particle accelerator

particle accelerator
accelerator (def. 7).

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Device that accelerates a beam of fast-moving, electrically charged atoms (ions) or subatomic particles.

Accelerators are used to study the structure of atomic nuclei (see atom) and the nature of subatomic particles and their fundamental interactions. At speeds close to that of light, particles collide with and disrupt atomic nuclei and subatomic particles, allowing physicists to study nuclear components and to make new kinds of subatomic particles. The cyclotron accelerates positively charged particles, while the betatron accelerates negatively charged electrons. Synchrotrons and linear accelerators are used either with positively charged particles or electrons. Accelerators are also used for radioisotope production, cancer therapy, biological sterilization, and one form of radiocarbon dating.

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 (See the figure—>.) any device that produces a beam of fast-moving, electrically charged atomic or subatomic particles (subatomic particle). Physicists use accelerators in fundamental research on the structure of nuclei, the nature of nuclear forces, and the properties of nuclei not found in nature, as in the transuranium elements (transuranium element) and other unstable elements. Accelerators are also used for radioisotope (isotope) production, industrial radiography (photography, technology of), radiation therapy, sterilization of biological materials, and a certain form of radiocarbon dating (carbon-14 dating). The largest accelerators are used in research on the fundamental interactions (fundamental interaction) of the elementary subatomic particles.

      This article reviews the development of accelerators and delineates the various types and their distinguishing features. For specific information about the particles accelerated by these devices, see atom and subatomic particle.

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 (antimatter) 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.” (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.

Accelerating (acceleration) particles
      The key feature of any particle accelerator is the accelerating electric field. The simplest example is a uniform static field between positive and negative electric potentials (voltages), much like the field that exists between the terminals of an electric battery. In such a field an electron, bearing a negative charge, feels a force that directs it toward the positive potential (akin to the positive terminal of the battery). This force accelerates the electron, and if there is nothing to impede the electron, its velocity and its energy will increase. Electrons moving toward a positive potential along a wire or even in air will collide with atoms and lose energy, but if the electrons pass through a vacuum, they will accelerate as they move toward the positive potential.

      The difference in electric potential between the position where the electron begins moving through the field and the place where it leaves the field determines the energy that the electron acquires. The energy an electron gains in traveling through a potential difference of 1 volt is known as 1 electron volt (eV). This is a tiny amount of energy, equivalent to 1.6 × 10−19 joule. A flying mosquito has about a trillion times this energy. However, in a television tube, electrons are accelerated through more than 10,000 volts, giving them energies above 10,000 eV, or 10 kiloelectron volts (keV). Many particle accelerators reach much higher energies, measured in megaelectron volts (MeV, or million eV), gigaelectron volts (GeV, or billion eV), or teraelectron volts (TeV, or trillion eV).

      Some of the earliest designs for particle accelerators, such as the voltage multiplier and the Van de Graaff generator, used constant electric fields created by potentials up to a million volts. It is not easy to work with such high voltages, however. A more-practical alternative is to make repeated use of weaker electric fields set up by lower voltages. This is the principle involved in two common categories of modern particle accelerators— linear accelerators (or linacs) and cyclic accelerators (principally the cyclotron and the synchrotron). In a linear accelerator the particles pass once through a sequence of accelerating fields, whereas in a cyclic machine they are guided on a circular path many times through the same relatively small electric fields. In both cases the final energy of the particles depends on the cumulative effect of the fields, so that many small “pushes” add together to give the combined effect of one big “push.”

      The repetitive structure of a linear accelerator naturally suggests the use of alternating rather than constant voltages to create the electric fields. A positively charged particle accelerated toward a negative potential, for example, will receive a renewed push if the potential becomes positive as the particle passes by. In practice the voltages must change very rapidly. For example, at an energy of 1 MeV a proton is already traveling at very high speeds—46 percent of the speed of light—so that it covers a distance of about 1.4 metres (4.6 feet) in 0.01 microsecond. (One microsecond is a millionth of a second.) This implies that in a repeated structure several metres long, the electric fields must alternate—that is, change direction—at a frequency of at least 100 million cycles per second, or 100 megahertz (MHz). Both linear and cyclic accelerators generally accelerate particles by using the alternating electric fields present in electromagnetic waves, typically at frequencies from 100 to 3,000 MHz—that is, ranging from radiowaves to microwaves.

      An electromagnetic wave (electromagnetic radiation) is in effect a combination of oscillating electric and magnetic fields vibrating at right angles to each other. The key with a particle accelerator is to set up the wave so that, when the particles arrive, the electric field is in the direction needed to accelerate the particles. This can be done with a standing wave—a combination of waves moving in opposite directions in an enclosed space, rather like sound waves vibrating in an organ pipe. Alternatively, for very fast-moving electrons, which travel very close to the speed of light (in other words, close to the speed of the wave itself), a traveling wave can be used for acceleration.

      An important effect that comes into play in acceleration in an alternating electric field is that of “phase stability.” In one cycle of its oscillation, an alternating field passes from zero through a maximum value to zero again and then falls to a minimum before rising back to zero. This means that the field passes twice through the value appropriate for acceleration—for example, during the rise and fall through the maximum. If a particle whose velocity is increasing arrives too soon as the field rises, it will not experience as high a field as it should and so will not receive as big a push. However, when it reaches the next region of accelerating fields, it will arrive late and so will receive a higher field—in other words, too big a push. The net effect will be phase stability—that is, the particle will be kept in phase with the field in each accelerating region. Another effect will be a grouping of the particles in time, so that they will form a train of bunches rather than a continuous beam of particles.

Guiding particles
       magnetic fields also play an important role in particle accelerators, as they can change the direction of charged particles. This means that they can be used to “bend” particle beams around a circular path so that they pass repeatedly through the same accelerating regions. In the simplest case a charged particle moving in a direction at right angles to the direction of a uniform magnetic field feels a force at right angles both to the particle's direction and to the field. The effect of this force is to make the particle move on a circular path, perpendicular to the field, until it leaves the region of magnetic force or another force acts upon it. This effect comes into play in cyclic accelerators such as cyclotrons and synchrotrons. In the cyclotron a large magnet is used to provide a constant field in which the particles spiral outward as they are fed energy and thereby accelerate on each circuit. In a synchrotron, by contrast, the particles move around a ring of constant radius, while the field generated by electromagnets around the ring is increased as the particles accelerate. The magnets with this “bending” function are dipoles—magnets with two poles, north and south, built with a C-shaped profile so that the particle beam can pass between the two poles.

      A second important function of electromagnets in particle accelerators is to focus the particle beams in order to keep them as narrow and intense as possible. The simplest form of focusing magnet is a quadrupole, a magnet built with four poles (two norths and two souths) arranged opposite each other. This arrangement pushes particles toward the centre in one direction but allows them to spread in the perpendicular direction. A quadrupole designed to focus a beam horizontally, therefore, will let the beam go out of focus vertically. In order to provide proper focusing, quadrupole magnets must be used in pairs, each member arranged to have the opposite effect. More-complex magnets with larger numbers of poles—sextupoles and octupoles—are also used for more-sophisticated focusing.

