The term “accelerators” usually refers to particle accelerators, devices for increasing the velocity of particles such as protons, electrons, positrons, and ions. Particle accelerators were originally invented for the purpose of studying the basic structure of matter, although they later found a number of practical applications.
Particle accelerators can be subdivided into two basic groups, linear and circular. Linear accelerators accelerate particles in a straight line, sometimes over miles. Circular accelerators move particles along circular or spiral paths in machines that vary in size from less than a few feet to miles in diameter.
The earliest particle accelerator was invented by Alabama-born physicist Robert Jemison Van de Graaff (1901–1967) in about 1929. The machine that now bears his name illustrates the fundamental principles on which all particle accelerators are based.
In the Van de Graaff generator, a nonconducting conveyor belt (silk, in the earliest versions) collects positive charges from a high-voltage source at one end of the belt and transfers those charges to the outside of a hollow metal dome at the other end of the belt located at the top of the machine. The original Van de Graaff accelerator operated at a potential difference of 80,000 volts, although later improvements raised that value to 25,000,000 volts.
The Van de Graaff generator can be used as a particle accelerator by attaching a source of positively charged ions, such as protons or He+ ions, to the hollow dome. The ions are released from their source and travel away from the dome at high velocities, repelled by the positive charge on the dome. If these rapidly-moving particles are directed at a target, the ions may collide with atoms in the target and break them apart. An analysis of ion-atom collisions such as these can provide information about the structure of the target atoms and about the ions themselves.
In a Van de Graaff accelerator, an electrically charged particle is accelerated by exposing that particle to an electric field. The velocity of a proton, for example, may go from near zero to 100,000 miles per second (160,000 km per second) as the particle feels a strong force of repulsion from the positive charge on the generator dome. Linear accelerators (linacs) operate on the same general principle, except that a particle is exposed to a series of electrical fields, each of which increases the velocity of the particle.
A typical linac consists of a few hundred or a few thousand cylindrical metal tubes arranged one in front of another. The tubes are electrically charged so that each carries a charge opposite that of the tube on either side of it. The odd-numbered tubes might, for example, be positively charged, and the even-numbered tubes negatively charged.
Imagine that a negatively charged electron is introduced into a linac just in front of the first tube. In the circumstances described above, the electron is attracted by and accelerated toward the tube. The electron passes toward and then into that tube. Once inside the tube, the electron no longer feels any force of attraction or repulsion and merely keeps on moving through the tube passively—drifting—until it reaches the far end. It is because of this behavior that the cylindrical tubes in a linac are generally referred to as drift tubes.
At the moment that the electron leaves the first drift tube, the charge on all drift tubes is reversed. Odd-numbered plates are now negatively charged and even-numbered plates are positively charged. The electron exiting the first tube now finds itself repelled by the tube it has just left and attracted to
the second tube. These forces of attraction and repulsion provide a kind of “kick” that accelerates the electron in a forward direction. It passes through the space between the first and second tubes and enters the second tube. Once again, the electron drifts through this tube until it exits at the far end.
Again electrical charge on all drift tubes reverses, and the electron is repelled by the second tube and attracted to the third. The added energy it receives is manifested in a greater velocity. As a result, the electron is moving faster in the third tube than in the second and can cover a greater distance in the same amount of time. To make sure that the electron exits a tube at just the right moment, the tubes must be of different lengths. Each one is slightly longer than the one before it.
The largest linac in the world is the Stanford Linear Accelerator, located at the Stanford Linear Accelerator Center (SLAC) in Stanford, California. An underground tunnel 2 mi (3 km) in length passes beneath U.S. highway 101 and holds 82,650 drift tubes along with the magnetic, electrical, and auxiliary equipment needed for the machine’s operation. Electrons accelerated in the SLAC linac leave the end
of the machine traveling at nearly the speed of light with a maximum energy of about 32 GeV (gigaelectron volts).
The term electron volt (ev) is the standard unit of energy measurement in accelerators. It is defined as the energy lost or gained by an electron as it passes through a potential difference of one volt. Most accelerators operate in the megaelectron volt (million electron volt; MeV), gigaelectron volt (billion electron volt; GeV), or teraelectron volt (trillion electron volt; TeV) range.
Linear accelerators are limited by certain basic physical constraints. For example, the SLAC linac is so long that engineers had to take into consideration the Earth’s curvature when they laid out the drift tube sequence. One way of avoiding the problems associated with the construction of a linac is to accelerate particles in a circle. Machines that operate on this principle are known as circular accelerators.
The earliest circular accelerator, the cyclotron, was invented by University of California professor of physics Ernest Orlando Lawrence in the early 1930s. Lawrence’s cyclotron added to the design of the linac one new fundamental principle from physics: a charged particle that passes through a magnetic field travels in a curved path. The shape of the curved path depends on the velocity of the particle and the strength of the magnetic field.
The cyclotron consists of two hollow metal containers that look as if a tuna fish can had been cut in half vertically. Each half resembles a uppercase letter D, so the two parts of the cyclotron are known as dees. At any one time, one dee in the cyclotron is charged positively and the other negatively. The dees are connected to a source of alternating current so that the signs on both dees change back and forth many times per second.
