useful, and this stimulated attempts to build an accelerator that could provide a beam of particles suitable for nuclear research.

Other developments of that period demonstrated principles still employed in the design of particle accelerators. The first successful experiments with artificially accelerated ions were performed in England at the University of Cambridge by John Douglas Cockcroft and E.T.S. Walton in 1932. Using a voltage multiplier, they accelerated protons to energies as high as 710 keV and showed that these react with the lithium nucleus to produce two energetic alpha particles. By 1931, at Princeton University in New Jersey, Robert J. Van de Graaff had constructed the first belt-charged electrostatic high-voltage generator. Cockcroft-Walton-type voltage multipliers and Van de Graaff generators are still employed as power sources for accelerators.

The principle of the linear resonance accelerator was demonstrated by Rolf Wideröe in 1928. At the Rhenish-Westphalian Technical University in Aachen, Ger., Wideröe used alternating high voltage to accelerate ions of sodium and potassium to energies twice as high as those imparted by one application of the peak voltage. In 1931 in the United States, Ernest O. Lawrence and his assistant David H. Sloan, at the University of California, Berkeley, employed high-frequency fields to accelerate mercury ions to more than 1.2 MeV. This work augmented Wideröe’s achievement in accelerating heavy ions, but the ion beams were not useful in nuclear research.

The magnetic resonance accelerator, or cyclotron, was conceived by Lawrence as a modification of Wideröe’s linear resonance accelerator. Lawrence’s student M.S. Livingston demonstrated the principle of the cyclotron in 1931, producing 80-keV ions; in 1932 Lawrence and Livingston announced the acceleration of protons to more than 1 MeV. Later in the 1930s, cyclotron energies reached about 25 MeV and Van de Graaff generators about 4 MeV. In 1940 Donald W. Kerst, applying the results of careful orbit calculations to the design of magnets, constructed the first betatron, a magnetic-induction accelerator of electrons, at the University of Illinois.

Following World War II there was a rapid advance in the science of accelerating particles to high energies. Progress was initiated by Edwin Mattison McMillan at Berkeley and by Vladimir Iosifovich Veksler at Moscow. In 1945 both men independently described the principle of phase stability. This concept suggested a means of maintaining stable particle orbits in the cyclic accelerator and thus removed an apparent limitation on the energy of resonance accelerators for protons (see below Cyclotrons: Classical cyclotrons) and made possible the construction of magnetic resonance accelerators (called synchrotrons) for electrons. Phase focusing, the implementation of the principle of phase stability, was promptly demonstrated by the construction of a small synchrocyclotron at the University of California and an electron synchrotron in England. The first proton linear resonance accelerator was constructed soon thereafter. The large proton synchrotrons that have been built since then all depend on this principle.

In 1947 William W. Hansen, at Stanford University in California, constructed the first traveling-wave linear accelerator of electrons, exploiting microwave technology that had been developed for radar during World War II.

The progress in research made possible by raising the energies of protons led to the building of successively larger accelerators; the trend was ended only by the cost of fabricating the huge magnet rings required—the largest weighs approximately 40,000 tons. A means of increasing the energy without increasing the scale of the machines was provided by a demonstration in 1952 by Livingston, Ernest D. Courant, and H.S. Snyder of the technique of alternating-gradient focusing (sometimes called strong focusing). Synchrotrons incorporating this principle needed magnets only 1/100 the size that would be required otherwise. All recently constructed synchrotrons make use of alternating-gradient focusing.

In 1956 Kerst realized that, if two sets of particles could be maintained in intersecting orbits, it should be possible to observe interactions in which one particle collided with another moving in the opposite direction. Application of this idea requires the accumulation of accelerated particles in loops called storage rings (see below Colliding-beam storage rings). The highest reaction energies now obtainable have been produced by the use of this technique.

Principles Of Particle Acceleration
Particle accelerators exist in many shapes and sizes (even the ubiquitous television picture tube is in principle a particle accelerator), but the smallest accelerators share common elements with the larger devices. First, all accelerators must have a source that generates electrically charged particles—electrons in the case of the television tube and electrons, protons, and their antiparticles in the case of larger accelerators. All accelerators must have electric fields to accelerate the particles, and they must have magnetic fields to control the paths of the particles. Also, the particles must travel through a good vacuum—that is, in a container with as little residual air as possible, as in a television tube. Finally, all accelerators must have some means of detecting, counting, and measuring the particles after they have been accelerated through the vacuum.

Generating particles
Electrons and protons, the particles most commonly used in accelerators, are found in all materials, but for an accelerator the appropriate particles must be separated out. Electrons are usually produced in exactly the same way as in a television picture tube, in a device known as an electron “gun.” The gun contains a cathode (negative electrode) in a vacuum, which is heated so that electrons break away from the atoms in the cathode material. The emitted electrons, which are negatively charged, are attracted toward an anode (positive electrode), where they pass through a hole. The gun itself is in effect a simple accelerator, because the electrons move through an electric field, as described below. The voltage between the cathode and the anode in an electron gun is typically 50,000–150,000 volts, or 50–150 kilovolts (kV).

As with electrons, there are protons in all materials, but only the nuclei of hydrogen atoms consist of single protons, so hydrogen gas is the source of particles for proton accelerators. In this case the gas is ionized—the electrons and protons are separated in an electric field—and the protons escape through a hole. In large high-energy particle accelerators, protons are often produced initially in the form of negative hydrogen ions. These are hydrogen atoms with an extra electron, which are also formed when the gas, originally in the form of molecules of two atoms, is ionized. Negative hydrogen ions prove easier to handle in the initial stages of large accelerators. They are later passed through thin foils to strip off the electrons before the protons move to the final stage of acceleration.