      As the energy of the circulating particles increases, the strength of the magnetic field guiding them is increased, which thus keeps the particles on the same path. A “pulse” of particles is injected into the ring and accelerated to the desired energy before it is extracted and delivered to experiments. Extraction is usually achieved by “kicker” magnets, electromagnets that switch on just long enough to “kick” the particles out of the synchrotron ring and along a beam line. The fields in the dipole magnets are then ramped down, and the machine is ready to receive its next pulse of particles.

Colliding particles
      Most of the particle accelerators used in medicine and industry produce a beam of particles for a specific purpose—for example, for radiation therapy or ion implantation. This means that the particles are used once and then discarded. For many years the same was true for accelerators used in particle physics research. However, in the 1970s rings (colliding-beam storage ring) were developed in which two beams of particles circulate in opposite directions and collide on each circuit of the machine. A major advantage of such machines is that when two beams collide head-on, the energy of the particles goes directly into the energy of the interactions between them. This contrasts with what happens when an energetic beam collides with material at rest: in this case much of the energy is lost in setting the target material in motion, in accord with the principle of conservation of momentum (momentum, conservation of).

      Some colliding-beam machines have been built with two rings that cross at two or more positions, with beams of the same kind circulating in opposite directions. More common yet have been particle-antiparticle colliders. An antiparticle has opposite electric charge to its related particle. For example, an antielectron (or positron) has positive charge, while the electron has negative charge. This means that an electric field that accelerates an electron will decelerate a positron moving in the same direction as the electron. But if the positron is traveling through the field in the opposite direction, it will feel an opposite force and will be accelerated. Similarly, an electron moving though a magnetic field will be bent in one direction—left, say—while a positron moving the same way will be bent in the opposite direction—to the right. If, however, the positron moves through the magnetic field in the opposite direction to the electron, its path will still bend to the right, but along the same curve taken by the leftward-bending electron. Taken together, these effects mean that an antielectron can travel around a synchrotron ring guided by the same magnets and accelerated by the same electric fields that affect an electron traveling the opposite way. Many of the highest-energy colliding-beam machines have been particle-antiparticle colliders, as only one accelerator ring is needed.

      As is pointed out above, the beam in a synchrotron is not a continuous stream of particles but is clustered into “bunches.” A bunch may be a few centimetres long and a tenth of a millimetre across, and it may contain about 1012 particles—the actual numbers depending on the specific machine. However, this is not very dense; normal matter of similar dimensions contains about 1023 atoms. So when particle beams—or, more accurately, particle bunches—cross in a colliding-beam machine, there is only a small chance that two particles will interact. In practice the bunches can continue around the ring and intersect again. To enable this repeated beam crossing, the vacuum in the rings of colliding-beam machines must be particularly good so that the particles can circulate for many hours without being lost through collisions with residual air molecules. The rings are therefore also referred to as storage rings, as the particle beams are in effect stored within them for several hours.

Detecting particles
      Most uses of the beams from particle accelerators require some way of detecting what happens when the particles strike a target or another particle beam traveling in the opposite direction. In a television picture tube, the electrons shot from the electron gun strike special phosphors on the inside surface of the screen, and these emit light, which thereby re-creates the televised images. With particle accelerators similarly specialized detectors respond to scattered particles, but these detectors are usually designed to create electrical signals that can be transformed into computer data and analyzed by computer programs. Only electrically charged particles create electrical signals as they move through a material—for example, by exciting or ionizing the atoms—and can be detected directly. Neutral particles, such as neutrons or photons, must be detected indirectly through the behaviour of charged particles that they themselves set in motion.

      There are a great variety of particle detectors, many of which are most useful in specific circumstances. Some, such as the familiar Geiger counter, simply count particles, whereas others are used, for example, to record the tracks of charged particles or to measure the velocity of a particle or the amount of energy it carries. Modern detectors vary in size and technology from small charge-coupled devices (CCDs) to large gas-filled chambers threaded with wires that sense the ionized trails created by charged particles.

      Most of the development of particle accelerators has been motivated by research into the properties of atomic nuclei and subatomic particles (subatomic particle). Starting with British physicist Ernest Rutherford (Rutherford, Ernest, Baron Rutherford of Nelson, of Cambridge)'s discovery in 1919 of a reaction between a nitrogen nucleus and an alpha particle, all research in nuclear physics (physics) until 1932 was performed with alpha particles released by the decay of naturally radioactive (radioactivity) 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 (Gamow, George) (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 (Cockcroft, Sir John Douglas) and E.T.S. Walton (Walton, Ernest Thomas Sinton) 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 (Van de Graaff, Robert Jemison) 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 (linear 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 (Lawrence, Ernest Orlando) 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 (McMillan, Edwin Mattison) 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 (particle accelerator)) and made possible the construction of magnetic resonance accelerators (called synchrotrons (synchrotron)) 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 (Hansen, William Webster), at Stanford University in California, constructed the first traveling-wave linear accelerator (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 (colliding-beam storage ring) (see below Colliding-beam storage rings (particle accelerator)). The highest reaction energies now obtainable have been produced by the use of this technique.

Constant-voltage accelerators
      The simplest type of particle accelerator is constructed by mounting a particle source on one end of an insulated, evacuated tube and creating a high voltage between the ends, with the polarity such that the particles are impelled from the source toward the far end of the tube. Such an accelerator is necessarily linear, and the electrostatic field can be applied to a given particle only once (unless, as in the tandem accelerator described below, the charge of the particle undergoes a change in sign). The simplicity of concept becomes complex in execution when the electric potential exceeds one million volts (1 megavolt, or 1 MV); these high voltages produce corona discharges and lightninglike sparks outside the accelerator, which dissipate the potential needed to accelerate the particles. Even more difficult to control are sparks within the equipment and, in positive-ion accelerators, unwanted secondary beams produced when the accelerated ions strike the end of the tube.

Voltage multipliers (cascade generators)
      The source of the high voltage for Cockcroft (Cockcroft, Sir John Douglas) and Walton (Walton, Ernest Thomas Sinton)'s pioneering experiments was a four-stage voltage multiplier assembled from four large rectifiers (rectifier) and high-voltage capacitors (electricity). Their circuit in effect combined four rectifier-type direct-voltage power supplies in series. The alternating voltage supplied by a high-voltage transformer was transmitted to the higher stages through an array of capacitors; a second group of capacitors kept the direct voltage constant. The final direct voltage would have been four times the peak voltage available from the transformer (200,000 volts) if corona discharge had not drained away considerable power. Nevertheless, the apparatus did accelerate protons to energies of 710 keV, sufficient to bring about the hoped-for result, a reaction with lithium nuclei. This achievement, the first nuclear reaction effected by artificially accelerated particles, was recognized by the award of the Nobel Prize for Physics in 1951.

      Cockcroft and Walton's system for building up high direct voltages can be extended to multiplication factors many times that originally demonstrated. Commercially available accelerators that reach 4 MeV are based on this circuitry.