The second major component of a cyclotron is a large magnet that is situated above and below the dees. The presence of the magnet means that any charged particles moving within the dees will travel not in straight paths, but in curves.
Imagine that an electron is introduced into the narrow space between the two dees. The electron is accelerated into one of the dees, the one carrying a positive charge. As it moves, the electron travels in a curved path because it is in a magnetic field.
After a fraction of a second, the current in the dees changes signs. The electron is then repelled by the dee toward which it first moved, reverses direction, and heads toward the opposite dee with an increased velocity. Throughout this process, the electron’s path is curved because of the magnetic field surrounding the dees, so the electron does not simply collide with the one dee or the other.
Just as a particle in a linac passes through one drift tube after another, always gaining energy, so does a particle in a cyclotron. As the particle gains energy, it picks up speed and spirals outward from the center of the machine. Eventually, the particle reaches the outer circumference of the machine, passes out through a window, and strikes a target.
Lawrence’s original cyclotron was a modest piece of equipment—only 4.5 in (11 cm) in diameter—capable of accelerating protons to an energy of 80,000 electron volts (80 kiloelectron volts). It was assembled from coffee cans, sealing wax, and leftover laboratory equipment. The largest accelerators of this design ever built were the 86 in (218 cm) and 87 in (225 cm) cyclotrons at the Oak Ridge National Laboratory and the Nobel Institute in Stockholm, Sweden, respectively.
At first, improvements in cyclotron design were directed at the construction of larger machines that could accelerate particles to greater velocities. Soon, however, a new problem arose. Nothing can travel faster than the speed of light. Thus, adding more and more energy to a particle will not make that particle’s speed increase indefinitely. Instead, as the particle’s velocity approaches the speed of light, additional energy supplied to it appears in the form of increased particle mass. A particle whose mass is constantly increasing, however, travels in a path different from that of a particle with constant mass. The practical significance of this fact is that as the velocity of particles in a cyclotron begins to approach the speed of light, those particles start to fall out of sync with the alternating current change that drives them back and forth between the dees.
Two different modifications—or a combination of the two—in the basic cyclotron design can deal with this problem. One approach is to gradually change the rate at which the electrical field alternates between the dees. The goal here is to have the sign change occur at exactly the moment that particles have reached a certain point within the dees. As the particles speed up and gain weight, the rate at which electrical current alternates between the two dees slows down to catch up with the particles.
In the 1950s, a number of machines containing this design element were built in various countries. Those machines were known as frequency modulated (FM) cyclotrons, synchrocyclotrons, or, in the Soviet Union, phasotrons. The maximum particle energy attained with machines of this design ranged from about 100 MeV to about 1 GeV.
A second solution for the mass-increase problem is to alter the magnetic field of the machine in such a way as to maintain precise control over the particles’ paths. This principle has been incorporated into the machines that are now the most powerful cyclotrons in the world, the synchrotrons.
A synchrotron consists essentially of a hollow circular tube (the ring) through which particles are accelerated. The particles are actually accelerated to velocities close to the speed of light in smaller machines before they are injected into the main ring. Once they are within the main ring, particles receive additional jolts of energy from accelerating chambers placed at various locations around the ring. At other locations around the ring, very strong magnets control the path followed by the particles. As particles pick up energy and tend to spiral outward, the magnetic fields are increased, pushing particles back into a circular path. The most powerful synchrotrons now in operation can produce particles with energies of at least 400 GeV.
In the 1970s, nuclear physicists proposed the design and construction of the most powerful synchrotron of all, the superconducting super collider (SSC). The SSC was expected to have an accelerating ring 51 mi (82.9 km) in circumference with the ability to produce particles having an energy of 20 TeV. Estimated cost of the SSC was originally set at about $4 billion. Shortly after construction of the machine at Waxahachie, Texas, began, however, the U.S. Congress decided to discontinue funding for the project.
By far the most common use of particle accelerators is basic research on the composition of matter. The concentrations of energy released in such machines are unmatched anywhere on Earth. At these energy densities, forms of matter are produced that do not exist under ordinary conditions or even in the hearts of stars. These forms of matter provide clues about the ultimate structure of matter and the origins of the universe.
Electron— A fundamental particle of matter carrying a single unit of negative electrical charge.
Ion— An atom or molecule that has acquired electrical charge by either losing electrons (positively charged ion) or gaining electrons (negatively charged ion).
Positron— A positively charged electron.
Potential difference— The work that must be done to move a unit charge between two points.
Proton— A fundamental particle matter carrying a single unit of positive electrical charge.
Accelerators have also found some important applications in medical and industrial settings. As particles travel through an accelerator, they give off a form of electromagnetic radiation known as synchrotron radiation. This form of radiation is somewhat similar to x rays and has been used for similar purposes.
Looking ahead The most powerful accelerator ever built, the Large Hadron Collider, is due to begin operation at CERN (the European Organization for Nuclear Research) in late 2007. (A hadron is any subatomic particle that is influenced by the strong nuclear force; protons and neutrons are hadrons.) This machine is a circular accelerator that will, in its first version, produce particle energies of up to 6 TeV. Since particles traveling in opposite directions can be made to collide, collision energies of up to 12 TeV will be possible. By observing the results of such collisions in the Large Hadron Collider, physicists intend to answer fundamental questions about matter and gravity.
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David E. Newton