Synchrotrons

Type of physical science: Elementary particle (high-energy) physics

Field of study: Techniques

A synchrotron is a particle accelerator of uniform radius that uses an oscillating magnetic field to move charged particles in a circular motion. Synchrotrons are used to create nuclear collisions in order to produce elementary particles.

Overview

Synchrotrons are large particle accelerators that move charged particles in a fixed circular path. They were developed to solve the problem of how to gain higher energy without using a single large magnet; instead, they use many smaller magnets that are located around the orbital path. Particles such as protons are first accelerated to high kinetic energies and then injected into the synchrotron. Additional acceleration is provided on each revolution by a radio-frequency generator. This voltage is synchronized with the orbital frequency, thus increasing the particle's energy on each turn. To maintain a fixed orbit with increasing energy, the magnetic field must also be incremented. After reaching the desired speed and energy, the particles are deflected from their circular orbit by a magnet to an experimental area containing targets and detectors.

Prior to the synchrotron, other accelerators were used with varying degrees of success. The Cockcroft-Walton voltage multiplier and the Van de Graaff electrostatic generator had been used to accelerate electrons, but they were limited to energies of a few million electronvolts (megaelectronvolts). The cyclotron experienced problems with electron acceleration because of a large relativistic mass increase at low energies, which caused electrons to become out of step with the accelerating voltage and quickly slow down. Synchrocyclotrons compensated for the mass increase by using a variable oscillating voltage that progressively slowed down with the mass increase. This permitted the particles to get back into phase or step with the voltage and slowly build up their energies. Like cyclotrons, however, these machines were limited by the size and cost of their huge single magnets.

The theoretical principles of the synchrotron were first announced by Vladimir Veksler and Edwin M. McMillan. These principles maintain that by increasing the magnetic field while reducing the frequency of the cyclotron voltage, one can increase the particle's orbit size and energy. If the change is accomplished slowly, then phase stability is maintained during the acceleration. Ions or charged particles may also be accelerated by keeping the frequency of the electrical field frequency and changing the magnetic field. The term "synchrotron" was proposed by McMillan because the machine behaves like a synchronous motor.

Electrons can be injected into a circular chamber at speeds high enough to remain in a stable orbit, which is accomplished by first accelerating the electrons to one or two megaelectronvolts in another type of accelerator, such as a betatron or a linear accelerator. The betatron relies on the same principle as the transformer, whereby an alternating current that is applied to a primary coil induces an oscillating secondary coil current. With the betatron acting as a transformer, a swarm of electrons inside a doughnut-shaped vacuum chamber serves as the secondary coil. If the chamber is placed between the poles of an electromagnet activated by a pulsed current, then a strong magnetic flux permeates the region. The electrons move in response to the changing magnetic field and gain energy by induction.

The betatron, like the cyclotron and synchrocyclotron, has the disadvantage of using a large magnet to supply the required variable magnetic flux for acceleration. The synchrotron addressed this problem by using a radio-frequency voltage to create a variable electric field, allowing the size of the magnet to be reduced. When electrons reach energies of around two megaelectronvolts, a speed very close to that of light, then speed and revolution frequency change very little, permitting the application of a nearly constant voltage. With increasing revolutions, the electron energy and mass become larger and the magnetic field must also be increased. All of this can be accomplished with a fixed-radius machine and a ring-shaped magnet. This type of synchrotron is called an electron synchrotron, as it is designed specifically for accelerating electrons. Electron synchrotrons use flux bars inside the orbit as an aid for the preliminary acceleration. These small bars, made of a high-permeability metal, initially serve to block the magnetic field at low current values and then reach a saturation point at high values, thus aiding the field.

The energy limitations for electrons that are accelerated in a synchrotron are dependent on the radiation losses. The electron, when accelerated, suffers a loss in energy that is proportional to the fourth power of its energy. This limit is reached when essentially all the additional energy that is supplied to the electron is radiated away during each revolution. The greatest particle energy ever achieved by an electron synchrotron was approximately one hundred billion electronvolts (gigaelectronvolts), accomplished by the electron synchrotron at the European Organization for Nuclear Research, also called CERN.

The proton synchrotron differs from the electron synchrotron in two important areas. First, radiation problems that are associated with electron acceleration become significant at high energies but are negligible with protons. Second, protons do not achieve relativistic velocities until their energies reach the gigaelectronvolt range. The rest mass of a proton is about two thousand times that of the electron, which would allow it to reach an energy of ten gigaelectronvolts, radiating as much energy as a five-megaelectronvolt electron. Energy loss by an electron in this range is not high, meaning that protons will not suffer large radiation losses until energies considerably beyond ten gigaelectronvolts are reached.