Van de Graaff generators
 In Van de Graaff (Van de Graaff, Robert Jemison) generators, electric charge is transported to the high-voltage terminal on a rapidly moving belt of insulating material driven by a pulley mounted on the grounded end of the structure; a second pulley is enclosed within a large, spherical high-voltage terminal, as shown in the figure—>. The belt is charged by a comb of sharp needles with the points close to the belt a short distance from the place at which it moves clear of the grounded pulley. The comb is connected to a power supply that raises its potential to a few tens of kilovolts. The gas near the needle points is ionized (ionization) by the intense electric field, and in the resulting corona discharge the ions are driven to the surface of the belt. The motion of the belt carries the charge into the high-voltage terminal and transfers it to another comb of needles, from which it passes to the outer surface of the terminal. A carefully designed Van de Graaff generator insulated by pressurized gas can be charged to a potential of about 20 megavolts. An ion source within the terminal then produces positive particles that are accelerated as they move downward to ground potential through an evacuated tube.

 In most constant-voltage accelerators, Van de Graaff generators are the source of high voltage, and most of the electrostatic proton accelerators still in use are two-stage tandem accelerators. These devices provide a beam with twice the energy that could be achieved by one application of the high voltage. For the first stage of a tandem accelerator, an ion source yields a beam of protons, which are accelerated to a low energy by an auxiliary high-voltage supply. This beam passes through a region containing a gas at low pressure, where some of the protons are converted to negative hydrogen ions by the addition of two electrons. As the mixture of charged particles moves through a magnetic field, those with positive charge are deflected away. Those with negative charge are deflected into the accelerator tube (shown in the figure—>, top), and the beam of negative ions is then accelerated toward a positive high-voltage terminal. In this terminal the particles pass through a thin carbon foil that strips off the two electrons, changing many of the negative ions back into positive ions (protons). These, now repelled by the positive terminal, are further accelerated through the second part of the tube. At the output end of the accelerator, the protons are magnetically separated, as before, from other particles in the beam and directed to the target. In three- or four-stage tandem accelerators, two Van de Graaff generators are combined with the necessary additional provisions for changing the charge of the ions.

      Van de Graaff and Cockcroft-Walton generators also are utilized for accelerating electrons. The rates at which charge is transported in electron beams (electron beam) correspond to currents of several milliamperes; the beams deliver energy at rates best expressed in terms of kilowatts. These intense beams are used for sterilization, industrial radiography (photography, technology of), cancer therapy (radiation therapy), and processing of plastics (radiation).

Betatrons (betatron)
      A betatron is a type of accelerator that is useful only for electrons (electron), which sometimes are called beta particles (beta particle)—hence the name. The electrons in a betatron move in a circle under the influence of a magnetic field that increases in strength as the energy of the electrons is increased. The magnet that produces the field on the electron orbit also produces a field in the interior of the orbit. The increase in the strength of this field with time produces an electric field that accelerates the electrons. If the average magnetic field inside the orbit is always twice as strong as the magnetic field on the orbit, the radius of the orbit remains constant, so that the acceleration chamber can be made in the shape of a torus, or doughnut. The poles of the magnet are tapered to cause the field near the orbit to weaken with increasing radius. This focuses the beam by causing any particle that strays from the orbit to be subjected to forces that restore it toward its proper path. The theory of this focusing was first worked out for the betatron; by analogy, the oscillations of particles about their equilibrium orbits in all cyclic accelerators are called betatron oscillations.

      Just after the sinusoidally varying strength of the magnetic field has passed through zero and starts increasing in the direction proper to guide the electrons in their circular orbit, a burst of electrons is sent into the doughnut, where—in a 20-MeV betatron—they gain about 100 eV per revolution and traverse the orbit about 200,000 times during the acceleration. The acceleration lasts for one-quarter of the magnet cycle until the magnetic field has reached its greatest strength, whereupon the orbit is caused to shrink, deflecting the electrons onto a target—for example, to produce a beam of intense X-rays (X-ray).

      The practical limit on the energy imparted by a betatron is set by the emission of electromagnetic energy from electrons moving in curved paths. The intensity of this radiation, commonly called synchrotron radiation (see below Synchrotrons: Electron synchrotrons (particle accelerator)), rises rapidly as the speed of the electrons increases. The largest betatron accelerates electrons to 300 MeV, sufficient to produce pi- mesons in its target; the energy loss by its electrons through radiation (a few percent) is compensated by changing the relation between the field on the orbit and the average field inside the orbit. At higher energies this compensation would not be feasible.

      Betatrons are now commercially manufactured, principally for use as sources of X-rays for industrial radiography (photography, technology of) and for radiation therapy in medicine. X-ray beams are produced when an electron beam is directed onto a target material with a heavy atomic nucleus, such as platinum.

Cyclotrons (cyclotron)
      The magnetic resonance accelerator, or cyclotron, was the first cyclic accelerator and the first resonance accelerator that produced particles energetic enough to be useful for nuclear research. For many years the highest particle energies were those imparted by cyclotrons modeled upon Lawrence (Lawrence, Ernest Orlando)'s archetype. In these devices, commonly called classical cyclotrons, the accelerating electric field (electron tube) oscillates at a fixed frequency, and the guiding magnetic field has a fixed intensity.

Classical cyclotrons
 The key to the operation of a cyclotron is the fact that the orbits of ions in a uniform magnetic field are isochronous; that is, the time taken by a particle of a given mass to make one complete circuit is the same at any speed or energy as long as the speed is much less than that of light. (As the speed of a particle approaches that of light, its mass increases as predicted by the theory of relativity (relativity).) This isochronicity makes it possible for a high voltage, reversing in polarity at a constant frequency, to accelerate a particle many times. An ion source, as shown in the figure—>, is located at the centre of an evacuated chamber that has the shape of a short cylinder, like a pillbox, between the poles of an electromagnet that creates a uniform field perpendicular to the flat faces. The accelerating voltage is applied by electrodes, called dees from their shape: each is a D-shaped half of a pillbox. The source of the voltage is an oscillator—similar to a radio transmitter—that operates at a frequency equal to the frequency of revolution of the particles in the magnetic field. The electric fields (electric field) caused by this accelerating voltage are concentrated in the gap between the dees; there is no electric field inside the dees. The path of the particle inside the dees is therefore circular. Each time the particle crosses the gap between the dees, it is accelerated, because in the time between these crossings the direction of the field reverses. The path of the particle is thus a spiral-like series of semicircles of continually increasing radius.

      Some means of focusing is required; otherwise, a particle that starts out in a direction making a small angle with the orbital plane will spiral into the dees and be lost. While the energy of the particle is still low, this focusing is supplied by the accelerating electric fields; after the particle has gained significant energy, focusing is a consequence of a slight weakening of the magnetic field toward the peripheries of the dees, as in the betatron.

