A particle accelerator is a device for increasing the energy of electrically charged atomic particles. The particles may be electrons, protons, or charged atomic nuclei. (The particles called neutrons cannot be accelerated individually because they have no charge. They can be given higher energy, however, by accelerating deuterons, the nuclei of heavy hydrogen; the deuteron comprises one proton and one neutron and can be accelerated because the proton carries a charge.) The purpose of increasing the energy of charged particles is to make them useful in studies of nuclear and particle physics, by shooting them at atoms and studying the resulting products with \Ldetector\ls. At very high energies, the charged particles can break up the nuclei of other atoms and interact with other particles, producing transformations that make it possible to study the nature and behavior of the fundamental units of matter. Particle accelerators are also important tools in the effort to develop nuclear fusion devices (see \Tfusion, nuclear\t). The energy of a charged particle is measured in electron volts, where one electron volt is the energy gained by an electron when it passes between electrodes having a potential difference of one volt. Electrons have a negative charge; protons have the same quantity of charge as an electron, but their polarity is positive. Thus, because it includes a proton and an electron, the hydrogen atom is electrically neutral. Other nuclei with several protons in their nuclei have equal numbers of electrons circulating; so they too are neutral. If one or more electrons are stripped from an atom, the atom becomes an ion and has a net positive charge equal to the number of electrons removed. A charged particle can be accelerated only by the application of an electric field, which moves it toward the opposite charge. Beams of particles may be focused by magnets; the effectiveness of the magnets is further enhanced if they are superconducting (see \Tsuperconductivity\t). Early machines in nuclear physics used static, or direct, electric fields; most modern machines, particularly those for the highest particle energies, use alternating fields. The latter are arranged so that particles are exposed to the field only when the field is in the accelerating direction. When the field is reversed and is in a decelerating direction, the particles are shielded from the field by various electrode configurations. DIRECT VOLTAGE ACCELERATORS The first accelerator to be used in nuclear physics was the machine designed by J. D. \TCockcroft\t and E. T. S. \TWalton\t and built in 1932 at the Cavendish Laboratory in Cambridge, England. The machine comprised a high-voltage \Ttransformer\t in which the voltage was multiplied by standard engineering techniques to a final value of 700,000 volts (700 kV). An evacuated tube led from the high-voltage terminal to the grounded end of the system. A hydrogen-gas discharge ion source in the terminal was used to direct protons into the tube, where they were accelerated by the high voltage to an energy of 700,000 electron volts (700 keV). These protons caused the first artificially produced nuclear disintegrations. Cockcroft and Walton received the 1951 Nobel Prize for physics. Van de Graaff Electrostatic Generator During the early 1930s another type of direct voltage generator was developed by R. J. \TVan de Graaff\t at Princeton University. In this machine a charge is carried on an insulating belt from a "spray charger," comprising points charged to a relatively high voltage, to a high-voltage terminal where the belt is discharged and the voltage builds up on the terminal. Eventually, if the process is not limited, an electric discharge similar to a lightning bolt will discharge the terminal. With great care, voltages as high as 3 million volts can be achieved. As in the machine built by Cockcroft and Walton, an evacuated tube carries charged particles from an ion source in the terminal to the grounded end of the system, where they are used to disintegrate nuclei. Later it was shown that much higher voltages are achievable if the machine is enclosed in a tank where a pressure of several atmospheres is maintained. The gas in the tank can be air, or it can be any one of several other gases that seem more suitable for maintaining high voltages. The art of building electrostatic \LVan de Graaff generator\ls continues to advance. An enormous machine currently being built in England is expected to be able to maintain 30 million volts on its terminal. Betatron Another type of direct-voltage accelerator is the \Tbetatron\t, a device in which a varying magnetic field generates the electric field that accelerates a beam of electrons, while the electrons are maintained in a circular orbit by a second magnetic field. The betatron can be thought of as a transformer in which the primary winding excites the magnetic field, and the secondary winding