Particle Accelerators: Cyclotrons, Synchrotrons, and Colliders

Particle accelerators, or ‘‘atom smashers’’ as they are popularly known, are devices that produce concentrated beams of charged particles of very high energy. These beams have been used to study nuclear and atomic structure (x-ray crystallography), to create radioactive isotopes, and to irradiate cancerous tumors with x-rays. After World War II they constituted the essential infrastructure for the new field of high-energy physics.

As accelerator power climbed to ever-higher energies, the laboratories where these microscopes probed to the heart of matter were transformed into huge industrial-scale centers of ‘‘big science.’’ Particle accelerators became particle factories supporting multidisciplinary teams of researchers whose size has increased from less than a dozen in the 1960s to as many as 1500 by the end of the twentieth century.

In the early 1930s a number of attempts were made to produce an energetic beam of charged particles under controlled conditions. One of the simplest devices was that devised by John Cockroft and Ernest Walton working in Ernest Rutherford’s famous Cavendish Laboratory in Cambridge. By charging a bank of capacitors in parallel at low potential and then discharging them through a load resistor to develop a high potential, they could multiply voltage by a factor of 4.

The British scientists attained a steady output of about 500 kilovolts. This was used to accelerate protons (positively charged hydrogen ions) that were then smashed into atoms in a metallic target. Another scheme was the electrostatic generator developed by Robert J. Van de Graaff working at Princeton University. Ions produced by a corona discharge from needle points were transported by motor- driven insulating belts to two spherical conductors mounted on insulating rods. There the belts were discharged into the terminals.

A potential difference of up to 1.5 million volts could be accumulated on the spheres, limited only by voltage breakdown, and sparking between them. This approach was so successful that Van de Graff and others formed their own company after the war to commercialize their device.

In these generators, charged particles picked up energy by falling through a large voltage difference. An alternative idea, also considered at the time, was to have the charged particles gain energy in several small steps. In this way, ‘‘the high-voltage energy would be accumulated on the particles, not on the apparatus’’ (Heilbron and Seidel, 1989).

It was Ernest O. Lawrence, working at the Radiation Laboratory at the University of California in Berkeley, along with his graduate student M. Stanley Livingston, who first successfully applied this concept in the autumn of 1931, accelerating protons to over 1 million volts in a cyclotron. Lawrence won the 1939 Nobel Prize in Physics for his invention.

The cyclotron comprised two hollow half-cylinders or D’s, with a small gap between them. A magnetic field was applied perpendicularly to the D’s. Ions were injected at the center, and were given a small kick by a radio oscillator as they crossed the gap. They described a circular trajectory in the magnetic field, incrementally increasing their energy (say, by 10 kilovolts), and the radius of their path, each time they crossed the gap (say, 100 times in all). Tracing a spiral as they moved from the center of the D’s to the circumference they thus emerged with 1 million volts of energy (in this case).

The key to increasing energy lay in the size of the D’s. The first experimental setup on which Livingston demonstrated the feasibility of the idea could be fitted in the palm of one’s hand. Within the decade it was followed by the 27-inch (69- centimeter) D, the 60-inch (152-centimeter) D, and then the huge 184-inch (467-centimeter) D; this measure being the diameter of the magnet face.

This machine, largely funded by the Rockefeller Foundation at a cost of $1.4 million in 1940, was designed to reach energies higher than 100 million volts, and required its own building to house it.

Cyclotron energies were restricted by the fact that, as the velocity of the particles approached that of the speed of light, the mass changed in accordance with Einstein’s relativistic principles, and the orbital frequency of the particles changed with it. The principles on which the cyclotron was based thus broke down. This limitation on achievable energy was removed with the discovery of phase stability in 1945. The implementation of this technique depended on the whether it was applied to a proton or an electron accelerator.

In the case of protons, the decreasing orbital frequency was compensated for by increasing both the strength of the magnetic field and the frequency of the accelerating voltage. Phase stability allowed engineers to do away with the huge pole faces required in a high-energy cyclotron. The particle beams circulated in evacuated beam pipes surrounded by magnets, radio-frequency generators and power supplies. Economic considerations were the only remaining constraint on particle energy, and the field of high-energy physics was born.

Many innovations have been exploited to increase accelerator energy while containing costs. Strong focusing, discovered in the U.S. in 1952, was an ingenious technique for limiting the crosssection of the particle beam, and therefore of the beam tube in which it circulated. The size (and therefore the cost) of the magnets confining the beam was thus sharply reduced.

Colliding beams of particles traveling in opposite directions is another important way of increasing the energy available for doing physics. The first practical demonstration of this principle occurred at the European Laboratory for Particle Physics (CERN, near Geneva) in 1971. The beams were produced in two different intersecting storage rings (the ISR machine).

Drawing on this experience, CERN scientists won the Nobel Prize for Physics in 1984 for colliding beams of opposite charge circulating in the same ring. The use of superconducting magnets became practicable in the 1980s, and these have opened yet another cost-reducing path to higher energy.

Most laboratories use circular accelerators. Linear accelerators have also been built, notably the 3.2-kilometer-long machine authorized in 1959 near Stanford University in California. Practicable length is, however, a limitation on energy, and linacs are generally used as injectors into ring systems. Today these are gigantic and very expensive.

The new machine under construction at CERN, the large hadron collider, will be housed in a tunnel 27 kilometers in circumference. In 2001 its cost to completion was estimated to be about $1,800 million. The superconducting super collider (SSC), cancelled by the Clinton administration in 1993 when its costs began to creep above $10,000 million, was designed to reach 20 million million volts in a 85-kilometer-long subterranean tunnel in Texas.

Only some governments have been able to afford tools of this kind for such esoteric research, and they have done so for reasons of state. CERN, established in 1954 by 12 European governments who pooled their resources, combined a determination to keep nuclear scientists in Europe with foreign policies sympathetic to the construction of a European community. In the U.S., particle accelerators were identified with world scientific and technological leadership, national prestige, and the ‘‘peaceful atom,’’ and used to promote international scientific exchange behind the Iron Curtain. Correlatively, some would argue, the demise of the SSC is simply one more response to the collapse of the Soviet bloc.