      The energy gained by a particle in a classical cyclotron is limited by the relativistic increase in the mass (relativistic mechanics) of the particle, a phenomenon that causes the orbital frequency to decrease and the particles to get out of phase with the alternating voltage. This effect can be reduced by applying higher accelerating voltages (relativity) to shorten the overall acceleration time. The highest energy imparted to protons in a classical cyclotron is less than 25 MeV, and this achievement requires the imposition of hundreds of kilovolts to the dees. The beam current in a classical cyclotron operated at high voltages can be as high as five milliamperes; intensities of this magnitude are very useful in the synthesis of radioisotopes (isotope).

Synchrocyclotrons (synchrocyclotron)
      Cyclotrons in which the frequency of the accelerating voltage is changed as the particles are accelerated are called synchrocyclotrons (synchrocyclotron), frequency-modulated (FM) cyclotrons, or phasotrons. Because of the modulation, the particles do not get out of phase with the accelerating voltage, so that the relativistic mass increase does not impose a limit on the energy. Moreover, the magnetic focusing can be made stronger, so that the magnetic field need not be so precisely shaped.

      Because of the phenomenon of phase stability, it is unnecessary to program the frequency of the accelerating voltage precisely to follow the decreasing frequency of revolution of the particles as they are accelerated. To see how phase stability affects the operation of a cyclotron, consider a particle moving in an orbit. Let the frequency of the accelerating voltage match the orbital frequency of this particle. If the particle crosses the accelerating gap at the time the accelerating voltage is zero, its energy and orbital radius will remain unchanged; it is said to be in equilibrium. There are two such times during each cycle of the accelerating voltage; only one of these (that at which the voltage is falling, rather than rising, through zero) corresponds to stable equilibrium. If a particle should arrive a short time before the voltage has fallen to zero, it is accelerated. Its speed therefore increases, but the radius of its orbit increases by an even larger proportion, so that the particle will take longer to reach the gap again and will next cross it at a time closer to that at which it would receive no acceleration. If, on the other hand, the particle reaches the gap a short time after the voltage has fallen through zero, its speed is diminished, and the radius of its orbit is diminished even more, so that it takes less time to reach the gap again, arriving—like the other particle—at a time closer to that at which it receives no acceleration. This phenomenon, by which the trajectories of errant particles are continually corrected, confers stability on the entire beam and makes it possible to accelerate the particles uniformly, by modulating the frequency, without dispersing them. The small periodic variations of the particles about the equilibrium values of phase and energy are called synchrotron oscillations.

      In the operation of a synchrocyclotron, particles are accelerated from the ion source when the frequency of the accelerating voltage is equal to the orbital frequency of the particles in the central field. As the frequency of the voltage falls, the particles, on the average, encounter an accelerating field. They oscillate in phase but around a value that corresponds to the average acceleration. The particles reach the maximum energy in bunches, one for each time the accelerating frequency goes through its program. The intensity of the beam is a few microamperes, much lower than that of a classical cyclotron.

      Large synchrocyclotrons have been constructed in many countries. They are used primarily for research with secondary beams of pi-mesons. The practical upper limit of the energy of a synchrocyclotron, set by the cost of the huge magnets required, is about 1 GeV.

Sector-focused cyclotrons
      The sector-focused cyclotron is another modification of the classical cyclotron that also evades relativistic constraint on its maximum energy (relativistic mechanics). Its advantage over the synchrocyclotron is that the beam is not pulsed and is more intense. The frequency of the accelerating voltage is constant, and the orbital frequency (electron tube) of the particles is kept constant as they are accelerated by causing the average magnetic field on the orbit to increase with orbit radius. This ordinarily would cause the beam to spread out in the direction of the magnetic field, but in sector-focused cyclotrons the magnetic field varies with the angular position as well as with the radius; this produces the equivalent of alternating-gradient focusing (see below Synchrotrons (particle accelerator)). This principle was discovered in 1938 by Llewellyn H. Thomas, then at Ohio State University, but was not applied until the alternating-gradient synchrotron was invented in 1952. Several of these devices, sometimes called azimuthally varying field (AVF) cyclotrons, have been built for use in nuclear and medical research. The world's largest cyclotron, at the TRIUMF laboratory in Vancouver, B.C., Can., is a sector-focused machine. Its magnet, which weighs 4,000 metric tons and is 18 metres (59 feet) in diameter, is divided into six equal sectors arranged like a “pinwheel.” Its maximum energy is 520 MeV, and it is used mainly for research in subatomic particle physics.

Linear resonance accelerators (linear accelerator)
      The technology required for designing a useful linear resonance accelerator was developed after 1940. These accelerators require very powerful sources of radio-frequency accelerating voltage. Further, a practical linear accelerator for heavy particles, such as protons (proton), must make use of the principle of phase stability.

      Linear accelerators fall into two distinct types: standing-wave linear accelerators (used for heavy particles) and traveling-wave linear accelerators (used to accelerate electrons (electron)). The reason for the difference is that, after electrons have been accelerated to a few megaelectron volts in the first few metres of a typical accelerator, they have speeds very close to that of light. Therefore, if the accelerating wave also moves at the speed of light, the particles do not get out of phase, as their speeds do not change. Protons, on the other hand, must reach much higher energies before their speeds can be taken as constant, so the accelerator design must allow for the prolonged increase in speed.

Linear electron accelerators
 The force that acts on electrons in a traveling-wave accelerator is provided by an electromagnetic field with a frequency near 3,000 MHz (1 MHz = 1,000,000 Hertz, or 1,000,000 cycles per second)—a microwave (electromagnetic radiation). The acceleration chamber is an evacuated cylindrical pipe that serves as a waveguide for the accelerating field. The phase velocity of an electromagnetic wave (electromagnetic radiation) in a cylindrical pipe is greater than the velocity of light in free space, so the wave must be slowed down by the insertion of metal irises a few centimetres apart in the pipe, as shown in the figure—>. In the intense field the electrons gain about 2 MeV every 30 centimetres (12 inches) or so. The microwaves are produced by large klystrons (klystron) (high-frequency vacuum-tube amplifiers) with power outputs of 20–30 megawatts. Because sources of radio-frequency power of this magnitude must be operated intermittently (they will not survive continuous service), the beams from these accelerators are delivered in short bursts.

      Pulses of electrons are injected at energies of a few hundred kiloelectron volts (that is, speeds about half that of light). The accelerator is so designed that, during the first part of the acceleration, the electrons are caused to gather into bunches, which then are accelerated nearly to the speed of light. Subsequently, the electrons move with the crest of the electromagnetic wave.

      Linear electron accelerators are manufactured commercially. They are used for radiography, for cancer treatment, and as injectors for electron synchrotrons (electron synchrotron).

 The 3.2-km (2-mile) linear electron accelerator at the Stanford Linear Accelerator Center ( SLAC; see the photograph—>) in California is the source of very energetic beams of electrons (electron beam) and positrons (positron), up to a maximum of 50 GeV. The positrons are produced as secondary particles when the electron beam is allowed to strike a target one-third of the distance along the accelerator, and they are later fed back into the machine, alternately with electrons, for acceleration along its full length. In the Stanford Linear Collider (SLC), which operated from 1989 to 1998, the electrons and positrons were directed into two separate arcs of magnets at the far end of the accelerator. The arcs formed a loop to bring the two beams into head-on collision at a total energy of about 100 GeV.