is the accelerating electron beam. ACCELERATORS USING ALTERNATING ELECTRIC FIELDS By using alternating electrical fields in the radio frequency (rf) range for acceleration, it becomes possible to accelerate a particle in a number of steps while maintaining the electric fields at manageable levels, thus avoiding problems connected with electrical breakdown. Linear Accelerator The simplest rf accelerator is the linear accelerator, or linac. It has different forms, depending on whether the purpose is to accelerate electrons or ions. For the acceleration of ions, an appropriate frequency is 200 million cycles per second (200 MHz) or less. The ions are injected along the axis of a long tank, which is excited by a high-powered rf system in a field pattern that includes an electric field along the axis. While the field is in the decelerating phase, the ions are shielded from it by drift tubes in the tank, which are pipes through which the beam passes. As the particles gain energy and velocity, they travel farther during the deceleration phase; therefore, the drift tubes must be longer toward the end of the tank to match the period of the accelerating field. The first linear accelerator was built in 1928 by Rolf Wideroe in Norway. Containing three drift tubes, it was used to accelerate sodium and potassium ions. The accelerator was used solely to demonstrate the principle of rf acceleration. During the 1930s further work was done on the ion linear accelerator at the University of California. But the lack of amplifiers for generating high power at high radio frequencies delayed application of the principle until after World War II, at which time rf sources became available through the development of radar. Shortly after the war, Luis Walter \TAlvarez\t built the first proton linear accelerator in which protons reached an energy of 32 million electron volts (MeV). The power required at a frequency of about 200 MHz was about 2 million watts (megawatts); this very high level limited operation of the machine to pulses less than 1 millisecond long. Since 1950 several proton and ion linear accelerators have been built, some as injectors for still larger machines and some for use in nuclear physics. The largest accelerator is the 800-MeV machine at the \TLos Alamos\t \Tscientific\t LABORATORY in New Mexico. It is used as a meson factory, producing for study the particles of mass that are intermediate between the electron and the proton and that seem to give the force necessary for holding the atomic nucleus together. Because electrons are much lighter than ions, their velocity at a given energy is significantly higher than that of ions. The velocity of a 1-MeV proton is less than 5 percent that of light, whereas a 1-MeV electron has reached 94 percent of the velocity of light. This makes possible the running of electron linacs at much higher frequencies--usually about 3,000 MHz--than those used in ion linacs. The accelerating system for electrons is a few centimeters in diameter, whereas diameters of a few meters are needed for ion linacs. Electron linacs having energies of 10 to 50 MeV are widely used in \Tradiation therapy\t as sources of X rays for treating tumors with intense radiation. The largest electron linac in existence today began operation in 1966 at the \TStanford Linear Accelerator Center\t (SLAC) in California. This machine, more than 3.2 km (2 mi) long, has been able to provide electrons with energies of more than 50,000 MeV, that is, 50 billion, or giga, electron volts (50 BeV or 50 GeV). The addition to SLAC of colliding-beam facilities (discussed below) culminated in the late 1980s in the Stanford Linear Collider (SLC), which can provide collision energies of more than 100 GeV between a beam of electrons and a beam of positrons. Cyclotron Although the ion linear accelerator evolved slowly, a research report by Wideroe in 1928 was the inspiration for another machine that has proved important: the \Tcyclotron\t. Ernest \TLawrence\t, of the University of California, read the report the next year and realized that an ion can be deflected in a circle by a magnetic field in such a way that it will return to a gap where it can receive additional acceleration. It was easily shown that, at least at low energies, in a magnetic field the angular velocity of an ion (see \Tmotion\t, ROTATIONAL) is independent of its energy. This makes it possible for the ion to return several times to be accelerated again and again by the same rf field at the same gap. As the energy is increased, the ion is deflected less by the magnetic field. The result is that the ion path is a spiral, beginning near the center of the magnet and eventually emerging with high energy at the edge of the magnet. The linac's drift tubes are replaced by half-pillboxes (called dees because they are shaped like the letter D), in which the ions are shielded during the deceleration phase of the rf