The radial and vertical deviations of the protons inside the proton synchrotron's acceleration chamber are subject to more precise control than those in the electron synchrotron. The oscillations experienced by the betatron were dependent on the initial injection conditions set up for the electrons. Injection in the Cosmotron, a proton synchrotron at Brookhaven National Laboratory on Long Island that was in operation from 1952 to 1966, was from a Van de Graaff electrostatic generator, which delivered a well-focused beam with amplitude deviations less than ten centimeters. The Cosmotron's magnet was built in sections consisting of 288 magnet blocks reaching twenty-three meters across. The magnet was capable of a maximum of fourteen thousand gauss but was constructed to allow straight sections of the vacuum chamber to be free from the magnetic fields for injection, acceleration, and finally ejection of the ions. The Van de Graaff generator provided protons in pulses of 3.5 megaelectronvolts. Acceleration is increased in intervals of one second, with some eight hundred electronvolts of energy added at each revolution. Protons end up making three million revolutions and travel more than 160,000 kilometers while reaching their highest energies. The radio-frequency voltage is changed from 0.37 million cycles per second to 4 million cycles per second during the operation.

At the high operating power levels of these machines, accurate timing of the magnetic pulses is not possible. The magnetic field at this stage must be used to control the cycles of the radio-frequency voltage. A signal proportional to the rate of change of the magnetic field is integrated electronically and supplied as a control device for the radio-frequency oscillator. The oscillator, through a resonance circuit with a magnetic core, is able to sense the required voltage from variations in the magnetic field. The highest energy level reached by protons in a proton synchrotron was 1.18 trillion electronvolts (teraelectronvolts), achieved by the Large Hadron Collider, also at CERN.

Improvements to the synchrotron design have been aided by the realization that an increase in the efficiency of ion focusing could greatly reduce the size of the magnet. The large magnet of the Cosmotron was required to keep the ions in the desired orbit by use of corrective forces that acted on these particles when they strayed from their path. Such deviations arise from collisions with air molecules in the tube or from fluctuations in the accelerating voltage. It was thought that if the straying of the particles could be more precisely controlled, then the circular pipe in which the particles move could be made much narrower, and only a small, thin magnet would be necessary to surround it.

A device called the alternating-gradient synchrotron was designed to compensate for ion-path variations. This machine operates on the principle of alternating-gradient focusing, which alternately focuses and defocuses the ion beam for a net focusing effect. The principle is similar to the use of both converging and diverging lenses to focus a beam of light. The design features a number of C-shaped magnets arranged in a circle, with alternate magnets facing opposite directions, so that the back of one magnet faces toward the center of the circle and the back of the next faces the outside. Since the ion beam can be contained in a much smaller pipe around the accelerator, the required magnets are also much smaller, considerably reducing the amount of steel and copper needed in construction.

In addition to providing a guide field for the particle path, the magnets of the alternating-gradient synchrotron have another important purpose. The poles of each magnet are shaped to allow the magnetic field to increase and then decrease in an outward direction. This alternation of the magnetic-field gradient permits the proton beam to focus and then defocus both vertically and horizontally, presenting a beam with greater focusing power than that of a conventional constant-gradient machine. The very large magnetic gradients employed require that all the component magnets be precisely built and accurately aligned to avoid errors that could lead to ion-path deviations and possible collisions with the vacuum-chamber walls.

The thirty-gigaelectronvolt Alternating Gradient Synchrotron (AGS) at Brookhaven National Laboratory accelerates protons with a succession of twelve radio-frequency oscillation stations. A high-frequency voltage is created across two gaps at each station. Protons that arrive at the gaps when the voltage and electric field are in the forward direction receive an acceleration. Provided that the applied radio frequency is correct, these protons will be accelerated at each gap. Very large energies can be reached after many cycles with small voltage increments.

The AGS uses a two-stage preliminary acceleration method involving both a Cockcroft-Walton generator and a linear accelerator operated in tandem. After emerging from the linear accelerator, protons travel with a velocity one-third the speed of light and an energy of fifty megaelectronvolts. The third stage of acceleration is performed in the main doughnut-shaped vacuum tube, which is 257 meters across. The magnet is divided into 240 units of mass equal to 14,500 kilograms. The actual vacuum tube measures only eighteen by seven centimeters and is serviced at a pressure equal to one hundred-millionth of an atmosphere.