      Linear electron accelerators constructed of superconducting (superconductivity) materials have been developed. Such structures dissipate far less energy than conventional metal structures, allowing a continuous electron beam, rather than a pulsed beam, to be accelerated. This principle is being exploited to good effect at the Continuous Electron Beam Accelerator Facility (CEBAF) in Newport News, Va. This consists of two 250-metre (820-foot) linear accelerators joined at each end by semicircular arcs to form an oval “racetrack.” Electrons are injected at 45 MeV and can be accelerated to energies of 4 GeV or more, the highest energies being reached after the beams have completed five circuits of the machine.

Linear proton accelerators
      The design principle applied in linear accelerators for protons was originated by Luis Alvarez (Alvarez, Luis W.) at Berkeley in 1946. It is based on the formation of standing electromagnetic waves (standing wave) in a long cylindrical metal tank or cavity. In the design that has been adopted, the electric field is parallel to the axis of the tank. Most of these accelerators operate at frequencies of about 200 MHz—lower than the frequencies employed in linear electron accelerators, owing to the lower velocity of the heavier protons.

 During the time required for a proton to traverse one of these tanks, the accelerating electric fields undergo many reversals of direction. In Alvarez's design the decelerating effect of the field during the intervals when it opposes the motion of the particles is prevented by installing on the axis of the tank a number of “drift tubes.” (See the figure—>.) The electric field is zero inside the drift tubes, and, if their lengths are properly chosen, the protons cross the gap between adjacent drift tubes when the direction of the field produces acceleration and are shielded by the drift tubes when the field in the tank would decelerate them. The lengths of the drift tubes are proportional to the speeds of the particles that pass through them.

      It would appear that any error in the magnitude of the accelerating voltages would cause the particles to lose the synchronism with the fields needed for proper operation of the device, but the principle of phase stability reduces to a manageable magnitude the need for precision in construction. It also makes possible an intense beam because protons can be accelerated in a stable manner even if they do not cross the gaps at exactly the intended times. The principle is the same as that of a synchrotron, except that the gap-crossing time for stable phase oscillations coincides with the rise, rather than the fall, of the voltage wave. If a proton arrives at the accelerating gap late, it receives a larger-than-normal increment of energy, enabling it to “catch up.”

      A very large amount of radio-frequency power is required for producing the accelerating voltages. This makes it necessary for linear proton accelerators to be operated in a pulsed mode. They are supplied with protons accelerated to about 750 keV by a Cockcroft-Walton generator. The entering beam passes through an accelerating radio-frequency cavity a short distance upbeam from the main linear accelerator, so that, as the particles pass through the first drift tubes, they are already bunched.

      As the particle energy increases in the Alvarez design, the drift tubes become longer, and an increasing proportion of the energy stored in the system is not used for acceleration. A more-efficient design, developed at the Los Alamos National Laboratory in New Mexico, is the side-coupled-cavity structure. In this design walls divide the long Alvarez tank into individual cavities that are linked by relatively short drift tubes. Smaller cavities along one side feed radio-frequency power to pairs of adjacent accelerating cavities in such a way that an alternating electric field is set up along the axis of the overall cylindrical structure. Particles traveling along the axis pass from one cell to the next just as the alternating electric field reverses direction, so they always experience an accelerating field. As the velocity of the particles increases, the lengths of the cavities must also increase along the accelerator.

      The highest-energy proton linear accelerator is at the Los Alamos National Laboratory. The protons are accelerated to 100 MeV in Alvarez-type tanks and then to 800 MeV in a standing-wave linear accelerator of the side-coupled-cavity type operated at a frequency of 805 MHz. The accelerator, 785 metres (2,500 feet) long, produces a beam carrying a current in excess of one milliampere, which delivers a power of more than 800 kilowatts. It was built in the late 1960s to provide beams for nuclear research, in particular intense secondary beams of pi-mesons, but it has since become more important as a source of protons to generate neutron beams. Since 1995 it has formed part of the Los Alamos Neutron Science Center (LANSCE), dedicated to research with neutrons.

      The intense pulses of protons produced by linear accelerators make them useful injectors for proton synchrotrons. The highest-energy injector of this kind is at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. The 150-metre- (500-foot-) long machine consists of five Alvarez-type tanks followed by a side-coupled-cavity linear accelerator that accelerates to a final energy of 400 MeV.

Synchrotrons (synchrotron)
      As the particles in a synchrotron are accelerated, the strength of the magnetic field is increased to keep the radius of the orbit approximately constant. This technique has the advantage that the magnet required for forming the particle orbits is much smaller than that needed in a cyclotron to produce the same particle energies. The acceleration is effected by radio-frequency voltages, while the synchronism is maintained by the principle of phase stability. The rate of increase of the energy of the particles is set by the rate of increase of the magnetic field strength. The peak accelerating voltage is ordinarily about twice as large as the average energy gain per turn would require, to provide the margin for phase stability. Particles can be stably accelerated with a range of energies and phases with respect to the accelerating voltage, and very intense beams can be produced.

      The magnetic field must be shaped so as to focus the beam of particles. In early synchrotrons the field was caused to decrease slightly with increasing radius, as in a betatron. This arrangement resulted in a weak focusing effect that was adequate for machines in which the dimensions of the magnet gap could be appreciable in comparison with the radius of the orbit. The magnitude of the magnetic fields that may be used is limited by the saturation (magnet) of the iron components that shape the field and provide a path for the magnetic flux (magnetic field). Therefore, if the energy of accelerators is to be increased, their radius must be increased correspondingly. For relativistic particles the radius is proportional to the kinetic energy. The magnet of a synchrotron with weak focusing, designed to have a reasonable intensity, would have a mass proportional to the cube of the radius. It is clear that increasing the energy beyond some point—in practice, about 10 GeV—would be very expensive.

      The introduction of alternating-gradient focusing provided the solution to this problem and made possible the development of synchrotrons with much higher energies. The idea was promptly incorporated in the design of the 33-GeV proton synchrotron at the Brookhaven National Laboratory in Upton, N.Y., and the 28-GeV machine at the European Organization for Nuclear Research ( CERN), near Geneva.

 The magnetic fields in an alternating-gradient synchrotron vary much more strongly with radius than those used for weak focusing. A magnet with pole-tips shaped as shown in cross section ab in the figure—> produces a magnetic field that sharply decreases with increasing radius. To the particle beam, this magnetic field acts like a lens with a very short focal length (optics). In the vertical direction (the orbital plane is horizontal) it focuses the beam, but in the radial direction it is almost equally defocusing. A magnet with the pole-tip shapes shown in cross section cd in the figure produces a field that strongly increases with increasing radius. This field is defocusing in the vertical direction and focusing in the radial direction. Although pairing such magnetic fields results in partial cancellation, the overall effect is to provide focusing in both directions. The ring of magnetic field is created by a large number of magnets, with the two types of pole-tips alternating, as shown at the top of the figure. The beam, in effect, passes through a succession of lenses as the particles move around the ring, producing a large beam current in a vacuum chamber of small cross section.