field. With M. S. Livingston, Lawrence built a working cyclotron in 1931. During the 1930s and '40s larger and larger machines were built at the University of California and elsewhere, primarily for the acceleration of protons and deuterons to energies that would reach into the hundreds of MeV. At energies of a few tens of MeV, relativistic effects begin to appear. Instead of velocity increasing with energy, the ion mass increases. This spoils the operating principle of the cyclotron. In the late 1930s this seemed to set an upper limit on the energy that could be achieved by cyclotrons. In 1945, however, E. M. McMillan in California and V. I. Veksler in the \TUSSR\t, working independently, discovered the principle of "phase stability." According to this principle, if the frequency of the accelerating field is changed as the radius of the orbit increases, the energy limit of a cyclotron can be increased indefinitely. Such frequency-modulated cyclotrons, also called synchrocyclotrons, were built and soon reached energies of more than 700 MeV. Synchrotron At high energies, the magnet required for guiding the particles became massive. Pole diameters approached 6 m (20 ft), which necessitated magnet weights of several thousand tons. To achieve still higher energies, a new device was required. In the machine subsequently developed, which was another result of the principle of phase stability, particles traveled on a roughly circular orbit of constant radius. The magnetic field that maintained the particles in orbit was increased as a radio-frequency field increased the energy of the particles. Called a \Tsynchrotron\t in the Western world and a synchrophasotron in the \TUSSR\t, this machine seemed to be limited in the energy it could attain only by size. Early synchrotrons were used to accelerate electrons, and a number have reached energies of several GeV, but the largest synchrotrons are designed to accelerate protons. The first proton synchrotron, Brookhaven National Laboratory's Cosmotron, became operational in 1952 and was the first accelerator capable of producing particle energies above 1 GeV. The largest synchrotrons are the 500-GeV machine at the \TCERN\t Laboratory in Geneva, Switzerland (see \TEuropean Organization for Nuclear Research\t), and the 500-GeV machine at the \TFermi National Accelerator Laboratory\t (Fermilab) in Chicago. The use of superconducting magnets at Fermilab in 1983 raised that machine's potential to 1 TeV (1 trillion eV) and gained it the name of Tevatron (see \Tsuperconductivity\t). Storage Rings The most recent development in the field has been the storage ring, in which accelerated beams of particles circulate in opposite directions for periods of several hours,if necessary. Using an antiproton storage ring, Fermilab is achieving collisions energies of nearly 1.8 TeV; at \TCERN\t, a similar system provides energies of more than 600 GeV. Both U.S. and European scientists are planning still larger accelerators. The proposed U.S. machine, the Superconducting Super Collider (SSC), would yield collision energies of 40 TeV and will require construction of an oval 87 km (54 mi) in circumference. In 1988, Ellis County, Tex., was chosen as the site for the SSC. First ground for the project was broken the following year, despite continuing political objections to the possible cost of the project before it would be completed in 1999. Some scientific observers also felt that the SSC would absorb funds that would be better distributed among several fields than devoted solely to high-energy particle physics. \TCERN\t's large project, the Large Electron-Positron Collider (LEP), was completed in 1989. Starting energies for the LEP were in the 100-GeV range, but \TCERN\t scientists hope to double that capacity within a few years. Meanwhile, the Soviet Union is working to complete a 600-GeV proton accelerator at Protvino, south of Moscow. With the addition of superconducting magnets, the \TUSSR\t hopes to boost this capacity to 3 TeV by the mid-1990s. The development, in 1952, of alternating-gradient focusing--the alternate focusing and defocusing of particle beams--drastically reduced the size of the magnets used in synchrotrons and storage beams and lowered the cost of accelerators. John P. Blewett Bibliography: Anderson, Ian, "A Production Line for Particle Physics," New Scientist, May 19, 1988; Close, Frank, et al., The Particle Explosion (1987); Dawson, J. M., "Plasma Particle Accelerators," Scientific American, March 1989; Humphries, Stanley, Principles of Charged Particle Acceleration (1986); Month, Melvin, and Turner, S., eds., Frontiers of Particle Beams (1988); Sessler, A. M., "New Particle Acceleration Techniques," Physics Today, January 1988; Trefil, J. S., From Atoms to Quarks (1980) and "Beyond the Quark," New York Times Magazine, Apr. 30, 1989. See also: \Tdetector, particle\t; \Tfundamental particles\t; \Tsynchrotron radiation\t.