Applications

Electromagnetic radiation is emitted by charged particles moving in a circular motion at relativistic energies. The emission of light by electrons as they spiral around the celestial magnetic field is an example of synchrotron radiation. The background light of the Crab Nebula and the pulsed beam of light from a pulsar are examples of this type of radiation. The construction of electron synchrotrons and other types of synchrotrons for nuclear physics has permitted the production of synchrotron radiation in the x-ray and ultraviolet regions of the spectrum.

Synchrotron radiation has such features as high intensity, small source size, high polarization, a pulse time structure, and a broad spectral bandwidth. Important research in such fields as biochemistry, materials science, surface science, and crystallography makes use of the techniques of x-ray absorption and scattering, photoemission spectroscopy, and x-ray microscopy.

Synchrotron radiation from protons is some thirteen orders of magnitude weaker than that from electrons of the same energy level and orbital radius. For multi-gigaelectronvolt accelerators, also known as storage rings, the rate of energy loss is so high that the electrons would lose all of their energy very quickly. In contrast, synchrotron radiation from the twenty-eight-gigaelectronvolt proton storage rings located in Geneva, Switzerland, is weak enough that protons could orbit for years without significant losses.

Cosmic-ray research became centered on synchrotrons when it was discovered that protons that strike a stationary target at an energy of several hundred megaelectronvolts produce small particles called pions and muons, which were first observed in cloud chambers. The collision of a proton beam with target nuclei creates pions. As the beam continues a short distance, some of the pions decay into muons. A magnetic field deflects the positive particles of the beam in one direction and the negative particles in the opposing direction. The length of the track that each particle leaves is a function of its energy and momentum. Because the exact initial momentum is known, the mass of each particle may be calculated by comparing the length and density of their tracks.

X-ray absorption is used to investigate atomic structure in complex solids, liquids, and gases. The intense, continuous synchrotron radiation from the high-energy storage rings provides the necessary photon energy for studies of amorphous and other noncrystalline materials, including catalysts and proteins that are not determinable by other research methods. The high intensity and strong collimation of synchrotron radiation make it an ideal source for the small-angle scattering and diffraction of x-rays. Research in the timed contraction cycles of muscular tissue has been undertaken to resolve the associated molecular changes. X-ray diffraction has also been used to study the structures of biological macromolecules such as viruses and proteins, as the arrangements of such molecules have direct bearing on their functions and can aid in the development of new treatments.

In 2013, a group of British scientists reported that such atomic-level study had allowed them to develop a purely synthetic, stable vaccine for foot-and-mouth disease. By using the synchrotron at the Diamond Light Source facility in Oxfordshire to analyze the structure of the virus, they were able to recreate the virus's outer shell without the interior RNA that would allow it to replicate. The body reacts to the synthesized shell as it would to any other vaccine—by producing antibodies to fight the infection—but without the risk of introducing infection itself. The synthetic vaccine is also safer because it does not require facilities to maintain stores of live pathogens.

Context

The desire to accelerate particles to higher energies has led to the development of accelerators of increasing dimensions and technological advancement. The older accelerators are sometimes converted into facilities for preliminary acceleration. The cyclotrons of the 1930s and 1940s evolved into synchrocyclotrons and finally into synchrotrons, which were first built in the early 1950s. The accelerators of the 1930s were no more than a few meters in size; by the 1970s and 1980s, machines that stretched for several kilometers were able to accelerate protons to thousands of gigaelectronvolts. Construction and operation costs have run into the billions of dollars, requiring both national and international efforts.

Energies in the gigaelectronvolt range were first reached in the 1950s with the construction of two large proton synchrotrons. The Cosmotron, built in 1952 at the Brookhaven National Laboratory in Long Island, New York, reached energies of 3 gigaelectronvolts. The second machine, activated in 1954 at the University of California, Berkeley, reached energies of 6.4 gigaelectronvolts, sufficient to create proton-antiproton pairs. By the 1960s, a 28-gigaelectronvolt proton synchrotron was in operation at CERN and involved the cooperation of twelve nations. A superproton synchrotron with a circumference of seven kilometers was brought online at CERN in 1976 and placed in an underground tunnel. Energies in the teraelectronvolt range were first reached in 1984 by the Tevatron, a proton synchrotron located at the Fermi National Accelerator Laboratory (Fermilab) near Batavia, Illinois.