      Particles accelerated in a large synchrotron are commonly injected by a linear accelerator and are steered into the ring by a device called an inflector. They begin their acceleration in the ring when the magnetic field is small. As the field created by the ring magnets increases, the injection pulse is timed so that the field and the energy of the particles from the linear accelerator are properly matched. The radio-frequency accelerating devices, usually called cavities, operate on the same principle as a short section of a linear accelerator. The useful beam may be either the accelerated particles that have been extracted from the ring by special magnets or secondary particles ejected from a target that is introduced into the beam.

Electron synchrotrons (electron synchrotron)
      The invention of the synchrotron immediately solved the problem of the limit on the acceleration of electrons that had been imposed by the radiation of electrons moving in circular orbits. This radiation has been named synchrotron radiation because it was first observed during the operation of a 70-MeV electron synchrotron built at the General Electric Company Research and Development Center laboratory in Schenectady, N.Y. A betatron can accelerate electrons to 300 MeV only if the radiation is carefully compensated, but a synchrotron needs only a modest increase in the radio-frequency accelerating voltage. As the particles lose energy by radiation, their average phase with respect to the accelerating voltage simply shifts slightly so as to increase their average energy gain per revolution.

      Electron synchrotrons (electron synchrotron) with energies near 300 MeV have been constructed in several countries, the first being the one built in 1949 at Berkeley under Edwin McMillan (McMillan, Edwin Mattison)'s direction. In these accelerators the electrons were injected by a pulsed electron gun, and the initial acceleration from 50–100 keV to 2–3 MeV was induced as in a betatron. The magnets were specifically designed to provide the accelerating flux in the initial part of the magnet cycle; during this time the speed of the electrons increased from about 50 percent of the speed of light to more than 95 percent. At this point, acceleration by the radio-frequency cavity supervened, and the small further change in speed was accommodated by a 5 percent change in the radius of the orbit.

      Strong focusing was first applied to the electron synchrotron in the 1.2-GeV device built in 1954 at Cornell University in Ithaca, N.Y. All large electron synchrotrons now are equipped with linear accelerators (linear accelerator) as injectors. The practical limit on the energy of an electron synchrotron is set by the cost of the radio-frequency system needed to restore the energy the electrons lose by radiation. To minimize this energy loss, the acceleration time is made as short as possible (a few milliseconds), and the magnetic fields are kept weak. The weak fields keep down the energy loss by guiding the electrons on gently curved paths. However, because synchrotron radiation losses increase as the fourth power of the energy, small increases in energy lead to large increases in radius.

      The largest electron synchrotrons, used in particle physics research, operate as colliding-beam storage rings (colliding-beam storage ring) (see below Colliding-beam storage rings (particle accelerator)). At CERN the Large Electron-Positron (LEP) collider was designed to accelerate electrons (electron) and positrons (positron) initially to 50 GeV and later to about 100 GeV in a ring with a circumference of 27 km (17 miles). This is probably the practical limit for such machines.

      Another way to reduce the energy used in an electron synchrotron is to employ superconducting (superconductivity) radio-frequency accelerating cavities. These have no electrical resistance (resistance) and hence much lower losses due to current heating effects. They are used, for example, to accelerate electrons in the 6.3-km (3.9-mile) ring of the electron-proton collider at the DESY (German Electron Synchrotron) laboratory in Hamburg, Ger. (see below Colliding-beam storage rings: Electron-proton storage rings (particle accelerator)). Superconducting cavities were also used to double the energy of the beams in LEP from 50 GeV per beam with copper cavities to a little over 100 GeV with superconducting cavities.

Proton synchrotrons
      The mode of operation of a proton synchrotron is very similar to that of an electron synchrotron, but there are two important differences. First, because the speed of a proton does not approach the speed of light until its energy is well above 1 GeV, the frequency of the accelerating voltage must be modulated to keep it proportional to the speed of the particle during the initial stage of the acceleration. Second, protons do not lose a significant amount of energy by radiation at energies attainable by present-day techniques. The limit on the energy of a proton synchrotron is therefore set by the cost of the magnet ring, which increases only as the first power of the energy or even more slowly. The highest-energy particle accelerators yet built are proton synchrotrons.

      The first proton synchrotron to operate (1952) was the 3-GeV Cosmotron at Brookhaven. It, and other accelerators that soon followed, had weakly focusing magnets. The 28-GeV proton synchrotron at CERN and the 33-GeV machine at Brookhaven made use of the principle of alternating-gradient focusing, but not without complications. Such focusing is so strong that the time required for a particle to complete one orbit does not depend strongly on the energy of the particle. Therefore, for the energy range (which may extend to several GeV) within which acceleration appreciably affects the speed of the particle, phase stability operates as it does in a linear accelerator: the region of stable phase is on the rising side of the time curve of the accelerating voltage. At higher energies, however, the speed of the proton is substantially constant, and the region of stable phase is on the falling side of the voltage curve, as it is in a synchrocyclotron. At the point that divides these regions, called the transition energy, there is no phase stability. At Brookhaven a model electron accelerator was built to demonstrate that the beam could be accelerated through the transition energy in a stable manner.

      In 1972 a large proton synchrotron went into operation at Fermilab (Fermi National Accelerator Laboratory). This machine had a magnet ring occupying a circular tunnel 6.3 km (3.9 miles) in circumference. At first it accelerated protons to 200 GeV, but by 1976 it had reached 500 GeV. In the same year, a similar accelerator, the Super Proton Synchrotron (SPS), began operation at CERN. The SPS was fed protons by the 28-GeV proton synchrotron (PS) and accelerated them to 400 GeV, reaching 450 GeV at a later date.

      To reach still higher energies, Fermilab built a second synchrotron in the 6.3-km tunnel. The Tevatron was designed to operate at nearly 1,000 GeV, or 1 TeV, the energy that gives the device its name. The intense magnetic fields needed to guide and focus such an energetic proton beam are provided by 1,000 magnets (magnet) with windings made of a superconducting alloy, and the whole ring is kept at 4.5 kelvins by liquid helium (helium). The original synchrotron at Fermilab, based on conventional magnets, served as injector for the Tevatron until 1997. In 1999 the Main Injector, a new synchrotron with a 3.3-km (2.1-mile) magnet ring, replaced the earlier machine to provide a more-intense beam for the Tevatron.

      At Fermilab the proton beam, initially in the guise of negative hydrogen ions (each a single proton with two electrons), originates in a 750-kV Cockcroft-Walton generator and is accelerated to 400 MeV in a linear accelerator. A carbon foil then strips the electrons from the ions, and the protons are injected into the Booster, a small synchrotron 150 metres (500 feet) in diameter, which accelerates the particles to 8 GeV. From the Booster the protons are transferred to the Main Injector, where they are further accelerated to 150 GeV before being fed to the final stage of acceleration in the Tevatron.

