The first particle accelerators built were linear accelerators 6(linacs). These machines were constructed on the notion of producing particles in one part of a machine and then using a strong electric field to accelerate them to high energies across the machine. At the opposite end of the machine, the particles were caused to collide with a target. Linacs possess one inherent disadvantage, however. To increase the energy of particles, the machine must be made longer and longer. For example, in order to build the two mile (3.2 km) linac at Stanford University in an exactly straight line, the engineers who designed it had to make sure it did not follow the Earth's curvature. Scientists were aware of this problem early on and looked for ways to deal with it. The most successful solution was that proposed by E. O. Lawrence in the early 1930s. Lawrence suggested that particles be subjected to electric and magnetic fields that would force them to travel in curved paths rather than straight lines. This type of cyclotron, or circular accelerator, consists of two hollow D-shaped compartments with a gap between them. One compartment (called a dee) is charged positively and the other, negatively. Particles to be accelerated are introduced in the center of the gap between the two dees. They are attracted to the dee whose charge is opposite their own and are repelled by the dee whose charge is the same as their own. As the particles move toward and then into the oppositely-charged dee, the charge on the dees is reversed. The particles are now repelled by the dee into which they had traveled and are attracted to the opposite dee. They reverse their path and head back to the opposite dee. Their path is not, however, a straight, back and forth line between the two dees. Large magnets above and below the dees cause the particles to move in a curved path, a spiral. Each time the electric field changes polarity, the particles change direction, pick up energy, and move through the machine in a larger and larger spiral. At some point, the particles attain maximum energy, reach the circumference of the cyclotron, are directed out of the machine, and strike a target. The first cyclotron was built by Lawrence at the University of California. It was made out of coffee cans, sealing wax, and left-over laboratory equipment. The machine was 4.3 inches (11 cm) in diameter and produced particles with energies of 80 keV (kiloelectron, or thousand-electron, volts). The next year, Lawrence and M. Stanley Livingston (1905-1986) built a larger machine, twenty-seven inches (69 cm) in diameter that they upgraded to thirty-seven inches (94 cm) in 1937. Within a short time, dozens of cyclotrons were being constructed in research centers all over the world. The two largest of these machines were a 85.8 inch (218 cm) cyclotron at the Oak Ridge National Laboratory and a 88.6 inch (225 cm) machine at the Nobel Institute in Stockholm, Sweden. Both of these cyclotrons could accelerate particles to energies of 22 MeV (megaelectron, or million-electron, volts). The way to build more powerful cyclotrons would seem to be obvious: simply make them larger and larger. That solution eventually breaks down, however. Albert Einstein (1879-1955) had showed as early as 1905 that mass and energy are closely related. As a particle moves through an accelerator and picks up energy, some of that energy is converted to mass. The more energy the particle receives, the more massive it becomes. At low velocities, this mass increase is modest. A particle moving at one-fifth the speed of light, for example, gains only two percent over its rest mass. But at velocities that are easily attained in large cyclotrons, the mass increase becomes very large. The consequence of increasing mass is that particles slow down as they spiral through the machine.
With each revolution, they tend to fall a little behind the alternating electric field, which is changing at a regular rate. Eventually, they are completely "out of synch" with the electric field, and they become lost within the machine. The solution to this problem was devised independently by Vladmir I. Veksler in what was then the Soviet Union and Edwin McMillan at the University of California in the 1940s. The solution is to gradually reduce the speed at which the electric field alternates. When that happens, particles within the machine will automatically adjust to the field and will stay "in synch" with it. The term phase stability is used to describe the principle discovered by Veksler and McMillan. The first machine to employ this principle, called a synchrocyclotron, was put into operation at the University of California at Berkeley in November 1946. It was 184 inches (4.67 m) in diameter and soon produced protons with energies of 720 MeV. Synchrocyclotrons are capable of accelerating protons to energies up to 1 GeV (gigaelectron, or billion-electron, volts). The practical upper limit for such machines is the size and cost of the magnets they require. A synchrocyclotron four times larger than the 4.67 meter machine requires magnets fifty times as large to keep particles in their proper path. At this larger size, it becomes less expensive to adopt another cyclotron design than it is to keep making bigger synchrocyclotrons. Fortunately, a second solution exists for the problem of mass increase at high energies. That solution is the sector-focusing cyclotron. In a sector-focusing cyclotron, it is the magnetic field, rather than the electric field, that is changed. Recall that the path taken by particles in a cyclotron is determined by the magnetic field imposed on them. By increasing the strength of the magnetic field at a regular rate, it is possible to make particles move in smaller circles each time they go around the machine. In this way, they can be kept "in synch" with the alternating electric field. The term sector-focusing refers to the technique by which the above process is accomplished. In 1938 Llewellyn H. Thomas, at Ohio State University, showed that the most efficient design for controlling the magnetic field was to divide the two dees into pie-shaped wedges, known as sectors. The magnetic fields in each sector alternate between strong and weak. The next stage in the development of circular accelerators was the betatron, invented by Donald Kerst (1911-), at the University of Illinois, in 1939. The betatron uses a doughnut-shaped hollow ring, rather than dees or pie-shaped sectors. It uses two sets of magnets. The first set acts as do magnets in other accelerators, guiding the path of the particles. The second magnet, placed in the center of the ring, is used to accelerate particles. Because of technical limitations, the betatron is used to accelerate electrons only. The final stage in the development of Lawrence's original cyclotron concept has been the synchrotron. The synchrotron, like the betatron, consists of a doughnut-shaped hollow ring. Particles are first accelerated by a linear accelerator and then injected into the synchrotron. Once inside the synchrotron, they are given additional energy when they pass through accelerating cavities placed at various positions along the ring. The particles' paths are carefully controlled by magnets that are also placed at various locations along the ring. Instead of giant magnets that surround the whole machine, much smaller magnets can be used in specific locations around the ring. The first electron synchrotron came into full operation at the University of California at Berkeley in 1949. It produced electron beams of 320 MeV energy. The first proton synchrotron was the 3-GeV Cosmotron opened at the Brookhaven National Laboratory in 1960. The largest synchrotron currently in use is the machine at the Fermi National Laboratory near Chicago, Illinois. It produces protons with a maximum energy of 1,000 GeV, or 1 TeV (teraelectron, or trillion-electron volts). Because of this energy output, the Fermi machine is also known as the Tevatron.
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