The major breakthroughs of the 1980s were made possible by two important developments in accelerator technology. The first technique for increasing the energy of synchrotrons was to replace existing magnets with powerful superconducting ones. Superconducting magnets can generate much stronger magnetic fields, and they boosted the energy of the Tevatron from four hundred to one thousand gigaelectronvolts, or one teraelectronvolt.

The second technique involves storing particles in intersecting storage rings. CERN's proton synchrotron accelerates protons to twenty-eight gigaelectronvolts and then alternately deflects them by large magnets into two storage rings. The two beams of protons traveling in opposite directions are allowed to intersect each other in an experimental area, thus gaining more energy than would be available from a single beam; the increase in energy comes from the difference between the initial and final kinetic energies of the particles traveling at relativistic speeds. The final energy from two twenty-eight-gigaelectronvolt protons colliding is equivalent to the collision of a fifteen-hundred-gigaelectronvolt proton with a stationary proton. Accelerators that employ this technology are called colliders.

The largest accelerator ring built to date is the Large Hadron Collider (LHC) at CERN. This collider can produce proton energies of up to seven teraelectronvolts, for a collision energy of fourteen teraelectronvolts. The ring is 26.7 kilometers in circumference and incorporates 9,593 superconducting magnets.

Principal terms

ELECTRONVOLT: the energy acquired by an electron or unit charge as it moves through a potential difference of one volt

GAUSS: a unit measure of magnetic-field strength

MAGNETIC FLUX: the passing of magnetic-field lines through an area

PHASE STABILITY: a condition in which a particle is kept in synchronous phase with the accelerating voltage

RADIO FREQUENCY: the oscillating voltage applied to the accelerator

RELATIVISTIC: moving at high speeds that are comparable to the speed of light

RESONANCE: a vibration in phase or unison with a reference signal

REST MASS: the normal mass of an object, measured in kilograms, when it is not moving at high speeds

SUPERCONDUCTING TEMPERATURE: the temperature at which a material loses its electrical resistance and becomes a perfect conductor

Bibliography

Chao, Alexander W., and Weiren Chou, eds. Reviews of Accelerator Science and Technology. 5 vols. Singapore: World Scientific, 2008–12. Print.

Chua, Dao Ming, and Huang Fu Toh, eds. Synchrotron: Design, Properties and Applications. New York: Nova Sci., 2012. Print.

Condon, E. U., and Hugh Odishaw. Handbook of Physics. 2nd ed. New York: McGraw, 1967. Print. A summary of the most important accelerator designs, with a detailed and well-illustrated discussion of both the electron and the proton synchrotron. Alternate gradient focusing is treated mathematically, and its applications are described.

Hatton, Bernard D., and Abelin R. Gilles, eds. Large Hadron Collider (LHC): Phenomenology, Operational Challenges and Theoretical Predictions. Hauppauge: Nova Sci., 2013. Print.

Kaplan, Irving. Nuclear Physics. 2nd ed. Reading: Addison, 1962. Print. An entire chapter is devoted to the acceleration of charged particles, covering ion sources, cyclotrons, synchrocyclotrons, sychrotrons, and linear accelerators. Presents accelerator and component dimensions, magnet sizes and weights, cycling frequencies, and energy levels.

Myers, Stephen, and Emilio Picasso. "The LEP Collider." Scientific American July 1990: 54–61. Print. A discussion of the building and results of the large electron-positron collider in Geneva, Switzerland. Includes full-color illustrations and photographs.

Neeman, Yuval, and Yoram Kirsh. The Particle Hunters. New York: Cambridge UP, 1983. Print. The physicist is compared to a detective in the hunt for particles created by the accelerators. Detecting methods are presented, along with an extensive listing of the elementary particles already found and yet to be discovered.

Parker, Sybil P., ed. McGraw-Hill Encyclopedia of Physics. New York: McGraw, 1982. Print. Diagrams and photographs adequately cover synchrotrons and their operating characteristics. Graphs illustrate phase stability and accelerating frequency. The total energy and electrical currents carried by the various accelerators are presented in table form.

Walsh, Fergus. "Synchrotron Yields 'Safer' Vaccine." BBC News. BBC, 27 Mar. 2013. Web. 16 Jan. 2014.

Wehr, M. Russell, James A. Richards Jr., and Thomas W. Adair III. Physics of the Atom. 4th ed. Reading: Addison, 1985. Print. A good qualitative presentation of accelerators that is well illustrated. Methods of increasing the effective energy of these machines and limitations are included. Suitable for the general reader.

Detectors on High-Energy Accelerators

Nuclear Reactions and Scattering

Radiation: Interactions with Matter

Storage Rings and Colliders