      Until 2000, protons at 800 GeV were extracted from the Tevatron and directed onto targets to yield a variety of particle beams for different experiments. The Main Injector then became the principal machine for providing extracted beams, at the lower energy of 120 GeV but at much higher intensities than the Tevatron provided. In 1987 the Tevatron began to operate as a proton-antiproton collider, and this has been its sole function since 2000.

      The SPS at CERN has also operated as proton-antiproton collider and has accelerated heavy ions (such as sulfur and lead ions), as well as electrons and positrons, for injection into the LEP collider (colliding-beam storage ring). Together with the smaller PS, it continues to form part of CERN's integrated complex of accelerators.

Colliding-beam storage rings (colliding-beam storage ring)
      Although particles are sometimes accelerated in storage rings, the main purpose of these rings is to make possible energetic interactions between beams of particles moving in opposite directions. When a moving object strikes an identical object that is at rest, at most half of the kinetic energy of the moving object is available to produce heat or to deform the objects; the remainder is accounted for by the motions of the objects after the encounter. If, however, the two objects are in motion in opposite directions with equal speeds, then all the kinetic energy is available to produce heat or deformation at the instant of collision. If the objects stick together, the combination is at rest after the collision. For particles with speeds close to that of light, the effect is accentuated. If a 400-GeV proton strikes a proton at rest, only 27.4 GeV are available for the interaction; the remainder produces motion of the particles. On the other hand, if two 31.4-GeV protons collide, 62.3 GeV are available for the interaction (the collision is not quite “head-on”).

      In a target of liquid or solid matter, the number of particles per unit volume accessible to an accelerated beam is large, but, when the target of one beam is another beam, the number of particles interacting is much smaller: the rate of interactions is proportional to the product of the currents in the two beams. Donald W. Kerst, builder of the first betatron, realized in 1956 that, though the beam current in a high-energy accelerator is small, the currents circulating in the magnet rings are effectively much larger because of the high orbital frequency of the particles. Thus, if the colliding beams are circulating in such rings, useful experiments on the interactions can be carried out. In a colliding-beam apparatus the two beams may be made up of identical particles (e.g., two beams of protons), in which case the installation consists of two separate rings of magnets. In one ring the magnetic fields guide the particles clockwise; in the other the fields are oriented in the opposite direction so as to guide the particles counterclockwise. The rings intersect at “interaction regions,” where the beams collide. In other cases the two beams are composed of particles of opposite charge (e.g., electrons and positrons, or protons and antiprotons). Such beams circulate in opposite directions in the same vacuum chamber, guided by the same magnets. The particles are bunched so that they collide only in the interaction regions.

      The highest interaction energies at present are, and in the future will be, achieved in colliding-beam storage rings (colliding-beam storage ring). This places the research with them at the very forefront of the quest for knowledge, even though many types of experiments cannot be conducted with storage rings. This is true partly because the number of interactions in a storage ring is a small fraction of that occurring in a stationary target and partly because storage beams do not produce intense beams of secondary particles.

Electron storage rings
      Many storage rings have been constructed to study the interactions of electrons (electron) with positrons (positron). The principal centres of this research are Cornell University; Stanford University; CERN; Tsukuba, Japan; Frascati, Italy; Beijing, China; and Novosibirsk, Russia.

      Electrons (electron) are emitted from a heated filament and accelerated first in a linear accelerator and then in a synchrotron before being injected into a storage ring. To make positrons (positron), a target such as a tungsten plate is inserted at a point along the linear accelerator. The energetic electrons radiate gamma rays (gamma ray) in the heavy target, and these gamma rays can create electron-positron pairs. The positrons, which have positive charge, are selected by a suitable magnetic field and accelerated along the remainder of the linear accelerator. They are then fed into the synchrotron for further acceleration and finally injected into the storage ring. Since they have opposite electric charges, the electrons and positrons circulate in opposite directions through the magnets of a single storage ring.

      Electron-positron storage rings are used principally for research into subatomic particles (subatomic particle). If a single storage ring is used, the two beams will always have the same energy. Because of the pulsed operation of the acceleration system, the particles are stored in bunches, which can be made to collide at only a few places around the ring. Detectors surround one or more of the collision points to record the particles produced when an electron and a positron annihilate (annihilation). Separate storage rings are sometimes used, in particular if the electrons and positrons are to have different energies. In the PEP-II storage rings at Stanford University and in the KEK-B facility at the National Laboratory for High Energy Physics (KEK) in Tsukuba, electrons and positrons are stored at different energies so that they have different values of momentum. When they annihilate, the net momentum is not zero, as it is with particles of equal and opposite momentum, so new short-lived particles (specifically, B-mesons (meson)) are created in motion; this gives them an apparently longer lifetime in the laboratory owing to the effect of time dilation in the theory of special relativity (relativity).

      The highest-energy electron-positron collider built so far is the LEP machine at CERN, which operated from 1989 to 2001. LEP reached a maximum of a little over 100 GeV per beam in a magnet ring that was 27 km (17 miles) in circumference and that occupied a 4-metre- (13-foot-) wide tunnel lying, on average, 100 metres (330 feet) underground. Other accelerators built earlier at CERN acted as injectors to LEP in a complex interlinked system. A purpose-built linear accelerator produced bunches of electrons and positrons at 600 MeV and fed them into the 28-GeV proton synchrotron, where they were accelerated to 3.5 GeV. They were then transferred to the SPS for acceleration to 20 GeV before injection into LEP. In the final stage LEP accelerated the counterrotating beams of electrons and positrons to a maximum energy of just over 100 GeV. The beams were then made to collide at four points around the ring where detectors were located.

      The electrons and positrons in a storage ring emit synchrotron radiation at very great rates—more than a megawatt in some installations. From a high-energy storage ring, the wavelength of this radiation extends into the X-ray region. These storage rings now constitute the brightest sources of electromagnetic radiation available in the ultraviolet (ultraviolet radiation) and X-ray regions. This radiation is proving to be increasingly useful for research in solid-state physics (physics), biophysics, and chemical physics (physics); a few electron storage rings of relatively low energy are equipped with magnetic structures specially designed to bend the beam to produce synchrotron radiation and are operated solely for this purpose.

Proton storage rings
      In 1971 CERN pioneered the storage of protons (proton) with the Intersecting Storage Rings (ISR), in which two interlaced rings each stored protons at 31 GeV. The two beams collided at eight crossing points, giving a total collision energy of 62 GeV. This was equivalent to a stationary target being struck by a beam of 2 TeV.

      A decade later CERN reached much higher energies with a radical new technique, colliding protons with antiprotons (antiproton) that were accelerated and stored together in the ring of the 450-GeV Super Proton Synchrotron. Protons and antiprotons, having opposite electric charge, circulate in opposite directions around the same synchrotron ring. The creation of an intense beam of antiprotons requires a technique known as “stochastic cooling,” developed by Simon Van der Meer (Meer, Simon van der) at CERN. Antiprotons are produced when a high-energy proton beam strikes a metal target, but they emerge from the target with a range of energies and directions, so the resulting antiproton beam is broad and diffuse. Stochastic cooling provides a means of successively applying small correcting forces to the particles in the broad beam until they have been “cooled”—focused into a narrow beam of uniform energy. The technique is to store the particles in a large-aperture ring and use electronic devices to sense the average deviations from the desired orbit and apply an appropriate average correction at a later stage around the ring. The correction signals cross the ring directly on straight paths, so they arrive in time to influence the particles, which are traveling along a longer curved path.

      The highest-energy proton-antiproton collider is the Tevatron at Fermilab (Fermi National Accelerator Laboratory). The antiprotons are produced by directing protons at 120 GeV from the Main Injector at Fermilab onto a nickel target. The antiprotons are separated from other particles produced in the collisions at the target and are focused by a lithium lens before being fed into a ring called the debuncher, where they undergo stochastic cooling. They are passed on first to an accumulator ring and then to the Recycler ring, where they are stored until there are a sufficient number for injection into the Main Injector. This provides acceleration to 150 GeV before transfer to the Tevatron. Protons and antiprotons are accelerated simultaneously in the Tevatron to about 1 TeV, in counterrotating beams. Having reached their maximum energy, the two beams are stored and then allowed to collide at points around the ring where detectors are situated to capture particles produced in the collisions.

      During storage in the Tevatron, the beams gradually spread out so that collisions become less frequent. The beams are “dumped” in a graphite target at this stage, and fresh beams are made. This process wastes up to 80 percent of the antiprotons, which are difficult to make, so, when the Main Injector was built, a machine to retrieve and store the old antiprotons was also built. The Recycler, located in the same tunnel as the Main Injector, is a storage ring built from 344 permanent magnets. Because there is no need to vary the energy of the antiprotons at this stage, the magnetic field does not need to change. The use of permanent magnets saves energy costs. The Recycler “cools” the old antiprotons from the Tevatron and also reintegrates with them a new antiproton beam from the accumulator. The more-intense antiproton beams produced by the Recycler double the number of collisions in the Tevatron.

      The difficulty in making intense beams of antiprotons has led CERN to return to the concept of a proton-proton collider. CERN began building the Large Hadron Collider, or LHC, in 2001, and test operations began in 2008. The LHC replaced LEP in its 27-km- (17-mile-) circumference tunnel in order to accelerate proton beams to 7 TeV. It uses a single ring of superconducting magnets of a special “2 in 1” design that bends protons in opposite directions in two separate beam pipes within the same structure. It is also designed to collide beams of heavy ions. At the Brookhaven National Laboratory in Upton, N.Y., the Relativistic Heavy Ion Collider (RHIC) came into operation in 2000. This has two rings of magnets that cross to accelerate beams of gold ions to 50 GeV and then bring them into head-on collision. The aim is to study quark-gluon plasma (Cosmos), a state of matter that is presumed to have existed in the very early universe.

Electron-proton storage rings
      The Hadron-Electron Ring Accelerator (HERA) at the DESY laboratory stores both electrons (electron) and protons (proton). It is the only machine that operates in this way with particles of different masses. To do so requires two interlaced rings: one to accelerate and store the electrons, the other to accelerate and store the protons. The machine, which began operation in 1992, occupies a tunnel 6.3 km (4 miles) in circumference. With high fields generated by superconducting magnets (conductive ceramics), the proton ring can reach energies up to 820 GeV. The electron energy, however, is limited by synchrotron radiation losses but reaches a maximum 30 GeV with the aid of low-loss superconducting accelerating cavities.

Impulse accelerators
      Primarily for use in research on thermonuclear fusion (nuclear fusion) of hydrogen isotopes (hydrogen), several high-intensity electron accelerators have been constructed. One type resembles a string of beads in which each bead is a torus of laminated iron and the string is the vacuum tube. The iron toruses constitute the cores of pulse transformers (transformer), and the beam of electrons (electron beam) in effect forms the secondary windings of all of the transformers, which are connected in series. The primaries are all connected in parallel and are powered by the discharge of a large bank of capacitors. These accelerators produce electron beams (electron beam) with energies between 1 and 9 MeV and currents between 200 and 200,000 amperes. The pulses are very brief, lasting about 50 nanoseconds. Besides their application to thermonuclear fusion, such accelerators are utilized for flash radiography, research on collective ion acceleration, microwave production, and laser excitation.

Christine Sutton

Additional Reading
The physics background is dealt with at a general level in Frank Close, Michael Marten, and Christine Sutton, The Particle Odyssey (2002); Gordon Fraser, The Quark Machines: How Europe Fought the Particle Physics War (1997); and (at a somewhat deeper level) Jonathan Allday, Quarks, Leptons, and the Big Bang (1998). Discussions of modern accelerator designs can be found in three articles from Scientific American: John R. Rees, “The Stanford Linear Collider,” 261(4):58–65 (October 1989); Stephen Myers and Emilio Picasso, “The LEP Collider,” 263(1):54–61 (July 1990); and Leon M. Lederman, “The Tevatron,” 264(3):48–55 (March 1991). Also useful are Andrew M. Sessler, “Gamma-Ray Colliders and Muon Colliders,” Physics Today, 51(3):48–53 (March 1998); and Justin Mullins, “Perfect Pitch,” New Scientist, 159(2144):52–53 (July 25, 1998). An extensive survey for nonspecialists is available in the article “Particle Accelerator,” in McGraw-Hill Encyclopedia of Science & Technology, 8th ed., vol. 13, pp. 126–155 (1997).More-technical works include Philip J. Bryant and Kjell Johnsen, The Principles of Circular Accelerators and Storage Rings (1993); D.A. Edwards and M.J. Syphers, An Introduction to the Physics of High Energy Accelerators (1993); AIP Conference Proceedings (irregular), papers from seminars and courses published by the American Institute of Physics; and IEEE Transactions on Nuclear Science (bimonthly), for new developments in accelerator technology.Classic research papers of historical interest include J.D. Cockcroft and E.T.S. Walton, “Experiments with High Velocity Positive Ions,” Proceedings of the Royal Society of London, Series A, vol. 137, pp. 229–242 (1932), on the cascade generator; and a selection of articles in Physical Review: Robert J. Van De Graaff, “A 1,500,000 Volt Electrostatic Generator,” 38:1919–20 (1931); D.W. Kerst, “Acceleration of Electrons by Magnetic Induction,” 60:47–53 (1941), on the betatron; Ernest O. Lawrence and M. Stanley Livingston, “The Production of High Speed Light Ions Without the Use of High Voltages,” 40:19–35 (1932), on the classical cyclotron; David H. Sloan and Ernest O. Lawrence, “The Production of Heavy High Speed Ions Without the Use of High Voltages,” 38:2021–32 (1931), on the development of the linear resonance accelerator idea; Edwin M. McMillan, “The Synchrotron: A Proposed High Energy Particle Accelerator,” 68:143–144 (1945); and papers in Claudio Pellegrini and Andrew M. Sessler (eds.), The Development of Colliders (1995).Christine Sutton

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